Title of Invention	"SYNERGISTIC LIGHTWEIGHT CEMENT SLURRY COMPOSITION"
Abstract	The present invention relates to a non-foamed high performance lightweight cement slurry compositions having low fluid loss, low rheology, nil free water, high stability, gas tight property and enhanced compressive strengths and lower permeability upon setting which makes it especially suitable for application in cementing against oi/gas bearing producing zones in varieties of well bore conditions comprising of a coarse hydralic cement, fume silica, lighweight aggregate, dispersant, set cement retarder, optionally fluid loss control additive and defoaming agent, water in a quantity sufficient to provide a slurry of density ranging from 1.00 to 1.50g/cm cube and the invention also relates to a proces for the preparation of said cement slurry composition by adding dispersant, cement set retarder, optionally fluid loss additive and defoaming agent to sufficient quantity of water, mixing, adding to ita a blend of fume silica and coarse hydraulic cement followed by lightweight aggregate and finally mixing in a Warring blender.

Title of Invention

"SYNERGISTIC LIGHTWEIGHT CEMENT SLURRY COMPOSITION"

Abstract

The present invention relates to a non-foamed high performance lightweight cement slurry compositions having low fluid loss, low rheology, nil free water, high stability, gas tight property and enhanced compressive strengths and lower permeability upon setting which makes it especially suitable for application in cementing against oi/gas bearing producing zones in varieties of well bore conditions comprising of a coarse hydralic cement, fume silica, lighweight aggregate, dispersant, set cement retarder, optionally fluid loss control additive and defoaming agent, water in a quantity sufficient to provide a slurry of density ranging from 1.00 to 1.50g/cm cube and the invention also relates to a proces for the preparation of said cement slurry composition by adding dispersant, cement set retarder, optionally fluid loss additive and defoaming agent to sufficient quantity of water, mixing, adding to ita a blend of fume silica and coarse hydraulic cement followed by lightweight aggregate and finally mixing in a Warring blender.

Full Text	Field of Invention The present invention relates to a synergistic lightweight cement slurry composition. The present invention relates to a non foamed high performance lightweight cement slurry compositions for cementing pipe in well bores, more . particularly compositions for cementing pipe against well bores penetrating oil/gas bearing subterranean zones or formations which readily fracture at low hydrostatic pressures. Background of the invention and Prior Art references Definition of certain terms used in following text: BHT- bottomhole temperature BHCT - bottomhole circulating temperature BHST - bottomhole static temperature BHP - bottomhole pressure Pv - plastic viscosity; Yp - yield point MD - measured depth TVD - true vertical depth In the production of hydrocarbons from the subterranean formation, the subterranean formation is typically cemented or sealed by pumping a queous hydraulic cement slurry into the annulus between the pipe and the formation. Typically, the cement slurry is pumped down the inside of the casing and back up the outside of the casing through the annular space. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe in the well bore and bonds the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented. In some locations, the subterranean oil/gas bearing zones or formations into or through which wells are drilled have high permeability and low compressive and tensile strengths. As a result, the resistances of the zones or formations to shear are low and they have very low fracture gradients. When a well fluid such as a hydraulic cement composition is introduced into a well bore penetrating such a subterranean zone or formation, the hydrostatic pressure exerted on the walls of the well bore can exceed the fracture gradient of the zone or formation and cause fractures to be formed in the zone or formation which may result in a significant loss of cement into the formation during cementing operations. This loss of cement composition is problematic because, inter alia, less of the cement composition will remain in the annular space to form the protective sheath to isolate the exposed producing zones and thereby causing undesirable migration of fluids between zones or formations penetrated by the well bore. It is known that cementing of final production casing in oil and gas wells is a very special sector of the art which requires cement slurries having a special set of properties. Thus in order to obtain effective cementing, it is necessary to have a slurry in which the following properties can be suitably adjusted: setting time; fluid loss; compressive strength; rheology; stability; permeability and gas channeling control. Though lightweight cement compositions have been developed and used heretofore, but it is known that it is particularly difficult to obtain a lightweight cement composition having adequate properties in all the above mentioned properties simultaneously so as to make it suitable for application in production casing cementation under all type of conditions. In this respect the study of the prior art is most revealing. It can be seen that the object of known slurries is to solve one of the problems mentioned above without being capable of solving all of them. Thus US Patent 4,933,031 describes of a low weight slurry composition consists of hydraulic cement, microsilica, hollow ceramic bubbles and having the gas tight property. The low weight slurry compositions (1.1 - 1.45 density) though gas tight but has very low compressive strength of maximum 600psi US Patent 5,588,489 claims a lightweight slurry composition comprising of slag cement, water sufficient to form pumpable slurry, a gas sufficient to foam the slurry and foaming agent. The patent claims lightweight, fast setting cement compositions which set into high compressive strength substantially impermeable masses. The slurries though claim to have high compressive strength however the 24 hour strength at 80°F for different samples of density in the range 10-12ppg has shown to achieve strength in the range of 200-600psi only. US Patent 6,516,883 relates to a low density cement compositions having enhanced compressive, tensile and bond strength upon setting which comprises of a hydraulic cement, sufficient water to form a slurry, hollow glass microspheres surface treated with a mixture of organosilane coupling agents present in an amount sufficient to produce a cement composition density of about 6 pounds per gallon, a gas present in an amount sufficient to foam the cement composition whereby the cement composition has a density of about 5 pounds per gallon or below and a mixture of foaming and foam stabilizing surfactants. The patent claims that the test cement compositions of this invention containing organosilane coupling agent surface treated hollow glass microspheres had significantly better tensile strengths, compressive strengths and shear bond strengths than did the same cement compositions containing untreated hollow glass microspheres. i A lightweight cement composition of US Patent 6,562,122 basically comprises of coarse particulate hydraulic cement, an ultrafine particulate hydraulic cement mixtureconsisting of slag cement and a Portland or equivalent cement, fly ash, fumed silica, hollow glass spheres and water. The cement compositions have excellent properties including high static gel strengths, low rheology, low fluid loss, high compressive strength upon setting, low permeability upon setting and resistance to chemical deterioration and failure due to sulfate degradation or the like. The lightweight cement compositions of this invention at density of 11.6ppg have 24 hours compressive strength in the range of 700 - 2400psi at temperatures ranging from 200.degree. F. to 275.degree F. Fluid loss of the slurry is low after addition of high dosage (5-6%) of fluid loss additive. Density of the slurry ranges from 9-13ppg. US Patent 6,626,991 relates to a cement having a density lying in the range 0.9 g/cm3 to 1.3 g/cm3 and being constituted by a solid fraction and a liquid fraction, having porosity (volume ratio of liquid fraction over solid fraction) lying in the range 38% to 50%. The solid fraction is constituted by a mixture of lightweight particles, microcement and optionally portland cement and gypsum. Such cements have remarkable mechanical properties due to their very low porosity in spite of having very low density. The patent Application EP0449360A1 relates to a lightweight cement composition for use in high pressure environments (greater than 3000 psi) comprising of an aqueous hydratable cement composition, ceramic microspheres and a foaming surfactant. In accordance with the invention, the cementing composition further includes microsilica. The compressive strength achieved for the lightweight slurry formulation at 70°C and 24 hours having 1.20 and 1.10 density are in the range of 1537 and 962psi respectively. US Patent 4,721,160. describes a lightweight cement slurry composition having a density in the range of 1.2 to 1.6, comprising of cement, an extender in the form of solid particle having density less than 2.2 gm/cc and in particular fly ashes, hollow silica alumina microspheres and coal derivatives and an SBR latex. The slurry is particularly gas tight. US Patent 4,415,366 provides foamed thyrotropic cement slurry comprising Portland cement and calcium sulphate hemihydrate in which a gas or a mixture of gases along with surfactant is injected to make it lightweight A lightweight, fast setting cement composition basically comprises of slag cement, water sufficient to form pumpable slurry, a gas sufficient to foam the slurry and a foaming agent has been described in US Patent 5, 711,801. US patent publication 2005076812: This pertains to methods of cementing, using a cement composition comprising a desegregating agent for inhibiting the segregation of the light weight bead in cement composition. WO publication 2004076796: this patent document also describes the prevention of segregation of light-weight beads. US patent 6457524: this document is related to a foamed light-weight cement composition having reduced transition time for deep water conductor casing application to prevent flow from any formation sands over pressured by water. CA patent 2111252: this document relates to the method of drilling a well-bore and subsequently disposing of the drilling fluid by converting it to a cementation composition. EP 0605114 relates to a process of cementing a well using a cementing composition containing a portion of the drilling fluid used to drill the well. A set actvated cementing composition comprised of water, silica fume, a dispersing agent, a set activator, a set delaying additive and a portion of the drilling fluid, is introduced into the well and permitted to set into a hard mass therein. US patent 53332041 deals with the method of cementing a well-bore by using set-activated cementation compositions whereas the cementitious compositions are basically comprised of water, particulate condensed silica, a dispersing agent, a set activator and a set delaying additive for increasing the time in which the composition sets after the set activator is combined therewith. The subject matter of the documents like Concrete Technology by M.S. Shetty, Concrete Technology by Gambhir, Reinforced Concrete by H.J. Shah, are not related to the slurry composition of the present invention. Also, the lightweight cement compositions utilized heretofore have often not had sufficient compressive strengths and low permeability upon setting which is a prerequisite for its use against cementation of producing zones. Recently the need for better economics, higher productivity and more efficient well operations has brought about new well drilling and completion techniques. One of the examples of such new techniques include the drilling of well horizontally inside the producing formation and extending the reservoir penetration by drilling wells at a high angle and thereby maximizing the area of producing zones in the well bore to increase the productivity of the reservoir. Subterranean formations transverses by such well bores are often weak and extensively fractured and have a very low margin between pore and fracture pressure because of high MD/TVD ratio and thus requiring improved lightweight cement compositions having low rheology, so that the cement compositions can be accurately placed without losses. In addition, the cement compositions must have high stability at bottom hole conditions as an unstable slurry may also cause zonal isolation problem in a deviated or horizontal well where free fluid can collect along the high side of the annulus and sedimentation can result in a highly porous, low strength & low density channel, which in turn may contribute to zonal communication and gas migration (fig -1) Another example of such new techniques include the reduction of the well bore diameter (referred to as a slim hole) and extending the reservoir penetration by drilling small lateral well bores which are completed using small diameter pipe such as coiled tubing to increase the productivity of the reservoir. The performance of primary cementing operations in the smaller annular spaces in the well bores requires improved lightweight cement compositions having low viscosities so that the cement compositions can be accurately placed. In addition, the cement compositions must have good static gel strength, low rheology, high compressive strength, low fluid loss, low permeability, good chemical resistance and a broad operating temperature range. For some wells the zones adjacent to the cement containing annulus, contain gas under substantial pressure. This gas can cause serious problems during cementations of such wells as the gas may penetrate through the cement slurry whereby an undesirable phenomenon referred to in the art as gas migration may occur. Gas migration may start during setting of the cement slurry. Shortly after the cementation process is finished the cement slurry will change from a hydrostatic fluid to a solid state body. If the cement slurry in this critical phase is not able to resist the gas pressure, channels will be formed in the partial cured cement slurry. Gas under pressure will then flow through the column during the setting of the cement slurry and/or at the boundary between the cement slurry and the wall of the well. This may result in channels which can reach the top of the well. Gas migration control is an especially difficult problem where cement slurry having a very low density has to be used. For oil wells which are drilled through low pressure formations where it is not advisable to expose the formations to high hydrostatic pressure, cement slurries having a low density have to be used, as a cement slurry having a too high density and thereby a high hydrostatic pressure may result in breakdown of the formation and loss of the cement slurry into the formation (lost circulation). In order to obtain a successful cementation with single lightweight hydraulic cement slurry in such varieties of well bore conditions it has to satisfy a number of specific requirements. The rheology of the cement slurry has to be low in order to reduce the pressure losses in the annulus for effective displacement of drilling mud and placement of the slurry in the annular space between the casing and the well. The set cement should have low permeability, right angle setting behaviour and gas tight property to prevent gas migration after placement. The set cement should also develop sufficient ultimate compressive strength so that it can be placed against the oil/gas bearing zones for zonal isolation. Further the cement slurry must have a low filtration loss in order to avoid loss of liquid from the cement slurry into the formation, as loss of liquid will increase the viscosity of the cement slurry. The cement compositions also should have high stability and smallest possible tendency of water separation at bottom hole conditions as any compromise on this parameters may also cause zonal isolation problem in a deviated or horizontal well. Finally, the cement slurry must have density low enough for placement in the annular space between casing and open hole without breaking down the formation. It is known that it is particularly difficult to obtain lightweight cement having adequate properties in all the above mentioned fields simultaneously. Up till now, such lightweight cement having a specific gravity lying in the range 1.0-1.5 has not been known. The cement slurries in the most widespread use have densities of about 1900 kg/m3, which is about twice the density desired for certain deposits. To lighten them, the simplest technique is to increase the quantity of water while adding stabilizing additives (known as "extenders") to the slurry for the purpose of avoiding particles settling and/or free water forming at the surface of the slurry. Manifestly, that technique cannot get down to a density close to 1000 kg/m3. Furthermore, hardened cements formed from such slurries have greatly reduced compression strength, a high degree of permeability, and poor adhesive capacity. For these reasons, that technique cannot be used to go below densities of about 1300 kg/m.3 while still conserving good isolation between geological layers and providing sufficient reinforcement for the casing. Another technique consists in lightening the cement slurry by injecting gas into it (generally air or nitrogen) before it sets. The quantity of air or nitrogen added is such as to reach the required density. That technique provides performance that is a little better than the preceding technique since the density of gas is lower than that of water, so less needs to be added. Foamed cements produced with densities below 1.0 g/cm3, but use of these kinds of cements for cementation of oil wells are, however, associated with a number of drawbacks and disadvantages. The entrained gas is more and more compressed as the hydrostatic pressure increases and, on the other hand, the entrained gas will expand as the cement slurry moves upwardly in the annulus between the casing and the wall of the well. Use of foamed cements therefore requires complicated and costly means for adjusting and controlling the gas content in the slurries and it is necessary to have an extensively control during the cementation process. In order to obtain effective cementing of the casing against producing zones in all type of wellbore conditions by a single type of lightweight slurry, the slurry must have optimum combination of properties like low fluid loss; high compressive strength; low rheology; high stability; low permeability and gas tight. The known cement slurries of the prior art have some adequate properties only and thus it is suitable only for application in production casing cementation of some types of well and not for any wellbore conditions. In the present invention lightweight cement slurry is provided which can be applied for cementation of any wellbore conditions because of its suitable properties in relation to the requirement of cementing. Major limitations of the Prior Art The cement slurry compositions have limited adequate properties and thus it cannot be used for cementation of the casing against producing zones in different wellbore conditions. The compressive strength of cement slurries are not adequate and though in some cases good but still less than the present invention None of the cement slurry compositions have good stability at bottom hole conditions which is a requirement for high angle wellbore cementing. Few cement slurry compositions possess high permeability. Thus, there are continuing needs for low density cement compositions which will be a single product with several significant advantages for improved primary well cementing of production casing in variety of wellbore conditions. The surprising results of the present invention is achieved by reducing void spaces between the solid particles in the cement composition by selecting and packing with common granular material and thus reducing its water requirement to form a slurry compared to the lightweight slurries of the prior art, which ultimately resulted in better performance of the slurry. SUMMARY OF THE INVENTION The present invention relates to a synergistic lightweight cement slurry composition. The present invention particularly relates to non-foamed high performance lightweight cement slurry compositions having low fluid loss, low rheology, nil free water, high stability, gas tight property and enhanced compressive strengths and lower permeability upon setting which makes it especially suitable for application in cementing against oil/gas bearing producing zones in varieties of well bore conditions comprising of a coarse hydraulic cement, fume silica, lightweight aggregate, dipersant, set cement retarder, optionally fluid loss control additive and defoaming agent, water in a quantity sufficient to provide a slurry of density ranging from 1.00 to 1.50g/cm3 and the invention also relates to a process for the preparation of said cement slurry compositions. Thus the present invention provides an improved lightweight slurry composition suitable for application in almost all well bore conditions such as high angle wells, gas wells, slim holes etc The objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon reading of the description of preferred embodiments which follows. Accordingly, the present invention provides a synergistic lightweight cement slurry composition for cementing pipe in well bores, said composition including: a. a hydraulic cement; b. fume silica 30-35% by weight of coarse particulate hydraulic cement, c. lightweight aggregate 30- 70% by weight of coarse particulate hydraulic cement, d. dispersing agent 0.2 - 2.0% by weight of coarse particulate hydraulic cement, e. set cement retarder 0.1 - 2.0% by weight of coarse particulate hydraulic cement, f. fluid loss control additive 0 - 1.0% by weight of coarse particulate hydraulic cement, and g. water present in a quantity sufficient to provide slurry of density ranging from about 1.0 and 1.5g/cm3. Brief description of the accompanying drawings Figure-1 represents effect of slurry stability on annular isolation in high angle wells wherein 11 is Free fluid; 12 is low strength permeability cement; 13 is medium strength cement; 14 is high strength cement; 15 - gas; 16 - oil zone B; 17 - shale and 18 - oil zone A; Figure 2 represents conventional slurry; Figure 3 represents high performance slurry; in figures 2 and 3, wherein 21 is cement, 22 is water, 23 is light weight material and 24 is strengthening material. Figure 4 represents effect of combining different size particles on packing densities Figure 5 represents gas migration test results Figure 6 represents cement setting behaviour DESCRIPTION OF PREFERRED EMBODIMENTS It is known that it is particularly difficult to obtain a lightweight cement slurry composition having all adequate properties. Till now lightweight cement composition having all required properties for cementing against oil/gas bearing subterranean zone penetrated by a well bore having density ranging from 1.0-to 1.5 g/cm3 has not been achieved. The density of the cement must be adjusted so that the pressure at the bottom of the well compensates at least for the pore pressure in the geological formations through which the well passes so as to avoid any risk of eruption. As well as this lower limit on density, there is also an upper limit. This upper limit is that the hydrostatic pressure generated by the column of cement plus the head losses due to the circulation of the fluids being pumped must remain below the fracturing pressure of the rocks in the section being cemented. Certain geological formations are very fragile and require densities close to that of water or even lower. The present invention provides a high performance lightweight cement slurry compositions having enhanced compressive strength and lower permeability upon setting for cementing pipe in well bores penetrating subterranean oil/gas bearing zones or formations which readily fracture at low hydrostatic pressures. The cement slurry compositions of the present invention also have other excellent slurry properties like low rheology, low fluid loss, zero free water & high stability at bottom-hole conditions. The composition is developed based on the innovative technology of maximizing the compactness of the solid mixture blend taking a lead from the technology, prevalent in concrete cement industry. In a preferred composition of the high performance lightweight cement slurry basically comprises essentially a coarse particulate hydraulic cement; fume silica having a bulk density in the range of 600-700kg/cm3 present in an amount in the range of from about 15% to about 60% based on the weight of said coarse particulate hydraulic cement; lightweight filler material based on the weight of cement with a real particle density between 0.1 and 1.5gm/cm3 present in an amount sufficient to impart a density to said cement composition in the range of from about 1.0 to about 1.5g/cm3 and water present in an amount sufficient to form a slurry. The cement compositions also preferably include a dispersing agent in an amount ranging from about 0% to about 2%, a set retarder in an amount ranging from about 0% to about 2.0% and optionally a fluid loss control additive present in an amount ranging from about 0.1% to about 1% by weight of the coarse particulate hydraulic cement. The compositions are particularly useful in cementing of depleted fields, low fracture gradient formations, loss prone zones, high angle wellbore, slim holes, gas wells and eliminating stage cementations when long cement column rise is required. The present method of the invention whereby high performance lightweight non foamed slurry formulation close to that of density of water has been designed considerably simplifies the cementing operation, since it avoids any need for logistics of the kind required for foaming. Improved methods of cementing pipe in well bores penetrating subterranean oil/gas bearing zones or formations which readily fracture at low hydrostatic pressures are provided by the present invention. The present invention provides universal non-foamed high performance lightweight cement compositions having low fluid loss, low rheology, nil free water, high stability, gas tight property and enhanced compressive strengths and lower permeability upon setting which makes it especially suitable for application in cementing against oil/gas bearing producing zones in varieties of well bore conditions. Thus the present invention provides a single lightweight slurry system having improved properties in all the known fields so as to make it a universal slurry system for application in almost all wellbore conditions. The present method of the invention of high performance lightweight slurry compositions for cementation of oil/gas bearing subterranean zones or formations which readily fracture at low hydrostatic pressures is based on the theories of optimum particle packing which is used in the civil engineering industry for optimization of granular dry blends to obtain ultrahigh performance concrete. The main principle of this technology is maximizing the packing of dry blend to exhibit easy miscibility at reduced water. The higher the packing or compactness of the solid mixture in the dry blend, the better is the performance of the slurry made out of the blend. Replacing excess mix water in the slurry with common granular materials is the key to this new technology. In conventional lightweight slurries, the void spaces between the solid particles are filled up with excess water thereby increasing its porosity, permeability and decreasing its compressive strength (fig-2). The principle of concrete technology is to reduce the void space between the solid particles in the slurry by packing with common granular material and thus reducing its water requirement. Maximizing the packing of a dry blend is possible only when there is a high ratio between the mean diameters (d50) for two consecutive granular classes because in that case the smaller particles will fill in the voids between the larger particles (fig-3). In figure 2 and 3, wherein 21 is cement, 22 is water, 23 is light weight material and 24 is strengthening material. The present method of the invention is characterized in that particulate additives are incorporated in the cement slurry, such that in combination with one another and with the other particulate components of the slurry, they give rise to a grain-size distribution that maximize the compactness of the solid mixture blend thus significantly altering the properties of the slurry. The reduction in slurry density is achieved only by reducing the blend specific gravity during designing of the lightweight slurry compositions. Nevertheless, Theological and mechanical properties will only be satisfactory if the size of the particles and the volume distribution thereof is selected in such a manner as to maximize the compactness of the solid mixture. For a solid mixture having two components will provide significant reductions in inter-particle volumes but mixtures having three or more components are preferred since they make it possible to obtain greater compactness (fig-4) if the mean sizes of the various components are significantly different. Accordingly, the present invention of lightweight cement slurry compositions for use in cementation of well penetrating a oil/gas bearing subterranean zone or formation which readily fractures at low hydrostatic pressure consist of three components. The cement slurry compositions are comprised of a hydraulic cement, 5 - 60% fume silica based on the weight of cement, 10-200%, lightweight filler material based on the weight of cement with a real particle density between 0.1 and 1.5gm/cm3, 0.2% - 2% of a thinner/dispersant (dry weight) based on the weight of cement, 0% -1% fluid loss additive (dry weight) optionally based on the weight of the cement, 0-2% set cement retarder (dry weight) based on the weight of the cement and water in such an amount that the cement slurry has a density between 1.0 and 1.5 g/cm3. The cement compositions of the present invention have excellent slurry properties including gas tight, low rheology, low fluid loss, zero free water, high compressive strength & high stability at bottom hole conditions. The cement compositions of the present invention have low rheology whereby when required they can be pumped at turbulent flow which aids in displacing drilling fluid from the well bore. The low rheology of the cement composition also produces low friction pressure when the composition is pumped which lowers the risk of fracturing easily fractured zones or formations. When the cement compositions of this invention are utilized in cementation of well penetrating a oil/gas bearing subterranean zone or formation which readily fractures at low hydrostatic pressure, low rheology is particularly important in preventing fracturing of weak subterranean zones or formations. The high stability of the low weight cement compositions will also minimize the risk of annular communication after cementation in high angle well. The gas tight property of the invented slurry will be of special use in cementation of gas wells for prevention of annular communication problem. The performance of the slurries made based on the technology of particle packing mainly depends on the degree of compactness or packing of the solid blends achieved by way of selection of proper size and proportion of constituents in the blends. The low density is obtained by combining lightweight particles and cement but the slurry properties will be satisfactory only if the size of the particles and the volume distribution thereof is selected in such a manner as to maximize the compactness of the solid mixture. The higher the packing of the blend, higher will be the performance of the slurries made out of the blend. Though through optimization of packing and by use of lightweight particles, micro cement and optionally Portland cement and gypsum (US patent 6,626,991) low density slurry having low porosity and improved mechanical properties are being designed, however superior slurry properties of the present invention are being achieved only because of attainment of optimized packing of the blend because of selection of proper constituent material, its size and its proportion in the blend. In accordance with the present invention that all the necessary properties of the slurry can be achieved only if weight ratio of liquid to solid in the slurry is maintained in the range of about 0.50 to 0.60 by way of selecting proper size and proportion of the constituent material. The high performance lightweight cement compositions of this invention can be used over a broad temperature range of from about 80.degree.F to about 270.degree.F, at densities in the range of from about 8.7 to about 12.20 pounds per gallon. Because the cement composition has a low density, i.e., a density such that the hydrostatic pressure of the cement composition exerted in the subterranean zone or formation being cemented is less than the fracture gradient of the subterranean zone or formation, fracturing of the zone or formation does not take place. Also, because the cement composition of this invention has enhanced compressive and lower permeability upon setting, a strong bond exists between the pipe and the walls of the well bore penetrating the subterranean zone or formation which prevents formation fluids from entering the annulus between the pipe and the well bore. The high overall strength of the cement composition also prevents it from being shattered by contact with the drill bit and drill string when the well is drilled to greater depths. The term "hydraulic cement" as used herein refers to those inorganic materials which set up to a hard monolithic mass under water. The coarse particulate hydraulic cement can be any of a variety of hydraulic cements having a maximum particle size of about 118 microns and a specific surface area of about 2800 square centimeters per gram. Portland cement is generally preferred, and the coarse cement can be, for example, one or more of the various Portland cements designated as API Classes A-H cements. These cements are identified and defined in the in API Specification for Cements and Materials for Well Cementing, API Spec 10 A, Twenty-Second Edition, January 1, 1995, which is incorporated herein by reference API Portland cements generally have a maximum particle size of about 90 microns and a specific surface of about 3900 square centimeters per gram. When an API Portland cement is utilized as the coarse hydraulic cement in accordance with this invention, it is preferably API Class G cement. Other hydraulic cements which are coarser than API Portland cement can also be used up to the maximum particle size set forth above. When more coarse cements are used, they preferably have properties which are the same or similar as API Class G cement. The fume silica used in the cement slurry composition of present invention is an ultra fine powder created during the production of silicon and ferrosilicon metal containing at least 85% by weight of silicon and LOI (Loss of ignition) of maximum 4.0%. The fume silica is densified for easier handling and having a bulk density in the range of 600-700kg/cm3. The individual particles are mainly spherical and have a diameter below 1 micron. A supplementary effect of strength improvement comes from the generally high pozzolanicity of fume silica. Moreover it also cause enhancement of rheological characteristics by the lubrication effect resulting from the perfect sphericity of the basic particles. The fumed silica is included in the cement compositions in an amount in the range of from about 15 to about 60% by weight of the hydraulic cement therein. The lightweight cement slurry compositions according to the present inventions preferably contain 10-200% lightweight particulate aggregate based on the weight of the cement. The lightweight particles in said composition typically have density between 0.1 and 1.5 gm/cm3. By way of example, it is possible to use at least one particle selected from the group consisting of hollow microspheres, silico-aluminate cenospheres, hollow glass beads, sodium-calcium-borosilicate glass beads, ceramic microspheres, silica-alumina microspheres, plastics materials and polypropylene beads. In general, the density of the slurry is adjusted essentially as a function of which lightweight particles are chosen. However the particular preferred lightweight particulate aggregates of this invention are HOLLOW GLASS MICROSPHERES (density range 0.5 - 0.3), HOLLOW CEREMIC MICROSPHERES (density range 0.6 -0.8) and PVC RESIN (bulk density range: 0.53 - 0.59 gms/ml). The type of lightweight aggregate and its amount used varies with the density of the formulated lightweight slurries. The water utilized in the cement compositions of the present invention should be only fresh water. Generally, the water can be from any source provided that it does not contain an excess of compounds, e.g., dissolved organics that may adversely affect other components in the cement composition. The water may be present in an amount sufficient to form a pumpable slurry i.e., an amount in the range of from about 80% to about 150% by weight of the coarse particulate hydraulic cement in the compositions. A variety of dispersing agents can also be utilized in accordance with this invention. The dispersing agent utilized is included in the cement composition in an amount in the range of from about 0.2%- 2.0% by weight of the coarse particulate hydraulic cement in the composition. A retarder is preferably used when the bottom hole circulating temperature exceeds about 100°F. Retarders satisfactory for use in the invention include those commercially available products commonly employed as retarders. The amount of retarder required will vary according to the bottom hole circulating temperature and variations in the makeup of the cement itself. The proper amount of retarder required in any particular case should be determined by running a "thickening time" test for the particular concentration of retarder and cement composition being used. Such tests should be run according to the procedures set by API RP 10B. In most applications the amount of retarder, if any, required will not exceed more than about 2.0 percent by weight of the cement. Additional additives may be added to the cement compositions of the present invention as deemed appropriate by one skilled in the art with the benefit of this disclosure. Examples of such additives include, inter alia, fluid loss control additives, defoamers, and the like. In general, where the fluid loss additives are used to provide the necessary fluid loss reduction, in an amount of from about 0.1-1.0% by weight based on cement used in the composition. To facilitate a better understanding of the present invention, the following examples of some of the preferred embodiments are given. It will be understood by those skilled in the art that considerable variation in the components of the cement compositions of the present invention as well as the methods of using the compositions can be made and this invention is not limited to the specific examples which are given herein for the purpose of disclosure. Examples EXAMPLE 1 A lightweight cement composition of the present invention was prepared comprising coarse particulate API Class G Portland cement (Digvijay), fumed silica, hollow glass spheres (Soda Lime Borosilicate Glass), fresh water, retarder and a dispersant. Test samples of the cement compositions were tested for density, thickening time, rheoiogy, fluid loss and free water in accordance with the procedures set forth in API Specification RP 10B. In addition, the compressive strength of the cement composition after setting was determined. All of the above listed tests were performed at BHCT of 104°F (40°C) and BHST of 140°F (60°C). The amounts of the components of the test cement composition as well as the test results are given in Table I below. The microspheres used are sold by 3M™ under the name Scotchlite K-37; such microspheres have a density of 0.37 g/cm3 and a grain-size distribution such that 10% of the particles (by volume) have a size of less than 20 microns, 50% less than 45-microns, and 90% less than 80microns; these particles were selected in particular because of their low true density. TABLE -1 (Table Removed) From Table-I, it can be seen that the cement composition of this invention having density of 1.05 which is almost equivalent to that of water had excellent properties at 104°F BHCT/140°F BHST. The cement composition of this invention inspite of its ultra low weight has high ultimate compressive strength of 1718psi. The cement composition also has inherent fluid loss control properties as evident from the fluid loss value of 416ml without the addition of any fluid loss control additive, nil free water separation and also has low rheology. EXAMPLE 2 A lightweight cement composition of the present invention was prepared comprising coarse particulate API Class G Portland cement (Digvijay), fumed silica, hollow glass spheres, fresh water, dispersant and cement set retarder. Test samples of the cement compositions were tested for density, thickening time, rheology, fluid loss and free water in accordance with the procedures set forth in API Specification RP 10B. In addition, the compressive strength of the cement composition after setting was determined. All of the above listed tests were performed at BHCT of 167°F (75°C) and BHST of 230°F (110°C). The amounts of the components of the test cement composition as well as the test results are given in Table II below. The microspheres used are sold by 3M™ under the name Scotchlite K-46; such microspheres have a density of 0.46 g/cm3 and a grain-size distribution such that 10% of the particles (by volume) have a size of less than 15 microns, 50% less than 40microns, and 90% less than 70microns; these particles were selected in particular because of their low true density and high crush strength of 6000psi (90% survival). TABLE - II (Table Removed) From Table-ll, it can be seen that the cement composition of this invention having density of 1.24 had excellent properties at 167°F BHCT/230°F BHST. The cement composition of this invention in spite of its low weight has high ultimate compressive strength of 2578psi. The cement composition also has inherent fluid loss control properties as evident from the fluid loss value of 593ml without the addition of any fluid loss control additive, nil free water separation and also has low rheology. EXAMPLE 3 A lightweight cement composition of the present invention was prepared comprising API Class G Portland cement (Digvijay), fumed silica, hollow ceramic microsphere (cenospheres) as lightweight particulate aggregate produced from flyash, fresh water, dispersant, fluid loss additive and cement set retarder. Test samples of the cement compositions were tested for density, thickening time, rheology, fluid loss and free water in accordance with the procedures set forth in API Specification RP 10B. In addition, the compressive strength of the cement composition after setting was determined. All of the above listed tests were performed at BHCT of 167°F (75°C) and BHST of 230°F (110°C). The amounts of the components of the test cement composition as well as the test results are given in Table III below. TABLE-III (Table Removed) From Table-Ill, it can be seen that the cement composition of this invention having density of 1.35 had excellent properties at 167°F BHCT/230°F BHST. The cement composition of this invention in spite of its low weight has adequate ultimate compressive strength of 1500psi. The cement composition has very low rheology which can be seen from the Pv & Yp value of 60 and 10 respectively and also has nil free water separation. The cenospheres used have a density of 0.77 g/cm3 and a grain-size distribution such that 10% of the particles (by volume) have a size of less than 10 microns, 50% less than 100microns, and 90% less than 300microns. EXAMPLE 4 A lightweight cement composition of the present invention was prepared comprising of API Class G Portland cement (Digvijay), fumed silica, PVC resin (a plastic material) with a real particle density of 1.24gm/cm3 as lightweight particulate aggregate , fresh water, dispersant and a cement set retarder. Test samples of the cement compositions were tested for density, thickening time, rheology, fluid loss and free water in accordance with the procedures set forth in API Specification RP 10B. In addition, the compressive strength of the cement composition after setting was determined. All of the above listed tests were performed at BHCT of 75°C and BHST of 110°C. The amounts of the components of the test cement composition as well as the test results are given in Table IV below. TABLE - IV (Table Removed) From Table-IV, it can be seen that the cement composition of this invention having density of 1.46 had excellent properties at 75°C BHCT/110°C BHST. The cement composition of this invention in spite of its low weight has high ultimate compressive strength of 2875psi. The cement composition also has inherent fluid loss control properties as evident from the fluid loss value of 278ml without the addition of any fluid loss control additive, nil free water separation and also has low rheology. EXAMPLE 5 A comparison of the set cement permeability, compressive strength and fluid loss control property of some high performance lightweight cement composition of this invention with that of a most widespread use conventional neat cement slurry of density 1900 kg/m3 comprised of 44% water, 0.2% fluid loss additive and 0.2% dispersant by weight of cement was carried out and the result of which are given in table V. Table -V (Table Removed) Without use of any fluid control loss additive From Table-V, it can be seen that the cement composition of this invention in spite of its very low density has set cement permeability equal to that of conventional normal density slurry. In comparison to this other type of lightweight slurries where the lower density is achieved either by addition of extra water or by use of foam have high degree of permeability. The high performance lightweight slurries of the present invention has inherent fluid loss control property as can be seen from the fluid loss values of these slurries in spite of using any fluid loss control additives are much lower than the conventional normal density slurry with fluid loss control additive. Moreover slurries of the invention because of optimized packing developing remarkably high compressive strength in spite of its low density. EXAMPLE 6 A study of the stability of the cement composition of this invention given in example 2, 3& 4 were carried out at bottom hole temperature condition of 75°C and 3000psi, the result of which are given in table VI. Table - VI (Table Removed) From Table-VI, it can be seen that the cement composition of this invention are highly stable at bottom conditions. Thus the high stability along with nil free water of the lightweight slurries of the present invention makes it very suitable for application in high angle and horizontal well bore cementation. EXAMPLE 7 Shortly after the cementation process is finished the cement slurry will change from a hydrostatic fluid to a solid state body. If the cement slurry in this critical phase is not able to resist the gas pressure, channels will be formed in the partial cured cement slurry. For determining the gas tight property of the lightweight slurries of the present invention one direct and three indirect tests were carried out with the cement composition of this invention given in example 4. The direct test cosist of testing in an industry approved gas migration test apparatus where the slurry is exposed to simulated wellbore conditions as being experienced in a high pressure gas well and any gas flow is monitored throughout its setting process. The indirect test consists of determination of the slurry parameters which are essential requirement for any gas tight slurry such as low fluid loss, right angle setting brehaviour and low set cement permeability. The result of the gas migration tests for cement slurriy of example 4 is shown on figure 5 which shows the measured gas flow through the cement slurries in ml/min as a function of time. From figure 5 it is seen that the cement slurry composition of example 4 according to the present invention is gas-tight, as no gas flow was measured through a column of cement slurry during its setting process. The fluid loss and set cement permeability of the cement slurry composition of example 4 are very low as shown in table V and its setting behaviour as shown in figure 6 is almost righr angle which further confirms the gas tight property of the lightweight slurries of the present invention. Thus the lightweight cement slurries of the present invention can be used for cementation of oil wells if the formatiom contain gas. Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned as well as those which are inherent therein. While numerous changes may be made by those skilled in the art, such changes are encompassed within the spirit of this invention as defined by the appended claims. Reference: 1. Richard, P.& Cheyrezy, M., 1995." Composition of Reactive powder concretes," Cement & Concrete Research, Vol.25, No.7, PP 1501-1511. 2. Ghosh., S.N., 1991, Cement and Concrete science technology, Volume-1, Part-1, ABI,Books Pvt. Ltd., New Deihi 3. API Specification for Cements and Materials for Well Cementing, API Spec 10 A, Twenty-Second Edition, January 1, 1995 4. API Recommended Practice 10B, Twenty second edition, December 1997. We Claim: 1. A synergistic lightweight cement slurry composition for cementing pipe in well bores, said composition characterized by: a. a coarse particulate hydraulic cement; b. fume silica is 30-35% by weight of coarse particulate hydraulic cement, c. lightweight aggregate is 30- 70% by weight of coarse particulate hydraulic cement, d. dispersing agent is 0.2 - 2.0 %by weight of coarse particulate hydraulic cement, e. set cement retarder is 0.1 - 2.0 % by weight of coarse particulate hydraulic cement, f. fluid loss control additive is 0 - 1.0 % by weight of coarse particulate hydraulic cement, and g. water is present in a quantity sufficient to provide a slurry of density ranging from about 1.0 to 1.5 g/cm3. 2. The composition as claimed in claim 1, wherein the coarse particulate hydraulic cement has a particle size not more than about 118 microns and a specific surface area not less than about 2800 square centimeters per gram. 3. The composition as claimed in claim 1, wherein the hydraulic cement is API Class G Portland or equivalent cement. 4. The composition as claimed in claim 1, wherein fume silica is an ultra fine powder comprising at least 85% by weight of silicon having loss of ignition up to 4.0%, and bulk density in the range of 600-700kg/cm3. 5. The composition as claimed in claim 1, wherein the lightweight aggregate with a real particle density between 0.1 and 1.5 gm/cm3 is present in an amount ranging from about 10% to about 200% by weight of said coarse particulate hydraulic cement. 6. The composition as claimed in claim 1, wherein the lightweight aggregate particle used is selected from the group consisting of hollow microspheres, silico-aluminate cenospheres, hollow glass beads, sodium-calcium-borosilicate glass beads, ceramic microspheres, silica-alumina microspheres, plastics materials and polypropylene beads. 7. The composition as claimed in claim 1, wherein water used has total dissolved solids not more than 1000ppm. 8. The composition as claimed in claim 7, wherein said water is present in an amount ranging from about 80% to about 150% by weight of said coarse particulate hydraulic cement. 9. The composition as claimed in claim 1 further comprises optionally a defoaming agent. 10. The composition as claimed in claim 1, having compressive strength ranging from 1500psi to 3000 psi 11. The composition as claimed in claim 1, having a weight ratio of liquid to solid ranging from 0.50 to about 0.60.

Full Text

Field of Invention
The present invention relates to a synergistic lightweight cement slurry composition. The present invention relates to a non foamed high performance lightweight cement slurry compositions for cementing pipe in well bores, more . particularly compositions for cementing pipe against well bores penetrating oil/gas bearing subterranean zones or formations which readily fracture at low hydrostatic pressures.
Background of the invention and Prior Art references Definition of certain terms used in following text:
BHT- bottomhole temperature
BHCT - bottomhole circulating temperature
BHST - bottomhole static temperature
BHP - bottomhole pressure
Pv - plastic viscosity;
Yp - yield point
MD - measured depth
TVD - true vertical depth
In the production of hydrocarbons from the subterranean formation, the subterranean formation is typically cemented or sealed by pumping a queous hydraulic cement slurry into the annulus between the pipe and the formation. Typically, the cement slurry is pumped down the inside of the casing and back up the outside of the casing through the annular space. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe in the well bore and bonds the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
In some locations, the subterranean oil/gas bearing zones or formations into or through which wells are drilled have high permeability and low compressive and tensile strengths. As a result, the resistances of the zones or formations to shear are low and they have very low fracture gradients. When a well fluid such as a hydraulic cement composition is introduced into a well bore penetrating such a subterranean zone or formation, the hydrostatic pressure
exerted on the walls of the well bore can exceed the fracture gradient of the zone or formation and cause fractures to be formed in the zone or formation which may result in a significant loss of cement into the formation during cementing operations. This loss of cement composition is problematic because, inter alia, less of the cement composition will remain in the annular space to form the protective sheath to isolate the exposed producing zones and thereby causing undesirable migration of fluids between zones or formations penetrated by the well bore.
It is known that cementing of final production casing in oil and gas wells is a very special sector of the art which requires cement slurries having a special set of properties. Thus in order to obtain effective cementing, it is necessary to have a slurry in which the following properties can be suitably adjusted: setting time; fluid loss; compressive strength; rheology; stability; permeability and gas channeling control. Though lightweight cement compositions have been developed and used heretofore, but it is known that it is particularly difficult to obtain a lightweight cement composition having adequate properties in all the above mentioned properties simultaneously so as to make it suitable for application in production casing cementation under all type of conditions.
In this respect the study of the prior art is most revealing. It can be seen that the object of known slurries is to solve one of the problems mentioned above without being capable of solving all of them.
Thus US Patent 4,933,031 describes of a low weight slurry composition consists of hydraulic cement, microsilica, hollow ceramic bubbles and having the gas tight property. The low weight slurry compositions (1.1 - 1.45 density) though gas tight but has very low compressive strength of maximum 600psi
US Patent 5,588,489 claims a lightweight slurry composition comprising of slag cement, water sufficient to form pumpable slurry, a gas sufficient to foam the slurry and foaming agent. The patent claims lightweight, fast setting cement compositions which set into high compressive strength substantially impermeable masses. The slurries though claim to have high compressive strength however the 24 hour strength at 80°F for different samples of density in the range 10-12ppg has shown to achieve strength in the range of 200-600psi only.
US Patent 6,516,883 relates to a low density cement compositions having enhanced compressive, tensile and bond strength upon setting which comprises of a hydraulic cement, sufficient water to form a slurry, hollow glass microspheres surface treated with a mixture of organosilane coupling agents present in an amount sufficient to produce a cement composition density of about 6 pounds per gallon, a gas present in an amount sufficient to foam the cement composition whereby the cement composition has a density of about 5 pounds per gallon or below and a mixture of foaming and foam stabilizing surfactants. The patent claims that the test cement compositions of this invention containing organosilane coupling agent surface treated hollow glass microspheres had significantly better tensile strengths, compressive strengths and shear bond strengths than did the same cement compositions containing untreated hollow glass microspheres.
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A lightweight cement composition of US Patent 6,562,122 basically comprises of coarse particulate hydraulic cement, an ultrafine particulate hydraulic cement mixtureconsisting of slag cement and a Portland or equivalent cement, fly ash, fumed silica, hollow glass spheres and water. The cement compositions have excellent properties including high static gel strengths, low rheology, low fluid loss, high compressive strength upon setting, low permeability upon setting and resistance to chemical deterioration and failure due to sulfate degradation or the like. The lightweight cement compositions of this invention at density of 11.6ppg have 24 hours compressive strength in the range of 700 - 2400psi at temperatures ranging from 200.degree. F. to 275.degree F. Fluid loss of the slurry is low after addition of high dosage (5-6%) of fluid loss additive. Density of the slurry ranges from 9-13ppg.
US Patent 6,626,991 relates to a cement having a density lying in the range 0.9 g/cm3 to 1.3 g/cm3 and being constituted by a solid fraction and a liquid fraction, having porosity (volume ratio of liquid fraction over solid fraction) lying in the range 38% to 50%. The solid fraction is constituted by a mixture of lightweight particles, microcement and optionally portland cement and gypsum. Such cements have remarkable mechanical properties due to their very low porosity in spite of having very low density.
The patent Application EP0449360A1 relates to a lightweight cement composition for use in high pressure environments (greater than 3000 psi)
comprising of an aqueous hydratable cement composition, ceramic microspheres and a foaming surfactant. In accordance with the invention, the cementing composition further includes microsilica. The compressive strength achieved for the lightweight slurry formulation at 70°C and 24 hours having 1.20 and 1.10 density are in the range of 1537 and 962psi respectively.
US Patent 4,721,160. describes a lightweight cement slurry composition having a density in the range of 1.2 to 1.6, comprising of cement, an extender in the form of solid particle having density less than 2.2 gm/cc and in particular fly ashes, hollow silica alumina microspheres and coal derivatives and an SBR latex. The slurry is particularly gas tight.
US Patent 4,415,366 provides foamed thyrotropic cement slurry comprising Portland cement and calcium sulphate hemihydrate in which a gas or a mixture of gases along with surfactant is injected to make it lightweight
A lightweight, fast setting cement composition basically comprises of slag cement, water sufficient to form pumpable slurry, a gas sufficient to foam the slurry and a foaming agent has been described in US Patent 5, 711,801.
US patent publication 2005076812: This pertains to methods of cementing, using a cement composition comprising a desegregating agent for inhibiting the segregation of the light weight bead in cement composition.
WO publication 2004076796: this patent document also describes the prevention of segregation of light-weight beads.
US patent 6457524: this document is related to a foamed light-weight cement composition having reduced transition time for deep water conductor casing application to prevent flow from any formation sands over pressured by water.
CA patent 2111252: this document relates to the method of drilling a well-bore and subsequently disposing of the drilling fluid by converting it to a cementation composition.
EP 0605114 relates to a process of cementing a well using a cementing composition containing a portion of the drilling fluid used to drill the well. A set actvated cementing composition comprised of water, silica fume, a dispersing agent, a set activator, a set delaying additive and a portion of the drilling fluid, is introduced into the well and permitted to set into a hard mass therein.
US patent 53332041 deals with the method of cementing a well-bore by using set-activated cementation compositions whereas the cementitious compositions are basically comprised of water, particulate condensed silica, a dispersing agent, a set activator and a set delaying additive for increasing the time in which the composition sets after the set activator is combined therewith.
The subject matter of the documents like Concrete Technology by M.S. Shetty, Concrete Technology by Gambhir, Reinforced Concrete by H.J. Shah, are not related to the slurry composition of the present invention.
Also, the lightweight cement compositions utilized heretofore have often not had sufficient compressive strengths and low permeability upon setting which is a prerequisite for its use against cementation of producing zones.
Recently the need for better economics, higher productivity and more efficient well operations has brought about new well drilling and completion techniques. One of the examples of such new techniques include the drilling of well horizontally inside the producing formation and extending the reservoir penetration by drilling wells at a high angle and thereby maximizing the area of producing zones in the well bore to increase the productivity of the reservoir. Subterranean formations transverses by such well bores are often weak and extensively fractured and have a very low margin between pore and fracture pressure because of high MD/TVD ratio and thus requiring improved lightweight cement compositions having low rheology, so that the cement compositions can be accurately placed without losses. In addition, the cement compositions must have high stability at bottom hole conditions as an unstable slurry may also cause zonal isolation problem in a deviated or horizontal well where free fluid can collect along the high side of the annulus and sedimentation can result in a highly porous, low strength & low density channel, which in turn may contribute to zonal communication and gas migration (fig -1)
Another example of such new techniques include the reduction of the well bore diameter (referred to as a slim hole) and extending the reservoir penetration by drilling small lateral well bores which are completed using small diameter pipe such as coiled tubing to increase the productivity of the reservoir. The performance of primary cementing operations in the smaller annular spaces in the well bores requires improved lightweight cement compositions having low viscosities so that the cement compositions can be accurately placed. In addition, the cement compositions must have good static gel strength, low rheology, high compressive strength, low fluid loss, low permeability, good chemical resistance and a broad operating temperature range.
For some wells the zones adjacent to the cement containing annulus, contain gas under substantial pressure. This gas can cause serious problems during cementations of such wells as the gas may penetrate through the cement slurry whereby an undesirable phenomenon referred to in the art as gas migration may occur. Gas migration may start during setting of the cement slurry. Shortly after the cementation process is finished the cement slurry will change from a hydrostatic fluid to a solid state body. If the cement slurry in this critical phase is not able to resist the gas pressure, channels will be formed in the partial cured cement slurry. Gas under pressure will then flow through the column during the setting of the cement slurry and/or at the boundary between the cement slurry and the wall of the well. This may result in channels which can reach the top of the well. Gas migration control is an especially difficult problem where cement slurry having a very low density has to be used.
For oil wells which are drilled through low pressure formations where it is not advisable to expose the formations to high hydrostatic pressure, cement slurries having a low density have to be used, as a cement slurry having a too high density and thereby a high hydrostatic pressure may result in breakdown of the formation and loss of the cement slurry into the formation (lost circulation).
In order to obtain a successful cementation with single lightweight hydraulic cement slurry in such varieties of well bore conditions it has to satisfy a number of specific requirements. The rheology of the cement slurry has to be low in order to reduce the pressure losses in the annulus for effective displacement of drilling mud and placement of the slurry in the annular space between the casing and the well. The set cement should have low permeability, right angle setting behaviour and gas tight property to prevent gas migration after placement. The set cement should also develop sufficient ultimate compressive strength so that it can be placed against the oil/gas bearing zones for zonal isolation. Further the cement slurry must have a low filtration loss in order to avoid loss of liquid from the cement slurry into the formation, as loss of liquid will increase the viscosity of the cement slurry. The cement compositions also should have high stability and smallest possible tendency of water separation at bottom hole conditions as any compromise on this parameters may also cause zonal isolation problem in a deviated or
horizontal well. Finally, the cement slurry must have density low enough for placement in the annular space between casing and open hole without breaking down the formation.
It is known that it is particularly difficult to obtain lightweight cement having adequate properties in all the above mentioned fields simultaneously. Up till now, such lightweight cement having a specific gravity lying in the range 1.0-1.5 has not been known.
The cement slurries in the most widespread use have densities of about 1900 kg/m3, which is about twice the density desired for certain deposits. To lighten them, the simplest technique is to increase the quantity of water while adding stabilizing additives (known as "extenders") to the slurry for the purpose of avoiding particles settling and/or free water forming at the surface of the slurry. Manifestly, that technique cannot get down to a density close to 1000 kg/m3. Furthermore, hardened cements formed from such slurries have greatly reduced compression strength, a high degree of permeability, and poor adhesive capacity. For these reasons, that technique cannot be used to go below densities of about 1300 kg/m.3 while still conserving good isolation between geological layers and providing sufficient reinforcement for the casing.
Another technique consists in lightening the cement slurry by injecting gas into it (generally air or nitrogen) before it sets. The quantity of air or nitrogen added is such as to reach the required density. That technique provides performance that is a little better than the preceding technique since the density of gas is lower than that of water, so less needs to be added. Foamed cements produced with densities below 1.0 g/cm3, but use of these kinds of cements for cementation of oil wells are, however, associated with a number of drawbacks and disadvantages. The entrained gas is more and more compressed as the hydrostatic pressure increases and, on the other hand, the entrained gas will expand as the cement slurry moves upwardly in the annulus between the casing and the wall of the well. Use of foamed cements therefore requires complicated and costly means for adjusting and controlling the gas content in the slurries and it is necessary to have an extensively control during the cementation process.
In order to obtain effective cementing of the casing against producing zones in all type of wellbore conditions by a single type of lightweight slurry,
the slurry must have optimum combination of properties like low fluid loss; high compressive strength; low rheology; high stability; low permeability and gas tight. The known cement slurries of the prior art have some adequate properties only and thus it is suitable only for application in production casing cementation of some types of well and not for any wellbore conditions. In the present invention lightweight cement slurry is provided which can be applied for cementation of any wellbore conditions because of its suitable properties in relation to the requirement of cementing. Major limitations of the Prior Art
The cement slurry compositions have limited adequate properties and thus it cannot be used for cementation of the casing against producing zones in different wellbore conditions.
The compressive strength of cement slurries are not adequate and though in some cases good but still less than the present invention
None of the cement slurry compositions have good stability at bottom hole conditions which is a requirement for high angle wellbore cementing. Few cement slurry compositions possess high permeability. Thus, there are continuing needs for low density cement compositions which will be a single product with several significant advantages for improved primary well cementing of production casing in variety of wellbore conditions.
The surprising results of the present invention is achieved by reducing void spaces between the solid particles in the cement composition by selecting and packing with common granular material and thus reducing its water requirement to form a slurry compared to the lightweight slurries of the prior art, which ultimately resulted in better performance of the slurry.
SUMMARY OF THE INVENTION
The present invention relates to a synergistic lightweight cement slurry composition. The present invention particularly relates to non-foamed high performance lightweight cement slurry compositions having low fluid loss, low rheology, nil free water, high stability, gas tight property and enhanced compressive strengths and lower permeability upon setting which makes it especially suitable for application in cementing against oil/gas bearing producing zones in varieties of well bore conditions comprising of a coarse
hydraulic cement, fume silica, lightweight aggregate, dipersant, set cement retarder, optionally fluid loss control additive and defoaming agent, water in a quantity sufficient to provide a slurry of density ranging from 1.00 to 1.50g/cm3 and the invention also relates to a process for the preparation of said cement slurry compositions.
Thus the present invention provides an improved lightweight slurry composition suitable for application in almost all well bore conditions such as high angle wells, gas wells, slim holes etc
The objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon reading of the description of preferred embodiments which follows.
Accordingly, the present invention provides a synergistic lightweight cement slurry composition for cementing pipe in well bores, said composition including:
a. a hydraulic cement;
b. fume silica 30-35% by weight of coarse particulate hydraulic cement,
c. lightweight aggregate 30- 70% by weight of coarse particulate hydraulic
cement,
d. dispersing agent 0.2 - 2.0% by weight of coarse particulate hydraulic cement,
e. set cement retarder 0.1 - 2.0% by weight of coarse particulate hydraulic
cement,
f. fluid loss control additive 0 - 1.0% by weight of coarse particulate hydraulic
cement, and
g. water present in a quantity sufficient to provide slurry of density ranging from
about 1.0 and 1.5g/cm3.
Brief description of the accompanying drawings
Figure-1 represents effect of slurry stability on annular isolation in high angle wells wherein 11 is Free fluid; 12 is low strength permeability cement; 13 is medium strength cement; 14 is high strength cement; 15 - gas; 16 - oil zone B; 17 - shale and 18 - oil zone A;
Figure 2 represents conventional slurry;
Figure 3 represents high performance slurry; in figures 2 and 3, wherein 21 is cement, 22 is water, 23 is light weight material and 24 is strengthening material.
Figure 4 represents effect of combining different size particles on packing densities
Figure 5 represents gas migration test results Figure 6 represents cement setting behaviour
DESCRIPTION OF PREFERRED EMBODIMENTS
It is known that it is particularly difficult to obtain a lightweight cement slurry composition having all adequate properties. Till now lightweight cement composition having all required properties for cementing against oil/gas bearing subterranean zone penetrated by a well bore having density ranging from 1.0-to 1.5 g/cm3 has not been achieved.
The density of the cement must be adjusted so that the pressure at the bottom of the well compensates at least for the pore pressure in the geological formations through which the well passes so as to avoid any risk of eruption. As well as this lower limit on density, there is also an upper limit. This upper limit is that the hydrostatic pressure generated by the column of cement plus the head losses due to the circulation of the fluids being pumped must remain below the fracturing pressure of the rocks in the section being cemented. Certain geological formations are very fragile and require densities close to that of water or even lower.
The present invention provides a high performance lightweight cement slurry compositions having enhanced compressive strength and lower permeability upon setting for cementing pipe in well bores penetrating subterranean oil/gas bearing zones or formations which readily fracture at low hydrostatic pressures. The cement slurry compositions of the present invention also have other excellent slurry properties like low rheology, low fluid loss, zero free water & high stability at bottom-hole conditions. The composition is developed based on the innovative technology of maximizing the compactness of the solid mixture blend taking a lead from the technology, prevalent in concrete cement industry.
In a preferred composition of the high performance lightweight cement slurry basically comprises essentially a coarse particulate hydraulic cement; fume silica having a bulk density in the range of 600-700kg/cm3 present in an amount in the range of from about 15% to about 60% based on the weight of said coarse particulate hydraulic cement; lightweight filler material based on the weight of cement with a real particle density between 0.1 and 1.5gm/cm3 present in an amount sufficient to impart a density to said cement composition in the range of from about 1.0 to about 1.5g/cm3 and water present in an amount sufficient to form a slurry.
The cement compositions also preferably include a dispersing agent in an amount ranging from about 0% to about 2%, a set retarder in an amount ranging from about 0% to about 2.0% and optionally a fluid loss control additive present in an amount ranging from about 0.1% to about 1% by weight of the coarse particulate hydraulic cement.
The compositions are particularly useful in cementing of depleted fields, low fracture gradient formations, loss prone zones, high angle wellbore, slim holes, gas wells and eliminating stage cementations when long cement column rise is required.
The present method of the invention whereby high performance lightweight non foamed slurry formulation close to that of density of water has been designed considerably simplifies the cementing operation, since it avoids any need for logistics of the kind required for foaming.
Improved methods of cementing pipe in well bores penetrating subterranean oil/gas bearing zones or formations which readily fracture at low hydrostatic pressures are provided by the present invention. The present invention provides universal non-foamed high performance lightweight cement compositions having low fluid loss, low rheology, nil free water, high stability, gas tight property and enhanced compressive strengths and lower permeability upon setting which makes it especially suitable for application in cementing against oil/gas bearing producing zones in varieties of well bore conditions. Thus the present invention provides a single lightweight slurry system having improved properties in all the known fields so as to make it a universal slurry system for application in almost all wellbore conditions.
The present method of the invention of high performance lightweight slurry compositions for cementation of oil/gas bearing subterranean zones or formations which readily fracture at low hydrostatic pressures is based on the theories of optimum particle packing which is used in the civil engineering industry for optimization of granular dry blends to obtain ultrahigh performance concrete. The main principle of this technology is maximizing the packing of dry blend to exhibit easy miscibility at reduced water. The higher the packing or compactness of the solid mixture in the dry blend, the better is the performance of the slurry made out of the blend.
Replacing excess mix water in the slurry with common granular materials is the key to this new technology. In conventional lightweight slurries, the void spaces between the solid particles are filled up with excess water thereby increasing its porosity, permeability and decreasing its compressive strength (fig-2). The principle of concrete technology is to reduce the void space between the solid particles in the slurry by packing with common granular material and thus reducing its water requirement. Maximizing the packing of a dry blend is possible only when there is a high ratio between the mean diameters (d50) for two consecutive granular classes because in that case the smaller particles will fill in the voids between the larger particles (fig-3). In figure 2 and 3, wherein 21 is cement, 22 is water, 23 is light weight material and 24 is strengthening material.
The present method of the invention is characterized in that particulate additives are incorporated in the cement slurry, such that in combination with one another and with the other particulate components of the slurry, they give rise to a grain-size distribution that maximize the compactness of the solid mixture blend thus significantly altering the properties of the slurry. The reduction in slurry density is achieved only by reducing the blend specific gravity during designing of the lightweight slurry compositions. Nevertheless, Theological and mechanical properties will only be satisfactory if the size of the particles and the volume distribution thereof is selected in such a manner as to maximize the compactness of the solid mixture.
For a solid mixture having two components will provide significant reductions in inter-particle volumes but mixtures having three or more components are preferred since they make it possible to obtain greater compactness (fig-4) if the mean sizes of the various components are significantly different.
Accordingly, the present invention of lightweight cement slurry compositions for use in cementation of well penetrating a oil/gas bearing subterranean zone or formation which readily fractures at low hydrostatic pressure consist of three components. The cement slurry compositions are comprised of a hydraulic cement, 5 - 60% fume silica based on the weight of cement, 10-200%, lightweight filler material based on the weight of cement with a real particle density between 0.1 and 1.5gm/cm3, 0.2% - 2% of a thinner/dispersant (dry weight) based on the weight of cement, 0% -1% fluid
loss additive (dry weight) optionally based on the weight of the cement, 0-2% set cement retarder (dry weight) based on the weight of the cement and water in such an amount that the cement slurry has a density between 1.0 and 1.5 g/cm3.
The cement compositions of the present invention have excellent slurry properties including gas tight, low rheology, low fluid loss, zero free water, high compressive strength & high stability at bottom hole conditions. The cement compositions of the present invention have low rheology whereby when required they can be pumped at turbulent flow which aids in displacing drilling fluid from the well bore. The low rheology of the cement composition also produces low friction pressure when the composition is pumped which lowers the risk of fracturing easily fractured zones or formations. When the cement compositions of this invention are utilized in cementation of well penetrating a oil/gas bearing subterranean zone or formation which readily fractures at low hydrostatic pressure, low rheology is particularly important in preventing fracturing of weak subterranean zones or formations. The high stability of the low weight cement compositions will also minimize the risk of annular communication after cementation in high angle well. The gas tight property of the invented slurry will be of special use in cementation of gas wells for prevention of annular communication problem.
The performance of the slurries made based on the technology of particle packing mainly depends on the degree of compactness or packing of the solid blends achieved by way of selection of proper size and proportion of constituents in the blends. The low density is obtained by combining lightweight particles and cement but the slurry properties will be satisfactory only if the size of the particles and the volume distribution thereof is selected in such a manner as to maximize the compactness of the solid mixture. The higher the packing of the blend, higher will be the performance of the slurries made out of the blend. Though through optimization of packing and by use of lightweight particles, micro cement and optionally Portland cement and gypsum (US patent 6,626,991) low density slurry having low porosity and improved mechanical properties are being designed, however superior slurry properties of the present invention are being achieved only because of attainment of
optimized packing of the blend because of selection of proper constituent material, its size and its proportion in the blend.
In accordance with the present invention that all the necessary properties of the slurry can be achieved only if weight ratio of liquid to solid in the slurry is maintained in the range of about 0.50 to 0.60 by way of selecting proper size and proportion of the constituent material. The high performance lightweight cement compositions of this invention can be used over a broad temperature range of from about 80.degree.F to about 270.degree.F, at densities in the range of from about 8.7 to about 12.20 pounds per gallon.
Because the cement composition has a low density, i.e., a density such that the hydrostatic pressure of the cement composition exerted in the subterranean zone or formation being cemented is less than the fracture gradient of the subterranean zone or formation, fracturing of the zone or formation does not take place. Also, because the cement composition of this invention has enhanced compressive and lower permeability upon setting, a strong bond exists between the pipe and the walls of the well bore penetrating the subterranean zone or formation which prevents formation fluids from entering the annulus between the pipe and the well bore. The high overall strength of the cement composition also prevents it from being shattered by contact with the drill bit and drill string when the well is drilled to greater depths.
The term "hydraulic cement" as used herein refers to those inorganic materials which set up to a hard monolithic mass under water. The coarse particulate hydraulic cement can be any of a variety of hydraulic cements having a maximum particle size of about 118 microns and a specific surface area of about 2800 square centimeters per gram. Portland cement is generally preferred, and the coarse cement can be, for example, one or more of the various Portland cements designated as API Classes A-H cements. These cements are identified and defined in the in API Specification for Cements and Materials for Well Cementing, API Spec 10 A, Twenty-Second Edition, January 1, 1995, which is incorporated herein by reference API Portland cements generally have a maximum particle size of about 90 microns and a specific surface of about 3900 square centimeters per gram. When an API Portland cement is utilized as the coarse hydraulic cement in accordance with this invention, it is preferably API Class G cement. Other hydraulic cements which
are coarser than API Portland cement can also be used up to the maximum particle size set forth above. When more coarse cements are used, they preferably have properties which are the same or similar as API Class G cement.
The fume silica used in the cement slurry composition of present invention is an ultra fine powder created during the production of silicon and ferrosilicon metal containing at least 85% by weight of silicon and LOI (Loss of ignition) of maximum 4.0%. The fume silica is densified for easier handling and having a bulk density in the range of 600-700kg/cm3. The individual particles are mainly spherical and have a diameter below 1 micron. A supplementary effect of strength improvement comes from the generally high pozzolanicity of fume silica. Moreover it also cause enhancement of rheological characteristics by the lubrication effect resulting from the perfect sphericity of the basic particles. The fumed silica is included in the cement compositions in an amount in the range of from about 15 to about 60% by weight of the hydraulic cement therein.
The lightweight cement slurry compositions according to the present inventions preferably contain 10-200% lightweight particulate aggregate based on the weight of the cement. The lightweight particles in said composition typically have density between 0.1 and 1.5 gm/cm3. By way of example, it is possible to use at least one particle selected from the group consisting of hollow microspheres, silico-aluminate cenospheres, hollow glass beads, sodium-calcium-borosilicate glass beads, ceramic microspheres, silica-alumina microspheres, plastics materials and polypropylene beads. In general, the density of the slurry is adjusted essentially as a function of which lightweight particles are chosen. However the particular preferred lightweight particulate aggregates of this invention are HOLLOW GLASS MICROSPHERES (density range 0.5 - 0.3), HOLLOW CEREMIC MICROSPHERES (density range 0.6 -0.8) and PVC RESIN (bulk density range: 0.53 - 0.59 gms/ml). The type of lightweight aggregate and its amount used varies with the density of the formulated lightweight slurries.
The water utilized in the cement compositions of the present invention should be only fresh water. Generally, the water can be from any source provided that it does not contain an excess of compounds, e.g., dissolved

organics that may adversely affect other components in the cement composition. The water may be present in an amount sufficient to form a pumpable slurry i.e., an amount in the range of from about 80% to about 150% by weight of the coarse particulate hydraulic cement in the compositions.
A variety of dispersing agents can also be utilized in accordance with this invention. The dispersing agent utilized is included in the cement composition in an amount in the range of from about 0.2%- 2.0% by weight of the coarse particulate hydraulic cement in the composition.
A retarder is preferably used when the bottom hole circulating temperature exceeds about 100°F. Retarders satisfactory for use in the invention include those commercially available products commonly employed as retarders. The amount of retarder required will vary according to the bottom hole circulating temperature and variations in the makeup of the cement itself. The proper amount of retarder required in any particular case should be determined by running a "thickening time" test for the particular concentration of retarder and cement composition being used. Such tests should be run according to the procedures set by API RP 10B. In most applications the amount of retarder, if any, required will not exceed more than about 2.0 percent by weight of the cement.
Additional additives may be added to the cement compositions of the present invention as deemed appropriate by one skilled in the art with the benefit of this disclosure. Examples of such additives include, inter alia, fluid loss control additives, defoamers, and the like. In general, where the fluid loss additives are used to provide the necessary fluid loss reduction, in an amount of from about 0.1-1.0% by weight based on cement used in the composition.
To facilitate a better understanding of the present invention, the following examples of some of the preferred embodiments are given. It will be understood by those skilled in the art that considerable variation in the components of the cement compositions of the present invention as well as the methods of using the compositions can be made and this invention is not limited to the specific examples which are given herein for the purpose of disclosure.
Examples EXAMPLE 1
A lightweight cement composition of the present invention was prepared
comprising coarse particulate API Class G Portland cement (Digvijay), fumed silica, hollow glass spheres (Soda Lime Borosilicate Glass), fresh water, retarder and a dispersant. Test samples of the cement compositions were tested for density, thickening time, rheoiogy, fluid loss and free water in accordance with the procedures set forth in API Specification RP 10B. In addition, the compressive strength of the cement composition after setting was determined. All of the above listed tests were performed at BHCT of 104°F (40°C) and BHST of 140°F (60°C). The amounts of the components of the test cement composition as well as the test results are given in Table I below.
The microspheres used are sold by 3M™ under the name Scotchlite K-37; such microspheres have a density of 0.37 g/cm3 and a grain-size distribution such that 10% of the particles (by volume) have a size of less than 20 microns, 50% less than 45-microns, and 90% less than 80microns; these particles were selected in particular because of their low true density.
TABLE -1

(Table Removed)
From Table-I, it can be seen that the cement composition of this invention having density of 1.05 which is almost equivalent to that of water had excellent properties at 104°F BHCT/140°F BHST. The cement composition of this invention inspite of its ultra low weight has high ultimate compressive strength of 1718psi. The cement composition also has inherent fluid loss control properties as evident from the fluid loss value of 416ml without the addition of any fluid loss control additive, nil free water separation and also has low rheology. EXAMPLE 2
A lightweight cement composition of the present invention was prepared comprising coarse particulate API Class G Portland cement (Digvijay), fumed silica, hollow glass spheres, fresh water, dispersant and cement set retarder. Test samples of the cement compositions were tested for density, thickening time, rheology, fluid loss and free water in accordance with the procedures set forth in API Specification RP 10B. In addition, the compressive strength of the cement composition after setting was determined. All of the above listed tests were performed at BHCT of 167°F (75°C) and BHST of 230°F (110°C). The amounts of the components of the test cement composition as well as the test results are given in Table II below.
The microspheres used are sold by 3M™ under the name Scotchlite K-46; such microspheres have a density of 0.46 g/cm3 and a grain-size distribution such that 10% of the particles (by volume) have a size of less than 15 microns, 50% less than 40microns, and 90% less than 70microns; these particles were selected in particular because of their low true density and high crush strength of 6000psi (90% survival).
TABLE - II

(Table Removed)
From Table-ll, it can be seen that the cement composition of this invention having density of 1.24 had excellent properties at 167°F BHCT/230°F BHST. The cement composition of this invention in spite of its low weight has high ultimate compressive strength of 2578psi. The cement composition also has inherent fluid loss control properties as evident from the fluid loss value of 593ml without the addition of any fluid loss control additive, nil free water separation and also has low rheology. EXAMPLE 3
A lightweight cement composition of the present invention was prepared comprising API Class G Portland cement (Digvijay), fumed silica, hollow ceramic microsphere (cenospheres) as lightweight particulate aggregate produced from flyash, fresh water, dispersant, fluid loss additive and cement set retarder. Test samples of the cement compositions were tested for density, thickening time, rheology, fluid loss and free water in accordance with the procedures set forth in API Specification RP 10B. In addition, the compressive strength of the cement composition after setting was determined. All of the above listed tests were performed at BHCT of 167°F (75°C) and BHST of
230°F (110°C). The amounts of the components of the test cement composition as well as the test results are given in Table III below.
TABLE-III

(Table Removed)
From Table-Ill, it can be seen that the cement composition of this invention having density of 1.35 had excellent properties at 167°F BHCT/230°F BHST. The cement composition of this invention in spite of its low weight has adequate ultimate compressive strength of 1500psi. The cement composition has very low rheology which can be seen from the Pv & Yp value of 60 and 10 respectively and also has nil free water separation.
The cenospheres used have a density of 0.77 g/cm3 and a grain-size distribution such that 10% of the particles (by volume) have a size of less than 10 microns, 50% less than 100microns, and 90% less than 300microns.
EXAMPLE 4
A lightweight cement composition of the present invention was prepared comprising of API Class G Portland cement (Digvijay), fumed silica, PVC resin
(a plastic material) with a real particle density of 1.24gm/cm3 as lightweight particulate aggregate , fresh water, dispersant and a cement set retarder. Test samples of the cement compositions were tested for density, thickening time, rheology, fluid loss and free water in accordance with the procedures set forth in API Specification RP 10B. In addition, the compressive strength of the cement composition after setting was determined. All of the above listed tests were performed at BHCT of 75°C and BHST of 110°C. The amounts of the components of the test cement composition as well as the test results are given in Table IV below.
TABLE - IV

(Table Removed)
From Table-IV, it can be seen that the cement composition of this invention having density of 1.46 had excellent properties at 75°C BHCT/110°C BHST. The cement composition of this invention in spite of its low weight has high ultimate compressive strength of 2875psi. The cement composition also has inherent fluid loss control properties as evident from the fluid loss value of
278ml without the addition of any fluid loss control additive, nil free water separation and also has low rheology.
EXAMPLE 5
A comparison of the set cement permeability, compressive strength and fluid loss control property of some high performance lightweight cement composition of this invention with that of a most widespread use conventional neat cement slurry of density 1900 kg/m3 comprised of 44% water, 0.2% fluid loss additive and 0.2% dispersant by weight of cement was carried out and the result of which are given in table V.
Table -V

(Table Removed)
Without use of any fluid control loss additive
From Table-V, it can be seen that the cement composition of this invention in spite of its very low density has set cement permeability equal to that of conventional normal density slurry. In comparison to this other type of lightweight slurries where the lower density is achieved either by addition of extra water or by use of foam have high degree of permeability. The high performance lightweight slurries of the present invention has inherent fluid loss control property as can be seen from the fluid loss values of these slurries in spite of using any fluid loss control additives are much lower than the conventional normal density slurry with fluid loss control additive. Moreover
slurries of the invention because of optimized packing developing remarkably high compressive strength in spite of its low density.
EXAMPLE 6
A study of the stability of the cement composition of this invention given in example 2, 3& 4 were carried out at bottom hole temperature condition of 75°C and 3000psi, the result of which are given in table VI. Table - VI

(Table Removed)
From Table-VI, it can be seen that the cement composition of this invention are highly stable at bottom conditions. Thus the high stability along with nil free water of the lightweight slurries of the present invention makes it very suitable for application in high angle and horizontal well bore cementation.
EXAMPLE 7
Shortly after the cementation process is finished the cement slurry will change from a hydrostatic fluid to a solid state body. If the cement slurry in this critical phase is not able to resist the gas pressure, channels will be formed in the partial cured cement slurry. For determining the gas tight property of the lightweight slurries of the present invention one direct and three indirect tests were carried out with the cement composition of this invention given in example 4. The direct test cosist of testing in an industry approved gas migration test apparatus where the slurry is exposed to simulated wellbore conditions as being experienced in a high pressure gas well and any gas flow is monitored throughout its setting process. The indirect test consists of determination of the slurry parameters which are essential requirement for any gas tight slurry such
as low fluid loss, right angle setting brehaviour and low set cement permeability.
The result of the gas migration tests for cement slurriy of example 4 is shown on figure 5 which shows the measured gas flow through the cement slurries in ml/min as a function of time. From figure 5 it is seen that the cement slurry composition of example 4 according to the present invention is gas-tight, as no gas flow was measured through a column of cement slurry during its setting process.
The fluid loss and set cement permeability of the cement slurry composition of example 4 are very low as shown in table V and its setting behaviour as shown in figure 6 is almost righr angle which further confirms the gas tight property of the lightweight slurries of the present invention. Thus the lightweight cement slurries of the present invention can be used for cementation of oil wells if the formatiom contain gas.
Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned as well as those which are inherent therein. While numerous changes may be made by those skilled in the art, such changes are encompassed within the spirit of this invention as defined by the appended claims. Reference:
1. Richard, P.& Cheyrezy, M., 1995." Composition of Reactive powder concretes," Cement & Concrete Research, Vol.25, No.7, PP 1501-1511.
2. Ghosh., S.N., 1991, Cement and Concrete science technology, Volume-1, Part-1, ABI,Books Pvt. Ltd., New Deihi
3. API Specification for Cements and Materials for Well Cementing, API Spec 10 A, Twenty-Second Edition, January 1, 1995
4. API Recommended Practice 10B, Twenty second edition, December 1997.

We Claim:
1. A synergistic lightweight cement slurry composition for cementing pipe in well bores, said composition characterized by:
a. a coarse particulate hydraulic cement;
b. fume silica is 30-35% by weight of coarse particulate hydraulic
cement,
c. lightweight aggregate is 30- 70% by weight of coarse particulate
hydraulic cement,
d. dispersing agent is 0.2 - 2.0 %by weight of coarse particulate
hydraulic cement,
e. set cement retarder is 0.1 - 2.0 % by weight of coarse particulate
hydraulic cement,
f. fluid loss control additive is 0 - 1.0 % by weight of coarse
particulate hydraulic cement, and
g. water is present in a quantity sufficient to provide a slurry of density
ranging from about 1.0 to 1.5 g/cm3.
2. The composition as claimed in claim 1, wherein the coarse particulate hydraulic cement has a particle size not more than about 118 microns and a specific surface area not less than about 2800 square centimeters per gram.
3. The composition as claimed in claim 1, wherein the hydraulic cement is API Class G Portland or equivalent cement.
4. The composition as claimed in claim 1, wherein fume silica is an ultra fine powder comprising at least 85% by weight of silicon having loss of ignition up to 4.0%, and bulk density in the range of 600-700kg/cm3.
5. The composition as claimed in claim 1, wherein the lightweight aggregate with a real particle density between 0.1 and 1.5 gm/cm3 is present in an amount ranging from about 10% to about 200% by weight of said coarse particulate hydraulic cement.
6. The composition as claimed in claim 1, wherein the lightweight aggregate particle used is selected from the group consisting of hollow microspheres, silico-aluminate cenospheres, hollow glass beads, sodium-calcium-borosilicate glass beads, ceramic microspheres, silica-alumina microspheres, plastics materials and polypropylene beads.
7. The composition as claimed in claim 1, wherein water used has total dissolved solids not more than 1000ppm.
8. The composition as claimed in claim 7, wherein said water is present in an amount ranging from about 80% to about 150% by weight of said coarse particulate hydraulic cement.
9. The composition as claimed in claim 1 further comprises optionally a defoaming agent.
10. The composition as claimed in claim 1, having compressive strength ranging from 1500psi to 3000 psi
11. The composition as claimed in claim 1, having a weight ratio of liquid to solid ranging from 0.50 to about 0.60.

Documents:

2564-del-2004-abstract.pdf

2564-del-2004-claims.pdf

2564-del-2004-complete specification (granted).pdf

2564-del-2004-correspondence-others.pdf

2564-del-2004-correspondence-po.pdf

2564-del-2004-description (complete).pdf

2564-del-2004-drawings.pdf

2564-del-2004-form-1.pdf

2564-del-2004-form-13.pdf

2564-del-2004-form-19.pdf

2564-del-2004-form-2.pdf

2564-del-2004-form-3.pdf

2564-del-2004-form-5.pdf

2564-del-2004-gpa.pdf

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Patent Number

218262

Indian Patent Application Number

2564/DEL/2004

PG Journal Number

22/2008

Publication Date

30-May-2008

Grant Date

31-Mar-2008

Date of Filing

27-Dec-2004

Name of Patentee

OIL AND NATURAL GAS CORPORATION.

Applicant Address

JEEVAN BHARTI, CANNAUGHT PLACE, NEW DELHI110001, INDIA

Inventors:

#	Inventor's Name	Inventor's Address
1	PAL, VIJENDRA	INSTITUTE OF DRILLING TECHNOLOGY, OIL AND NATURAL GAS CORPORATION, KAULAGARH ROAD, DEHRADUN, INDIA
2	DASGUPTA, DEBASHISH	INSTITUTE OF DRILLING TECHNOLOGY, OIL AND NATURAL GAS CORPORATION, KAULAGARH ROAD, DEHRADUN, INDIA
3	GARG, SATYA PRAKASH	INSTITUTE OF DRILLING TECHNOLOGY, OIL AND NATURAL GAS CORPORATION, KAULAGARH ROAD, DEHRADUN, INDIA
4	SUYAN, KALYAN MAL	INSTITUTE OF DRILLING TECHNOLOGY, OIL AND NATURAL GAS CORPORATION, KAULAGARH ROAD, DEHRADUN, INDIA
5	BANERJEE, SATINATH	INSTITUTE OF DRILLING TECHNOLOGY, OIL AND NATURAL GAS CORPORATION, KAULAGARH ROAD, DEHRADUN, INDIA

PCT International Classification Number

C04B 14/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA