Title of Invention	A METHOD OF DETERMINING PRODUCTION RATES AND FLOW RATES IN A WELL AND AN APPARATUS PERFORMING THE SAME
Abstract	A system and method is provided for determining flow rates of fluid in a well. The system and method utilize temperature measurements and a modeling technique that enable the determination of flow rates from one or more well zones via the temperature data.

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SYSTEM AND METHOD FOR DETERMINING FLOW RATES IN A WELL CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority from Provisional Application 60/510,595, filed October 10, 2003, which is incorporated herein by reference. BACKGROUND OF THE INVENTION Field of the Invention [0001] The present invention relates to a system and method for determining flow rates in a well, and particularly to determining flow rates from a sensed well parameter, such as temperature. Description of Related Art [0002] In a variety of wells, various parameters are measured to determine specific well characteristics. Typically, however, a logging string is lowered into a well to measure desired parameters at various points along a wellbore. The logging string is lowered into the wellbore separately from an actual production completion. [0003] Thus, diagnosis of the well involves a separate, physical intervention into the well which increases cost and consumes time. In many applications, the logging string is used to measure a variety of parameters in an attempt to accurately determine the desired well characteristic or characteristics. BRIEF SUMMARY OF THE INVENTION [0004] In general, the present invention provides a method and system for using a well model in utilizing well parameters sensed while an actual operational completion is deployed in a wellbore. For example, a model of temperature as a function of zonal rates can be utilized. Temperature measurements are taken along the wellbore, and the model is used as a tool in inverting the measured temperatures to allocate flow rates from one or more well zones. BRIEF DESCRIPTION OF THE DRAWINGS [0005] Certain embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: [0006] Figure 1 is a schematic illustration of a completion and sensing system deployed in a wellbore, according to an embodiment of the present invention; [0007] Figure 2 is an elevation view of an embodiment of the system illustrated in Figure 1 for determining flow rates from multiple formation layers with multiple phase liquids; [0008] Figure 3 is a flowchart generally representing an embodiment of the methodology used in determining flow rates in a well, according to an embodiment of the present invention; [0009] Figure 4 is a diagrammatic representation of a processor-based control system that can be used to carry out all or part of the methodology for determining flow rates in a given well, according to an embodiment of the present invention; [0010] Figure 5 is a flowchart generally representing use of a well model in combination with measured parameters, according to an embodiment of the present invention; [0011] Figure 6 is a diagrammatic chart generally representing error sources that may be determined and/or compensated for according to an embodiment of the present invention; [0012] Figure 7 is a diagrammatic representation of the system illustrated in Figure 1 in which flow rates are determined in a single layer, single phase well; [0013] Figure 8 is a diagrammatic representation of the system illustrated in Figure 1 in which flow rates are determined in a multi-layer, single-phase well; and [0014] Figure 9 is a diagrammatic representation of the system illustrated in Figure 1 in which flow rates are determined in a multi-layer, multi-phase liquid well. DETAILED DESCRIPTION OF THE INVENTION [0015] In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those of ordinary skill in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible, [0016] The present invention generally relates to a system and method for determining flow rates in a well. Temperature measurements are taken along a wellbore, and those measurements are used to determine fluid flow rates at distinct zones within the well. In some applications, the total flow at the wellhead is measured and this total flow is allocated among separate zones based on temperature measurements taken along the well. Additionally, the physical property contrasts between differing fluids, such as oil and water, can be used to allocate flow rates in multi-phase liquid wells. Accordingly, the present system and method enables the allocation of flow rates in multi-phase liquid, multi-layer wells. [0017] Furthermore, the temperature sensing system is deployed with an operational completion and enables temperature measurements to be taken during operation of the well. Thus, the operation of the well deep downhole can be diagnosed without separate physical intervention into the well. An operator can continually diagnose zonal flow rates during operation of the well. Depending on the specific application, operation of the well may comprise production of fluids or injection of fluids into the surrounding formation. [0018] Referring generally to Figure 1, a system 20 is illustrated in accordance with an embodiment of the present invention. System 20 comprises a completion 22 deployed in a well 24. Well 24 is defined by a wellbore 26 drilled in a formation 28 having, for example, one or more fluids, such as oil and water. Completion 22 extends downwardly into wellbore 26 from a wellhead 30 disposed, for example, along a seabed floor or a surface of the earth 32. In many applications, wellbore 26 is lined with a casing 34 having sets of perforations 36 through which fluid flows between formation 28 and wellbore 26. In the embodiment illustrated, wellbore 26 is generally vertical. However, the wellbore also may be a deviated wellbore. [0019] As further illustrated, system 20 comprises a temperature sensing system 38. For example, temperature sensing system 38 may comprise a distributed temperature sensor (DTS) 40 that is capable sensing temperature continuously along wellbore 26. Distributed temperature sensor 40 may be coupled to a control 42 able to receive and process the temperature data obtained from multiple locations along wellbore 26. As discussed further below, control 42 also may enable using the temperature data in conjunction with a model of the well to derive flow rates from one or more wellbore zones. [0020] By way of further explanation, completion 22 is representative of a variety of completions. Depending on the application, one or more production related completions may be utilized within wellbore 26. For example, valves, electric submersible pumping systems, and/or gas lift systems can be utilized in producing one or more fluid phases from one or more well layers, i.e. well zones. Other examples of completions include well treatment completions, such as injection systems for injecting fluids into formation 28 at one or more well zones. [0021] An example of a multizone production system is illustrated in Figure 2. In this embodiment, several sets of perforations 36 are disposed along casing 34 to enable the inflow of fluid from formation 28 into wellbore 26. Specifically, the perforations 36 are located to enable the flow of fluid from a plurality of layers or zones 44 that form well 24. The multiple layers or zones 44 may comprise, for example, an upper producing zone 48 and a lower producing zone 50. Wellbore 26 also is divided into corresponding zones 44 via a plurality of packers 46. Fluid, such as oil or a combination of oil and water, flows from upper producing zone 48 and lower producing zone 50 into wellbore 26 so that it may be produced upwardly to an appropriate collection location, such as the surface of the earth. In this embodiment, completion 22 comprises a plurality of completion devices 52 that produce the fluid from the two or more zones. As discussed above, the completion devices 52 may comprise a variety of components, including electric submersible pumping systems, valves, gas lift systems, or other appropriate devices. Depending on the specific application, the produced fluids may be commingled or produced separately through one or more production tubings 54 or through an annulus 56 surrounding the one or more tubings. Also, the produced fluids may comprise multiphase liquids, such as mixtures of oil and water. [0022] Referring generally to Figure 3, an example of the methodology associated with the present invention is illustrated in flow chart form. Determining flow rates within a given well comprises establishing a sensor system in a well with an operable completion, as illustrated by block 58. The sensor system may comprise a distributed temperature sensor designed to sense well parameters, e.g. temperature, along wellbore 26, as illustrated by block 60. In many applications, a total flow is measured at an easily accessible location, such as at the wellhead 30, as illustrated by block 62. For example, a surface multiphase flow meter can be used to measure total flow at the wellhead. A well model may then be applied to determine flow rates from distinct well zones 44 based on the multiple temperature measurements, as illustrated by block 64. [0023] Some or all of the methodology outlined with reference to Figures 1-3 may be carried out by controller 42 which comprises an automated system 66, such as the processing system diagrammatically illustrated in Figure 4. Automated system 66 may be a computer-based system having a central processing unit (CPU) 68. CPU 68 may be operatively coupled to a distributed temperature sensor system 40, a memory 70, an input device 72, and an output device 74. Input device 72 may comprise a variety of devices, such as a keyboard, mouse, voice-recognition unit, touchscreen, other input devices, or combinations of such devices. Output device 74 may comprise a visual and/or audio output device, such as a monitor having a graphical user interface. Additionally, the processing may be done on a single device or multiple devices at the well location, remote from the well location, or with some devices located at the well and other devices located remotely. [0024] In automatically determining flow rates from well zones 44, a model of temperature as a function of zonal rates for a specific well may be stored by automated system 66 in, for example, memory 70. The forward model is used as a tool to invert the measured temperatures along wellbore 26 and allocate the flow rates from the different producing zones. As illustrated best in Figure 5, the general approach involves determining a model of temperature as a function of flow rates, as illustrated by block 75. The temperatures at various locations along wellbore 26 are measured, as illustrated by block 76, and the data may be stored by automated system 66. Subsequently, an inversion of the measured temperatures is performed by applying the model to determine flow rates, as illustrated by block 77. Appropriate models enable the allocation of flow rates across multiple zones flowing multiple liquid phases. It should also be noted that in at least some applications gas holdup increases toward the surface in a production string. However, the theoretical basis of the modeling discussed herein is not violated in such wells when temperatures are measured in the region of the producing interval(s). [0025] In general, the inversion process begins with a model incorporating the physics of the well to the extent possible. Flow rates from the different layers or zones of the well are then applied to the model which provides temperatures. The calculated temperatures are compared to measured temperatures, and the model is adjusted (e.g. by adjusting the estimate of oil and water coming from each zone) so the calculated temperatures match the measured temperatures. Also, the total flow rate at the surface can be used as a control for the sum of the allocated flow rates. [0026] The process may also involve the evaluation of and/or compensation for potential errors in the model and the inversion process. Potential sources of error are set forth in the chart of Figure 6. The overall methodology can be used to determine under what conditions flow rates may be allocated with a desired degree of certainty or confidence. This is accomplished for a given well by estimating error in zonal rates due to, for example, model error (see block 78 of Figure 6), measurement error (block 79), and parameter error (block 80). The methodology of determining and compensating for errors may be incorporated into the inversion process illustratively set forth by block 77 of the flow chart illustrated in Figure 5. [0027] Referring again to Figure 6, the model error is a byproduct of the model being an approximate representation of the key physical processes taking place in the wellbore, such as Joule-Thomson cooling at the sandface. The determination and/or [0030] Furthermore, to facilitate an understanding of the mathematical basis for the model, brief explanations of symbols used in the subsequent description of the model are as follows: [0031] The material balance equations are written in general form as follows: Mass Balance Equation: Rate of increase of mass = rate of mass in - rate of mass out (1.0) Momentum Balance Equation: Rate of increase of momentum = rate of momentum in - rate of momentum out + external force on the fluid (1.00) Energy Balance Equation: Rate of change of (internal energy + K.E. + P.E. due to convection) + (net rate of heat addition by conduction) - (net rate of work done by the system on the surrounding) = (Rate of accumulation of internal energy + K.E + P.E) (1.000) The analytical solution to Eq. 1.29 depends upon the Joule-Thomson coefficient, which may be obtained for different applications, such as a single phase liquid (oil or water production), two phase liquid (oil and water production), single-phase gas, and multiphase (oil, water, and gas) as described below. In a single phase liquid example, consider black oil production below the bubble point pressure in which the distributed temperature sensor is analyzed very close to the producing intervals in which gas hold up is usually very small compared to that on the surface, so that the assumption of constant density and that the pressure is below the bubble point pressure is applicable at a very small interval close to the producing zone for a black oil production. From Eq. 1.4 for V = constant, the Joule-Thomson coefficient becomes: fluid temperature inside the well in front of the nonproducing zones as a function of the dimensionless depth and the solution is as follows: Eqs. 1.38 and 1.39 are used to convert the profile from the dimensionless domain to the real domain by knowing the fixed fluid temperature at the bottom hole of the well, Tfbh, and the depth of the well, L. [0036] It should be noted that certain assumptions have been made during the mathematical modeling described above. The assumptions for each zone are as follows: Producing Zone (node 1-2): -In production, temperature at the perforations (Teibh) is the same as the temperature of the earth calculated from the geothermal gradient. -Conduction heat transfer is neglected. -Work done by the fluid against the viscous force is neglected. -Steady State Problem (No energy accumulation in the system). -Change in P.E. is neglected. -Incompressible fluid and neglect the area change between the two nodes, so change in K.E. is neglected. In Front of the Producing Zone (node 2-2') -Steady state (No mass or energy accumulation). -Neglect change in P.E. and K.E. -No loss or gain of heat during mixing (adiabatic mixing). -Fluid is incompressible or compressibility is very small. -Work done by the fluid against the viscous force is neglected. -Mixing takes place at constant pressure. -The mixture heat capacity is constant. Nonproducing Zone (node 4-3) -Work done by the fluid against the viscous forces is neglected. -Thermal conductivity is constant. -Heat conducted from the producing zone is neglected. Well Path (node2'-5) -Steady state problem (No energy, mass, and momentum accumulation in the system). -Work done by the fluid against the viscous forces is neglected. -Constant heat flux from the tubing to the casing and from the casing to the surroundings at each control volume. -Thermal resistance of pipe and steel is neglected compared to that of the fluid in the tubing/casing annulus. -Incompressible fluid and no area change, so change in K.E. is neglected. [0037] The temperature forward modeling derived above also can be applied to multi-layer or multi-zone wells for both single and multi-phase liquid production. As illustrated in the example of Figure 8, well 24 is a single-phase, multi-layer production well having nonproducing zones 86, 88 and producing zones 90, 92. The completion 22 extends downwardly into wellbore 26 through a pair of packers 46. This schematic representation also illustrates a thermal nodal analysis used to develop a mathematical temperature model by determining the temperature at each of a plurality of nodes, labeled 1,2,2', 3,4,5,5', 6,7,8,and 9. [0038] The difference between the single layer and the two layer production is in the nodal analysis between nodes 5-5'- nodes 8-7, and nodes 5'-9, as well as a minor change between nodes 2' -5. The main differences between the single layer and the two layer production will be mentioned for each of these nodes. [0039] Node 2'-5 The ordinary differential equation given across these nodes for single layer production, Eq. 1.42, is the same as that used for the two layer production. However, the boundary condition that will be used is more general such as: TfD (ZD = Zdbh ) = Tfdbh This general boundary condition enables handling of the two or multi-layer production cases as the temperature at node 5' should be corrected due to the mixing between the two streams and also due to the change of the rate from q1 to q1+q2. In this case, the well is treated as having different sections, each having the same equation but different boundary condition depending upon the temperature of the previous section. The solution of Eq. 1.42 using the above general boundary condition has been performed using Mathematica® software, and the solution is as follows: Where, Tfdbh is the temperature of entry and Zdbh is the depth measured from the bottom of the well at the entry level. Eqs. 1.38 and 1.39 are used to convert the dimensionless temperature profile obtained from Eq. 2.1 to the real domain. [0040] Node 5 -5' The modeling between node 5 and 5' is very similar to that between node 2 and 2' for the single layer production presented above in that both mass and energy balance are applied. Also, the assumptions used between nodes 2 and 2' are the same as between nodes 5 and 5' except the last assumption where the heat capacity of the two streams are not the same and also the mixing rates are not equal. Similarly, by dividing the producing zone into equal intervals, each interval produces at an equal rate. The number of divisions depends upon the number of temperature measurements in the producing zone. By applying a macroscopic mass and energy balance due to the mixing of two streams from the upper producing zone and the total rate obtained from the lower zone, the temperature at any interval inside the producing zone can be obtained using the following derived equation: Where, i = 1, 2, , n (n is the number of divisions or the number of temperature measurements inside the upper producing zone); Tf(i) is the temperature at each interval inside the producing zone; Tf(o) is the wellbore temperature at node 5; CP2 is the specific heat capacity of the fluid in the upper producing zone; q1, q2 is the total production from the lower zone and upper zone, respectively; and Eq. 2.1 can be used to describe the temperature profile between node 5' and 9 by using the total rate (q1 + q2) instead of q1. Also, Cp between node 5' and 9 is equal to CP5' as calculated from Eq. 2.5. [0043] The temperature forward modeling derived above also can be applied to multi-layer, multi-zone wells, such as a two-phase (oil-water) liquid, two-layer production well. As illustrated in the example of Figure 9, well 24 is a multi-phase liquid, multi-layer production well having nonproducing zones 94, 96 and producing zones 98, 100. The completion 22 extends downwardly into wellbore 26 through a pair of packers 46. The schematic representation further illustrates a thermal nodal analysis used to develop a mathematical temperature model by determining the temperature at each of a plurality of nodes, labeled 1, 2, 2',3, 4, 5, 5', 6, 7, 8 and 9. [0044] The extension of the modeling to two-phase liquid flow depends upon recalculating the parameters of the modeling for the two-phase flow. The equation for each parameter will differ depending upon the nodal location, thus, the equation for each parameter will be given between each node with a special reference to the equation used in the temperature modeling. It should be noted that in the nonproducing zone as there is no fluid flow, only heat energy flow, the change from single-phase to two-phase liquid flow will not affect the temperature modeling between nodes 3 and 4 and nodes 8 and 7. Also, it should be mentioned that the correction of the temperature due to the pressure drop in front of the producing interval is neglected. [0045] Node 2-2' As seen from Eq. 1.11, the temperature modeling between nodes 2 and 2' depends only on the geothermal temperature, which does not depend upon the production phase. Therefore, the temperature modeling between nodes 2 and 2' is the same as for single-phase flow. [0046] Node 2'-5 The extension of the temperature modeling to multi-layer, two-phase liquid flow is trivial as it is only an extension of the equations discussed above following the same steps as in the extension from single layer to two layers. [0049] An appropriate temperature forwarding model, as discussed above, is used as the forward tool in inverting the temperature measurements inside an operating well. The operating well may be, for example, a producing well or a well under treatment. Inverting the temperature measurements enables allocation of fluid flow rates from producing layers. [0050] In a broad sense, inversion is finding the independent parameters in the forward model that minimize the error between the measured dependent parameter and the calculated dependent parameter from the forward model. Accordingly, it becomes an optimization problem in which it is desirable to minimize a certain objective function, which is the error between the measured and the calculated dependent parameters, by changing the independent parameters in a certain domain. In other words, the independent parameters can be changed according to specified constraints. [0051] As discussed above, with respect to the subject well applications, the dependent parameter is temperature and the independent parameters are mainly the zonal rates, although there could be other input parameters of the forward modeling. Accordingly, the mathematical description of the optimization problem is as follows: Where, m: is a vector of the independent parameters, mainly the zonal rates and/or other input parameters. [0052] The inversion process can be used to minimize, e.g. compensate for, various errors as discussed above. Several optimization algorithms may be used to determine the zonal rate or rates by minimizing the error between the temperature measured from, for example, distributed temperature sensor 40 and the calculated temperature from the forward modeling, such as the forward models discussed previously. However, one optimization algorithm that works well and is relatively straightforward is the "Generalized Reduced Gradient" algorithm that is coded in Excel® software available from Microsoft Corporation. The Excel® software can be, for example, loaded onto control 42 and utilized by an operator in determining fluid flow rates from well zones based on the temperature input data obtained from the well via distributed temperature sensor 40 and control 42. An inverse modeling with the Generalized Reduced Gradient optimization algorithm can thus be used to invert for the zonal rate allocation by minimizing the difference between the measured temperature from the distributed temperature sensor 40 and the calculated temperature from the forward model. [0053] Testing has shown a high level of accuracy in the zonal rate allocation based on distributed temperature sensor measurements in a variety of applications and under varying conditions. For example, in single-phase liquid production in an environment with high temperature contrast between producing zones, the zonal rates can be allocated with high accuracy, even without imposing the total rate as a constraint in the optimization. In a single-phase liquid production with low temperature contrast between producing zones, the zonal rates can be allocated with high accuracy, particularly if the total rate is added as a constraint in the optimization. Another example is two-phase liquid production in which oil and water are produced with high temperature contrast between producing zones. In this application the zonal rates were allocated with high accuracy when using the total rate for each production phase as a constraint in the optimization. If more than two zones are inverted, accuracy can sometimes be improved by determining the total phase rate above each two producing zones. However, this does not mean the inversion is not useful if only one total rate is imposed for each phase above more than two producing intervals. [0054] Although, only a few embodiments of the present invention have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this invention. Accordingly, such modifications are intended to be included within the scope of this invention as defined in the claims. CLAIMS What is claimed is: 1. A method of determining production rates in a well, comprising: determining a model of temperature as a function of zonal flow rates in the well; measuring temperatures at a plurality of locations in the well; and inverting the measured temperatures by applying the model to determine an allocation of production rates from different producing zones in the well. 2. The method as recited in claim 1, wherein determining comprises determining the model for a single-phase liquid producing well. 3. The method as recited in claim 1, wherein determining comprises determining the model for a multi-layer producing well. 4. The method recited in claim 1, wherein determining comprises determining the model for a multi-layer, single-phase liquid producing well. 5. The method as recited in claim 1, wherein determining comprises determining the model for a multi-layer, multi-phase liquid producing well. 6. The method as recited in claim 1, wherein measuring comprises measuring temperature with a distributed temperature sensor. 7. The method as recited in claim 1, wherein inverting comprises determining a degree of certainty in the production rates allocated. 8. The method as recited in claim 7, wherein determining the degree of certainty comprises determining a degree of error in the model. 9. The method as recited in claim 7, wherein determining the degree of certainty comprises determining a degree of error in the measured temperatures. 10. The method as recited in claim 7, wherein determining the degree of certainty comprises determining a degree of error in well parameter values. 11. The method as recited in claim 1, wherein inverting comprises utilizing a generalized reduced gradient optimization algorithm. 12. A method of determining flow rates in a well, comprising: measuring temperature at a plurality of points along the well having a plurality of well zones and a plurality of liquid phases; and determining flow rates of the plurality of liquid phases through each of the plurality of well zones via the measured temperatures. 13. The method as recited in claim 12, wherein measuring comprises utilizing a distributed temperature sensor. 14. The method as recited in claim 12, wherein determining comprises constructing a model of temperature as a function of zonal flow rates in the well, and using the model to invert the measured temperatures in allocating flow rates from the plurality of well zones. 15. The method as recited in claim 12, wherein determining comprises determining flow rates of oil and water phases during production. 16. The method as recited in claim 12, wherein determining comprises determining flow rates of fluid injected into each of the plurality of well zones. 17. The method as recited in claim 14, wherein inverting the temperatures comprises utilizing an optimization algorithm. 18. The method as recited in claim 12, wherein determining comprises measuring a total flow rate at a wellhead. 19. A system, comprising: a temperature sensor deployable with a production completion along a wellbore to sense temperature data at a plurality of wellbore locations during production; and a processor system able to receive the temperature data and allocate a flow rate from a plurality of wellbore zones based on the temperature data. 20. The system as recited in claim 19, wherein the processor system uses a temperature forward model, in which temperature is a function of zonal flow rates, to invert the temperature data and allocate flow rates from producing layers of a formation. 21. The system as recited in claim 19, wherein the temperature sensor comprises a distributed temperature sensor. 22. The system as recited in claim 19, wherein the processor system is able to allocate flow rates in a multi-layer, multi-phase liquid producing well. 23. The system as recited in claim 19, wherein the production completion comprises an electric submersible pumping system. 24. The system as recited in claim 19, wherein the production completion comprises a gas lift system. 25. The system as recited in claim 19, wherein the wellbore is oriented generally vertically. 26. A method, comprising: deploying a distributed temperature sensor along a wellbore; utilizing a model of temperature as a function of fluid flow rates into the wellbore; obtaining temperature data from the distributed temperature system; allocating a fluid flow rate in at least one wellbore zone using the temperature data in conjunction with the model; and determining error in the fluid flow rate. 27. The method as recited in claim 26, wherein allocating comprises inverting the temperature data to obtain the fluid flow rate. 28. The method as recited in claim 26, wherein deploying comprises deploying the distributed temperature system in a generally vertical wellbore. 29. The method as recited in claim 26, wherein deploying comprises deploying the distributed temperature system in a deviated wellbore. 30. The method as recited in claim 26, wherein allocating comprises determining fluid flow rates across a plurality of wellbore zones. 31. The method as recited in claim 26, wherein allocating comprises determining flow rates for a single-phase liquid producing well. 32. The method as recited in claim 26, wherein allocating comprises determining flow rates for a multi-phase liquid producing well. 33. The method as recited in claim 26, wherein determining comprises compensating for model error, measurement error, and well parameter error. 34. A system, comprising: means for measuring temperature at a plurality of points along a well having a plurality of well zones and a plurality of liquid phases; and means for determining flow rates of the plurality of liquid phases through each of the plurality of well zones via the measured temperatures. 35. The system as recited in claim 34, wherein the means for measuring comprises a distributed temperature sensor. 36. The system as recited in claim 34, wherein the means for determining comprises a processor system able to receive the temperature data and allocate a flow rate from a plurality of wellbore zones based on the temperature data. Dated this 7 day of April 2006

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