Iterative calculation method for determining dynamic reserves of fault control fracture-vuggy oil reservoir after fracture closure

Abstract

The invention discloses an iterative calculation method for determining dynamic reserves after fracture closure of a fault control fracture-vuggy oil reservoir, and the method comprises the following steps: S1, obtaining cumulative production data and flowing pressure data of a target well, and drawing an energy indication curve according to the cumulative production data and the flowing pressure data, dividing the energy indication curve into a stage I before crack closing and a stage II after crack closing according to slope change; s2, establishing a material balance equation, wherein a comprehensive compression coefficient in the material balance equation adopts a modified comprehensive compression coefficient which considers fluid item change and reservoir physical property parameter change at the same time; s3, collecting physical property parameters of the reservoir and the fluid of the target well, and performing iterative calculation in combination with a material balance equation; and S4, selecting a result with a minimum error as a dynamic reserve result of the stage II according to an iterative calculation result. According to the method, the dynamic reserves after the fracture is closed can be more conveniently and effectively obtained, meanwhile, the change of the porosity of the well control area caused by fracture closing can be evaluated, and reliable support is provided for dynamic analysis and efficient development of oil reservoirs of the type.

Description

Technical Field

[0001] This invention relates to the fields of oil and gas field development and reservoir engineering technology, and in particular to an iterative calculation method for determining the dynamic reserves after fracture closure in fracture-controlled fracture-vuggy reservoirs. Background Technology

[0002] Fault-controlled fractured-cavity carbonate reservoirs exhibit localized oil and gas enrichment, facilitating the construction of high-yield well clusters. The efficient development of such unconventional oil and gas resources and complex reservoirs is a current development priority in my country. Dynamic reserves refer to the underground reserves that can be utilized using current development models and methods. They are typically calculated and evaluated based on production dynamic data using specific methods, and are therefore closely related to production dynamics. Dynamic reserves are a crucial parameter for evaluating the current state of reservoir development, production management, and dynamic analysis, and are relevant to subsequent adjustments and optimization of production enhancement measures. Existing methods for calculating dynamic reserves mainly include the mass balance method, waterdrive curve method, production decline method, and well testing method.

[0003] Among the various methods, the water-drive curve method utilizes the linear relationship between cumulative oil production and cumulative water production to plot a semi-logarithmic curve and obtain the slope through linear regression, then uses a formula to calculate dynamic reserves. This method is only applicable to the high water-cut stage and cannot be used in the low water-cut production stage. The production decline method selects an appropriate decline pattern to fit a decline curve based on the decline law of production over time, and then uses integration to bring the cumulative production to the economic limit to obtain recoverable dynamic reserves. The limitation of this method is that the target well has already entered the decline stage and the law is obvious. For reservoirs with fracture closure, the production decline may not be the same as the natural production decline law, so its adaptability is not strong. The well testing method uses pressure recovery or pressure drop test data to select a model that matches the target reservoir, fits and inverts the reservoir parameters, and then calculates dynamic reserves. Although this method has high adaptability, the well testing cost is also high, and the pressure drop test requires a period of well shut-in.

[0004] The material balance method establishes a material balance equation based on the fundamental principle of conservation of fluid elastic expansion, pore elastic compression, natural water intrusion, and produced fluid volume. Then, a curve showing the relationship between cumulative produced fluid volume and pressure drop, also known as the production indicator curve, is plotted. Utilizing the linear relationship between cumulative produced fluid volume and pressure drop in the material balance equation, and substituting relevant physical property parameters and the slope of the production indicator curve, the dynamic reserves can be directly calculated. This method is simple and efficient, requiring only the collection of production data and relevant physical property parameters. It is also highly adaptable, applicable wherever the pressure wave covers the entire well control area.

[0005] However, the mass balance method essentially describes a specific equilibrium state at a certain stage. When reservoir or fluid properties change significantly, the original equilibrium is broken, and a new equilibrium is established. This is reflected in the production indicator curve as a distinct segmentation of the slope. This phenomenon is particularly pronounced when fracture closure occurs. In many field applications, the changed slope is simply substituted to calculate the dynamic reserves after fracture closure. However, the change in slope is essentially caused by changes in reservoir and fluid properties, making this approach inaccurate. The composite compressibility coefficient has the greatest impact on the calculation results and carries the most uncertainty in the mass balance equation. This parameter incorporates the properties of both the fluid and the reservoir rock. Therefore, the composite compressibility coefficient must be corrected when calculating the dynamic reserves after fracture closure to obtain more realistic results. To address this issue, some scholars have proposed iteratively correcting the water-oil ratio term in the composite compressibility coefficient. However, this only considers changes in the fluid term and not changes in reservoir properties. Furthermore, the error relationship controlling the iteration is problematic; this relationship describes the difference in dynamic reserves between two stages as the cumulative fluid production of the previous stage. When crack closure occurs, the difference in dynamic reserves between the two stages cannot be ignored due to the contribution of reduced porosity. Summary of the Invention

[0006] To address the aforementioned problems, this invention aims to provide an iterative calculation method for determining the dynamic reserves of fractured-vuggy reservoirs after fracture closure.

[0007] The technical solution of the present invention is as follows: An iterative calculation method for determining the dynamic reserves of fractured-vuggy reservoirs after fracture closure includes the following steps: S1: Obtain the cumulative production data and flowing pressure data of the target well, and use them to draw an energy indicator curve. The energy indicator curve is then divided into stage I before fracture closure and stage II after fracture closure based on the slope change. S2: Establish a material balance equation, wherein the comprehensive compressibility coefficient in the material balance equation is a modified comprehensive compressibility coefficient that simultaneously considers changes in fluid terms and changes in reservoir physical property parameters. S3: Collect the reservoir and fluid properties of the target well and perform iterative calculations in conjunction with the material balance equation; S4: Based on the results of the iterative calculation, select the result with the smallest error as the dynamic reserve result for stage II.

[0008] Preferably, in step S2, the mass balance equation is: (1) (2) In the formula: To accumulate oil production, m 3 ; This is the current crude oil volume factor; For dynamic reserves, m 3 ; The volume factor of the original crude oil; To correct the overall compression ratio, MPa -1 ; For production pressure drop, MPa; The compressibility coefficient of crude oil is expressed in MPa. -1 ; To bind water saturation; The water-oil ratio; The formation water compressibility coefficient is given in MPa. -1 ; Porosity; Poisson's ratio; Here is Young's modulus, in MPa.

[0009] Preferably, in step S3, the physical properties of the reservoir and fluid include: stage I cumulative oil production, stage I cumulative water production, stage I energy indicator curve slope, stage II energy indicator curve slope, formation water compressibility, Young's modulus, Poisson's ratio, original crude oil volume factor, stage I crude oil volume factor, stage II crude oil volume factor, stage I crude oil compressibility, stage II crude oil compressibility, initial porosity, and bound water saturation.

[0010] Preferably, step S3, the iterative calculation, specifically includes the following sub-steps: S31: Given any water-oil ratio R in stage I wo1 Based on the aforementioned material balance equation, the dynamic reserves N1 for stage I are calculated as follows: (3) In the formula: The slope of the energy indicator curve for stage I; The volume factor for crude oil in Stage I; The compression coefficient of crude oil in Stage I, in MPa -1 ; Initial porosity; S32: Based on the water-oil ratio R in stage I wo1 The water-oil ratio R in stage II is determined by the following formula. wo2 : (4) In the formula: The cumulative water production in stage I, in m 3 ; For the cumulative oil production of Phase I, m 3 ; S33: Initial porosity Based on this, the step size is reduced to obtain the stage II porosity. And according to the water-oil ratio R in stage II wo2 Based on the aforementioned material balance equation, the dynamic reserves N2 for stage II are calculated as follows: (5) In the formula: The slope of the energy indicator curve for stage II; The volume factor for crude oil in Stage II; The compression coefficient of crude oil in Stage II, in MPa -1 ; S34: Adjust the water-oil ratio in stage I. wo1 Repeat steps S31-S33 until N1 and N2 meet the relative error requirements.

[0011] Preferably, in step S34, the relative error is calculated using the following formula: (6) In the formula: The percentage represents the relative error.

[0012] Preferably, in step S34, the relative error is required to be less than 5%.

[0013] The beneficial effects of this invention are: This invention not only considers the changes in crude oil physical properties during the production process, but also the influence of two uncertain factors, the water-oil ratio and porosity, to correct the overall compressibility coefficient. This invention can more conveniently and effectively obtain the dynamic reserves after fracture closure, and at the same time, it can assess the changes in porosity in the well-controlled area caused by fracture closure, providing reliable support for the dynamic analysis and efficient development of this type of reservoir. Attached Figure Description

[0014] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0015] Figure 1 The theoretical energy indicator curve for an oil well with closed fractures; Figure 2 Flowchart for iterative calculation of dynamic reserves; Figure 3 A model diagram illustrating the porosity changes in the well-controlled region caused by fracture closure; Figure 4 Here is the energy indication curve of well SHB1-X in a specific embodiment; Figure 5 In one specific embodiment, the SHB1-X well is in different A schematic diagram showing the comparison results of iterative calculations of dynamic reserves. Detailed Implementation

[0016] The present invention will be further described below with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and technical features described in this application can be combined with each other. It should also be pointed out that, unless otherwise indicated, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. The terms "comprising" or "including" and similar words used in this invention refer to elements or objects preceding the word that encompass the elements or objects listed following the word and their equivalents, without excluding other elements or objects.

[0017] This invention provides an iterative calculation method for determining the dynamic reserves of fractured-vuggy reservoirs after fracture closure, comprising the following steps: S1: Obtain the cumulative production data and flowing pressure data of the target well, and use them to draw an energy indicator curve. The energy indicator curve is then divided into stage I before fracture closure and stage II after fracture closure based on the slope change.

[0018] A schematic diagram of the energy indicator curve is shown below. Figure 1 As shown, based on the obvious overall inflection point of the curve, it is divided into stage I before crack closure and stage II after crack closure.

[0019] S2: Establish a material balance equation, wherein the comprehensive compressibility coefficient in the material balance equation is a modified comprehensive compressibility coefficient that simultaneously considers changes in fluid terms and changes in reservoir physical parameters.

[0020] In one specific embodiment, the mass balance equation is: (1) (2) In the formula: To accumulate oil production, m 3 ; This is the current crude oil volume factor; For dynamic reserves, m 3 ; The volume factor of the original crude oil; To correct the overall compression ratio, MPa -1 ; For production pressure drop, MPa; The compressibility coefficient of crude oil is expressed in MPa. -1 ; To bind water saturation; The water-oil ratio; The formation water compressibility coefficient is given in MPa. -1 ; Porosity; Poisson's ratio; Here is Young's modulus, in MPa.

[0021] In the above embodiments, the formula (2) for calculating the corrected comprehensive compressibility factor, which simultaneously considers changes in the fluid term and changes in reservoir physical parameters, is derived through the following steps: The combined compressibility coefficient is a combination of the fluid and rock compressibility coefficients, and typically takes the following form: (7) In the formula: The overall compressibility factor is expressed in MPa. -1 ; The rock compressibility coefficient is expressed in MPa. -1 .

[0022] The compressibility of solid rock can be expressed as: (8) In the formula: The compressibility of a solid substance is expressed in MPa. -1 ; Let m be the volume of the rock. 3 ; For the skeleton stress, MPa.

[0023] Under elastic deformation conditions, the compressibility coefficient of solid rock can be calculated using Young's modulus and Poisson's ratio. For a certain type of rock, Young's modulus and Poisson's ratio can be assumed to be specific: (9) According to the effective stress theory, the external stress on a rock consists of skeletal stress and pore pressure: (10) In the formula: External stress, MPa; ρ represents pore pressure, in MPa.

[0024] The rock compression process is actually the compression of the rock skeleton. The pore space, as a non-material existence, does not produce compression phenomena on its own. Therefore, the reservoir pore compressibility coefficient can be represented by the skeleton compressibility coefficient. (11) In the formula: Let m be the reservoir pore volume. 3 .

[0025] During reservoir rock compression, the external stress typically remains constant. However, as the pore pressure decreases, the skeletal stress increases accordingly, leading to the initiation of compressive deformation in the rock. Therefore, differentiating the expression for the external stress of the rock yields: (12) Based on the expression for reservoir porosity compressibility: (13) Combining formulas (8) and (9), the reservoir porosity compressibility coefficient, expressed by porosity, Young's modulus, and Poisson's ratio, can be obtained: (14) Substituting the reservoir porosity compressibility coefficient shown in formula (14) into the comprehensive compressibility coefficient expression shown in formula (7), we can obtain the formula (2) for calculating the modified comprehensive compressibility coefficient that simultaneously considers changes in fluid terms (crude oil physical properties and water-oil ratio) and changes in reservoir physical properties (porosity).

[0026] S3: Collect the reservoir and fluid properties of the target well and perform iterative calculations in conjunction with the material balance equation.

[0027] In a specific embodiment, the physical properties of the reservoir and fluid include: stage I cumulative oil production, stage I cumulative water production, stage I energy indicator curve slope, stage II energy indicator curve slope, formation water compressibility, Young's modulus, Poisson's ratio, original crude oil volume factor, stage I crude oil volume factor, stage II crude oil volume factor, stage I crude oil compressibility, stage II crude oil compressibility, initial porosity, and bound water saturation.

[0028] In a specific embodiment, such as Figure 2 As shown, the iterative calculation specifically includes the following sub-steps: S31: Given any water-oil ratio R in stage I wo1 Based on the aforementioned material balance equation, the dynamic reserves N1 for stage I are calculated as follows: (3) In the formula: The slope of the energy indicator curve for stage I; The volume factor for crude oil in Stage I; The compression coefficient of crude oil in Stage I, in MPa -1 ; Initial porosity; S32: Based on the water-oil ratio R in stage I wo1 The water-oil ratio R in stage II is determined by the following formula. wo2 : (4) In the formula: The cumulative water production in stage I, in m 3 ; For the cumulative oil production of Phase I, m 3 ; S33: Initial porosity Based on this, the step size is reduced to obtain the stage II porosity. And according to the water-oil ratio R in stage II wo2 Based on the aforementioned material balance equation, the dynamic reserves N2 for stage II are calculated as follows: (5) In the formula: The slope of the energy indicator curve for stage II; The volume factor for crude oil in Stage II; The compression coefficient of crude oil in Stage II, in MPa -1 ; S34: Adjust the water-oil ratio in stage I. wo1 Repeat steps S31-S33 until N1 and N2 meet the relative error requirements.

[0029] In one specific embodiment, the relative error is calculated using the following formula: (6) In the formula: The percentage represents the relative error.

[0030] In the above embodiments, the formula for calculating the relative error is derived through the following steps: The difference in dynamic reserves between Stage I and Stage II is divided into two parts: one part is the cumulative oil production in Stage I, and the other part is the loss of dynamic reserves due to the loss of volume in the connecting area caused by fracture closure, resulting in lower porosity in the control area. Figure 3 As shown. Ignoring the production process, dynamic reserves can be approximated using the volumetric method: For stage I before crack closure: (15) In the formula: The total reservoir volume is m. 3 ; The volumetric reserves in stage I before fracture closure, m 3 .

[0031] For stage II after crack closure: (16) In the formula: The volumetric reserves (m) represent the stage II reserves after fracture closure. 3 ; Therefore, the difference in reserves caused by changes in porosity should be: (17) Based on the cumulative oil production of Stage I and converted to subsurface conditions, the difference in actual dynamic reserves between the two stages can be approximated as: (18) The relative error calculation formula shown in formula (6) can be obtained from formula (18). Using this relative error calculation formula, the water-oil ratio and porosity values ​​after fracture closure can be controlled so that the calculated dynamic reserves meet the error requirements, and finally more reliable unknown parameters such as dynamic reserves and fracture closure porosity can be obtained.

[0032] In one specific embodiment, the relative error requirement is a relative error of less than 5%. It should be noted that the 5% threshold is a manually set value; other thresholds, such as 1% or 10%, can also be applied to this invention. A smaller threshold allows for more iterations and higher calculation accuracy.

[0033] S4: Based on the results of the iterative calculation, select the result with the smallest error as the dynamic reserve result for stage II.

[0034] In a specific embodiment, taking a fault-controlled fracture-vuggy reservoir as an example, the iterative calculation method for determining the dynamic reserves after fracture closure of a fault-controlled fracture-vuggy reservoir, as described in this invention, is used to calculate its dynamic reserves after fracture closure. The specific steps include: (1) Obtain the cumulative production data and flowing pressure data of the target well, and use them to draw an energy indicator curve. The energy indicator curve is divided into stage I before fracture closure and stage II after fracture closure based on the slope change. In this embodiment, the energy indicator curve plotted based on the cumulative production data and flowing pressure data of well SHB1-X is as follows: Figure 4 As shown. The first stage before crack closure is divided as follows. Figure 4 As shown by the red line, the second stage after the crack closure is divided as follows: Figure 4 As shown by the blue line.

[0035] (2) Establish the mass balance equations shown in formulas (1)-(2); (3) Collect the reservoir and fluid properties of well SHB1-X and perform iterative calculations in conjunction with the material balance equation; In this embodiment, the reservoir and fluid properties of the SHB1-X well are shown in Table 1: Table 1. Reservoir and fluid properties of well SHB1-X

[0036] During iterative calculation, the steps S31-S34 are followed, and the relative error in step S34 is calculated using formula (6). The requirement for the relative error is that it is less than 5%. The iterative calculation results are as follows: Figure 5 As shown. From Figure 5 The group with the smallest error was selected as the dynamic reserves of well SHB1-X after fracture closure, and the results are shown in Table 2: Table 2. Iterative calculation results of dynamic reserves after fracture closure in well SHB1-X

[0037] As shown in Table 2, the dynamic reserves after crack closure, calculated by the iterative method of this invention, are 19.29 × 10⁻⁶. 4 m 3 The dynamic reserves in the second stage, as assessed using conventional material balance methods at the oilfield site, are 25 × 10⁻⁶. 4 m 3 It is evident that fracture closure significantly impacts the dynamic reserves of fracture-vuggy reservoirs. Failure to consider this impact during calculations can lead to overestimation of dynamic reserves, further affecting subsequent evaluation results.

[0038] In summary, this invention can more accurately determine the dynamic reserves of fractured-vuggy reservoirs after fracture closure. Compared with the prior art, this invention represents a significant advancement.

[0039] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. An iterative calculation method for determining the dynamic reserves of fractured-vuggy reservoirs after fracture closure, characterized in that, Includes the following steps: S1: Obtain the cumulative production data and flowing pressure data of the target well, and use them to draw an energy indicator curve. The energy indicator curve is then divided into stage I before fracture closure and stage II after fracture closure based on the slope change. S2: Establish a material balance equation, wherein the comprehensive compressibility coefficient in the material balance equation is a modified comprehensive compressibility coefficient that simultaneously considers changes in fluid terms and changes in reservoir physical property parameters. S3: Collect the reservoir and fluid properties of the target well and perform iterative calculations in conjunction with the material balance equation; S4: Based on the results of the iterative calculation, select the result with the smallest error as the dynamic reserve result for stage II.

2. The iterative calculation method for determining the dynamic reserves after fracture closure in fracture-controlled fractured-vuggy reservoirs according to claim 1, characterized in that, In step S2, the mass balance equation is: （1） （2） In the formula: To accumulate oil production, m 3 ; This is the current crude oil volume factor; For dynamic reserves, m 3 ; The volume factor of the original crude oil; To correct the overall compression ratio, MPa -1 ; For production pressure drop, MPa; The compressibility coefficient of crude oil is expressed in MPa. -1 ; To bind water saturation; The water-oil ratio; The formation water compressibility coefficient is given in MPa. -1 ; Porosity; Poisson's ratio; Here is Young's modulus, in MPa.

3. The iterative calculation method for determining the dynamic reserves after fracture closure in fracture-controlled fractured-vuggy reservoirs according to claim 2, characterized in that, In step S3, the physical properties of the reservoir and fluid include the cumulative oil production in stage I, the cumulative water production in stage I, the slope of the energy indicator curve in stage I, the slope of the energy indicator curve in stage II, the formation water compressibility coefficient, Young's modulus, Poisson's ratio, the volume factor of the original crude oil, the volume factor of the crude oil in stage I, the volume factor of the crude oil in stage II, the compressibility coefficient of the crude oil in stage I, the compressibility coefficient of the crude oil in stage II, the initial porosity, and the bound water saturation.

4. The iterative calculation method for determining the dynamic reserves after fracture closure in fracture-controlled fractured-vuggy reservoirs according to claim 2, characterized in that, Step S3, the iterative calculation specifically includes the following sub-steps: S31: Given any water-oil ratio R in stage I wo1 Based on the aforementioned material balance equation, the dynamic reserves N1 for stage I are calculated as follows: （3） In the formula: The slope of the energy indicator curve for stage I; The volume factor for crude oil in Stage I; The compression coefficient of crude oil in Stage I, in MPa -1 ; Initial porosity; S32: Based on the water-oil ratio R in stage I wo1 The water-oil ratio R in stage II is determined by the following formula. wo2 : （4） In the formula: The cumulative water production in stage I, in m 3 ; For the cumulative oil production in Phase I, m 3 ; S33: Initial porosity Based on this, the step size is reduced to obtain the stage II porosity. And according to the water-oil ratio R in stage II wo2 Based on the aforementioned material balance equation, the dynamic reserves N2 for stage II are calculated as follows: （5） In the formula: The slope of the energy indicator curve for stage II; The volume factor for crude oil in Stage II; The compression coefficient of crude oil in Stage II, in MPa -1 ; S34: Adjust the water-oil ratio in stage I. wo1 Repeat steps S31-S33 until N1 and N2 meet the relative error requirements.

5. The iterative calculation method for determining the dynamic reserves after fracture closure in fracture-controlled fractured-vuggy reservoirs according to claim 4, characterized in that, In step S34, the relative error is calculated using the following formula: （6） In the formula: The percentage represents the relative error.

6. The iterative calculation method for determining the dynamic reserves after fracture closure in fracture-controlled fractured-vuggy reservoirs according to claim 4 or 5, characterized in that, In step S34, the requirement for the relative error is that the relative error is less than 5%.

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