A method for evaluating deep ultra-low permeability rock reservoir compressibility

Abstract

The application discloses a deep ultra-low permeability rock reservoir compressibility evaluation method, which comprises the following steps: analyzing the relationship between main control factors of reservoir compressibility and reservoir fracturing difficulty, and determining a relationship formula for representing the influence degree of the main control factors on reservoir compressibility; quantitatively representing the influence degree of the main control factors on reservoir compressibility based on the relationship formula of the influence degree of each main control factor on reservoir compressibility; and evaluating the reservoir compressibility by using an analytic hierarchy process based on the influence degree of different main control factors. The method quantitatively represents the influence degree of elastic modulus, Poisson's ratio, fracture toughness, ground stress and natural fractures on reservoir compressibility, and realizes more accurate evaluation of reservoir compressibility.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of oil and gas field development, and particularly relates to a deep ultra-low permeability rock reservoir compressibility evaluation method. BACKGROUND

[0002] Accelerating the development of deep and ultra-deep (hereinafter collectively referred to as "deep") oil and gas resources is an important measure to alleviate energy supply contradictions and ensure national energy security. Deep rock reservoirs are dense, and the formation of a dense fracture network through fracturing is the key to the efficient development of deep resources. Fracturing is essentially a rock fracture mechanics process, and the core is rock reservoir fracture propagation driven by high-pressure fluid. As deep resource development enters the deep, the level of in-situ ground stress and formation temperature increases significantly, and rock fracturing fracture propagation under deep in-situ environment is increasingly difficult, and deep reservoir fracturing is facing great challenges.

[0003] The size and complexity of the fractures formed during reservoir fracturing (i.e., fracture propagation capacity) are important indicators for judging the pros and cons of the transformation effect, and the "fracturability index (FI)" is used in engineering for evaluation; the fracture toughness (KIC) is a quantitative indicator of the resistance of material fracture propagation, so establishing the relationship between the difficulty of reservoir fracturing and the rock fracture characteristics is the key to accurately characterizing the fracturability.

[0004] However, in the prior art, the rock fracture propagation characteristics under deep in-situ high ground stress and high temperature conditions (hereinafter referred to as "in-situ environment") are not systematic, and the rock fracture propagation resistance under deep in-situ environment is rarely studied. The existing fracturability strategies mainly have the following three main problems: (1) usually do not consider the rock fracture propagation characteristics, and this method obviously ignores the real mechanism of effective fracturing; (2) although the rock fracture characteristics are considered, they are regarded as a constant independent of the burial depth; (3) the fracturability of deep reservoirs is controlled by multiple factors such as rock intrinsic properties (such as elastic modulus, Poisson's ratio), fracture characteristics and geological conditions (such as ground stress), and different factors contribute differently to the reservoir fracturability, but some fracturability evaluation strategies have not quantitatively characterized the contribution of each main controlling factor, and further scientifically allocated the weight coefficients of each main controlling factor, resulting in distorted evaluation accuracy.

[0005] The above problems lead to the fact that the reservoir fracturability evaluation and fracturing process optimization based thereon do not distinguish the differences between deep rock fracturing behavior and shallow rock fracturing behavior, and cannot fundamentally determine the rock mechanics control mechanism of deep reservoir fracturability, resulting in that the deep reservoir fracturability evaluation theory and fracturing technology innovation are more dependent on engineering summary.

[0006] It can be seen that it is urgent to accurately obtain the rock fracture propagation resistance in the high temperature and high stress environment of the reservoir geology in situ during the focus fracturing reconstruction, to quantitatively characterize the influence degree of the in-situ geological conditions at different depths and the fracture propagation characteristics on the fracturing response of the reservoir, to propose a rock gas reservoir fracturability evaluation strategy adapting to the deep in-situ environment, and to provide important support for the development of deep oil and gas resources. SUMMARY

[0007] In view of the above problems in the prior art, the deep ultra-low permeability rock reservoir fracturability evaluation method provided by the present application solves the problems that the control factors of reservoir fracturability are not quantitatively analyzed in the prior art, and the influence of fracture toughness on reservoir fracturability is not considered, thereby affecting the accuracy of the reservoir fracturability evaluation result.

[0008] In order to achieve the above-mentioned purposes, the technical scheme adopted by the present application is as follows: a deep ultra-low permeability rock reservoir fracturability evaluation method, comprising the following steps:

[0009] S100, analyzing the relationship between the main control factors of reservoir fracturability and the reservoir fracturing difficulty, and determining a relationship formula for characterizing the influence degree of the main control factors on the reservoir fracturability; the main control factors include in-situ stress, elastic modulus, Poisson's ratio, fracture toughness and natural fracture;

[0010] S200, quantitatively characterizing the influence degree of the main control factors on the reservoir fracturability based on the relationship formula of the influence degree of each main control factor on the reservoir fracturability;

[0011] S300, evaluating the reservoir fracturability by using the analytic hierarchy process based on the influence degree of different main control factors.

[0012] Further, in the step S100, the relationship formula for characterizing the influence degree of the in-situ stress on the reservoir fracturability is as follows:

[0013]

[0014] In the formula, p denotes the fluid pressure in the fracturing fracture, K denotes the stress intensity factor at the tip of the fracture, and a denotes the length of the penny-shaped fracture.

[0015] Further, in the step S100, the relationship formula for characterizing the influence degree of the elastic modulus and Poisson's ratio on the reservoir fracturability is as follows:

[0016]

[0017]

[0018] In the formula, E denotes the elastic modulus, v denotes Poisson's ratio, and a denotes the length of the penny-shaped fracture.​ represents total fluid mass in the fracture, represents fluid pressure in the fracture, represents in-situ stress, represents penny crack length, represents wellbore radius, represents fluid density, represents distance from arbitrary point in the fracture to wellbore center point.

[0019] Further, when the fracture tip stress intensity factor KI reaches the fracture toughness KIC , the fracture initiates and propagates, and the fracture toughness KIC has a relationship with the degree of reservoir fracturability as follows:

[0020]

[0021]

[0022] wherein, represents penny crack length, represents wellbore radius, represents fluid flow rate, represents fluid density, represents elastic modulus, represents Poisson's ratio, represents time.

[0023] Further, in the step S200, based on the relationship between the degree of influence of each main control factor on reservoir fracturability, the influence of each normalized in-situ stress, elastic modulus, Poisson's ratio, and fracture toughness on injection pressure is analyzed using the control variable method; and the reservoir reconstruction volume under different natural fracture network density conditions is calculated to represent the influence of natural fractures on the degree of reservoir fracturability.

[0024] Through analysis of the influence data, it is determined that:

[0025] the elastic modulus, Poisson's ratio, and natural fractures are positive indicators of reservoir fracturability;

[0026] the in-situ stress and fracture toughness are negative indicators of reservoir fracturability.

[0027] Further, in the step S200, the degree of influence of each main control factor on reservoir fracturability is as follows:

[0028] natural fractures > fracture toughness > in-situ stress > elastic modulus > Poisson's ratio.

[0029] Further, the step S300 includes the following sub-steps: ​

[0030] S301. Construct the AHP structural hierarchy, including the inherent characteristics of the rock and the geological environment;

[0031] S302. Based on the degree of influence of each main control factor on reservoir compressibility, compare each main control factor pairwise and construct a judgment matrix for AHP structure hierarchy, including a brittleness index judgment matrix and a compressibility index judgment matrix.

[0032] Among them, the factors in the brittleness index judgment matrix include elastic modulus and Poisson's ratio, while the factors in the compressibility index judgment matrix include brittleness, fracture toughness, geostress, and natural cracks.

[0033] S303. Based on the constructed judgment matrix, determine the weight coefficients of each controlling factor;

[0034] S304. Construct a compressibility assessment model based on the weight coefficients of each main control factor.

[0035] S305. Based on the constructed compressibility assessment model, calculate the compressibility index of rock reservoirs at different burial depths to achieve reservoir compressibility evaluation.

[0036] Furthermore, in step S303, the weighting coefficient The calculation formula is:

[0037]

[0038] In the formula, Let represent the order of the judgment matrices, and let the orders of the brittleness index judgment matrix and the compressibility index judgment matrix be 2 and 4, respectively. This represents the scale by which controlling factor i contributes to the objective compared to controlling factor j.

[0039] Based on the weighting coefficients The calculation formula is used to determine the weight coefficients of each controlling factor in the fragility index judgment matrix. The weight coefficients of each controlling factor in the compressibility index judgment matrix are determined as follows: ;in, and These represent the weighting coefficients for the elastic modulus and Poisson's ratio, respectively. , , and These represent the weighting coefficients for brittleness, fracture toughness, geostress, and natural cracks, respectively.

[0040] Further, in step S304, the expression for the compressibility assessment model is:

[0041]

[0042] In the formula, represents compressibility index, represents brittleness, represents fracture toughness, represents geostress, represents natural fracture, and represents normalized elastic modulus, represents normalized Poisson's ratio.

[0043] The beneficial effects of the present application are:

[0044] (1) Accurate acquisition of fracture toughness under deep in-situ environment:

[0045] The existing compressibility evaluation method has two problems for fracture toughness, one is to regard fracture toughness as a constant independent of burial depth, and the other is to use logging parameters for fitting to obtain, resulting in low accuracy of compressibility evaluation when applied to compressibility evaluation. However, the method analyzes that fracture toughness increases significantly with increasing burial depth, and has a significant impact on reservoir compressibility. The fracture toughness used by the method is accurately determined in the laboratory under deep in-situ environment, ensuring the accuracy of the fracture toughness results used; secondly, the degree of influence of fracture toughness on rock reservoir compressibility is quantitatively calculated, and the response weight coefficient is given based on the calculation results in the compressibility evaluation strategy. Based on this, the compressibility evaluation strategy established by the method not only truly considers the nature of reservoir fracturing, but also realizes scientific and quantitative characterization of the degree of influence of fracture toughness on reservoir compressibility.

[0046] (2) Accurate identification of the controlling factor of reservoir compressibility:

[0047] The existing compressibility evaluation method is not accurate in identifying the main controlling factor, and there are problems of incomplete selection of main controlling factors or repeated calculation of similar controlling factors. The method first analyzes the compressibility research to determine that the reservoir compressibility is mainly affected by the inherent mechanical properties of rock (endogenous) and geological environment (exogenous) factors. The endogenous factors can be decomposed into rock brittleness index and fracture parameters, and the exogenous factors include natural fractures and geostress in the rock mass. By quantitatively calculating the degree of influence of elastic modulus, Poisson's ratio, fracture toughness, geostress and natural fracture on reservoir compressibility, the importance of main controlling factors is determined.

[0048] (3) Scientific distribution of weight coefficients of main controlling factors affecting compressibility

[0049] ​Because the influence degree of each main control factor on the rock reservoir compressibility is different, the weight coefficient of each main control factor in the compressibility evaluation strategy needs to be scientifically determined. In order to avoid the subjectivity of the weight of the main control factor determined by human, the weight coefficient of each factor is scientifically determined by using the analytic hierarchy process (AHP). Compared with the method of using equal weight coefficient in the existing compressibility evaluation method, the method is more scientific and meets the engineering requirements. BRIEF DESCRIPTION OF DRAWINGS

[0050] Figure 1 A flow chart of the deep ultra-low permeability rock reservoir compressibility evaluation method provided by the present application is provided.

[0051] Figure 2 The coin-shaped fracturing fracture model and parameter information provided by the present application.

[0052] Figure 3 The influence of the elastic modulus on the water injection pressure provided by the present application.

[0053] Figure 4 The influence of the Poisson's ratio on the water injection pressure provided by the present application.

[0054] Figure 5 The influence of the fracture toughness on the water injection pressure provided by the present application.

[0055] Figure 6 The influence of the ground stress on the water injection pressure provided by the present application.

[0056] Figure 7 The influence of the natural fracture density on the reservoir compressibility provided by the present application.

[0057] Figure 8 The influence degree of the main control factor on the reservoir compressibility provided by the present application.

[0058] Figure 9 The AHP hierarchical structure of the reservoir compressibility evaluation model provided by the present application. DETAILED DESCRIPTION

[0059] The specific embodiments of the present application are described below to facilitate the understanding of the present application by those skilled in the art, but it should be clear that the present application is not limited to the scope of the specific embodiments, and for those skilled in the art, it is obvious that various changes are within the spirit and scope of the present application defined and determined by the appended claims, and all the inventions utilizing the concept of the present application are within the scope of protection.

[0060] The embodiment of the present application provides a deep ultra-low permeability rock reservoir compressibility evaluation method, as shown in Figure 1 The method comprises the following steps:

[0061] S100, analyze the relationship between the main control factors of reservoir compressibility and the reservoir fracturing difficulty, and determine a relationship formula for representing the influence degree of the main control factors on the reservoir compressibility; the main control factors include the ground stress, the elastic modulus, the Poisson's ratio, the fracture toughness and the natural fracture;

[0062] S200, quantitatively represent the influence degree of the main control factors on the reservoir compressibility based on the relationship formula of the influence degree of each main control factor on the reservoir compressibility;

[0063] S300, evaluate the reservoir compressibility by using the analytic hierarchy process based on the influence degrees of different main control factors.

[0064] When the existing reservoir compressibility evaluation model evaluates the influence of the main control factors on the reservoir compressibility, it is usually determined based on subjective judgment, which obviously contradicts the fact that the influence degrees of the main control factors on the reservoir compressibility are different. The present application realizes the quantitative representation of the influence degrees of the main control factors on the reservoir compressibility by calculation, and then realizes the accurate evaluation of the reservoir compressibility.

[0065] In the present application, the high-pressure fluid injected in the hydraulic fracturing engineering forms an artificial fracturing fracture by fracturing the rock formation through the horizontal wellbore, so as to open the micro-lithologic trap oil and gas reservoir and obtain an ideal oil and gas drainage area. The fracturing fracture forms a coin-shaped fracture extending perpendicularly to the horizontal wellbore around the wellbore, as shown in FIG. 1. Figure 2 For this reason, the classical coin-shaped fracture theoretical model cannot directly represent the ground stress , the elastic modulus E, the Poisson's ratio v and the fracture toughness K IC on the reservoir fracturing difficulty (represented by the injected pressure p(x, t)). Therefore, in order to determine the influence of each main control factor on the reservoir fracturing difficulty by accurate theoretical calculation, it is necessary to first establish the theoretical relationship between the ground stress , the elastic modulus E, the Poisson's ratio v and the fracture toughness K IC and the fracturing difficulty. Before establishing the above relationship, the following assumptions are made: (1) the rock is a homogeneous isotropic material; (2) the fracturing fluid has a viscous flow behavior, and the fracture expansion speed is small enough, so that the fluid flow at any position in the fracture is uniform laminar flow; (3) for the purpose of simplifying the solution, the rock is regarded as impermeable material in the solution.

[0066] Based on this, in step S100 of the embodiment of the present application, the process of analyzing the relationship between the ground stress and the reservoir fracturing difficulty is as follows:

[0067] According to the elastic theory, the fluid pressure p(x, t) in the fracture and the stress intensity factor K I at the tip of the fracture have the following relationship:

[0068]

[0069] In the formula, R is the penny-shaped crack length, R0 is the wellbore radius, and r is the distance from an arbitrary point in the crack to the center point of the wellbore. The area wetted by the fracturing fluid is referred to as the fluid wetted zone; in this embodiment, it is assumed that the fluid wetted zone has a constant fluid pressure p(x,t) of , that is, p(x,t)= , and that the fracturing fluid lag zone has p(x,t)=0, so that:

[0070]

[0071]

[0072] In this embodiment, the fluid pressure in the fracturing crack is mainly considered to change under different rock material parameters and geostress conditions, so that and are obtained, and the relationship formula for characterizing the influence degree of geostress on the fracturing difficulty of the reservoir is:

[0073]

[0074] In the formula, represents the fluid pressure in the fracturing crack, represents the stress intensity factor at the crack tip, and represents the penny-shaped crack length.

[0075] In step S100 of the embodiment of the present application, the process of analyzing the relationship between the elastic modulus and Poisson's ratio and the fracturing difficulty of the reservoir is as follows:

[0076] It is assumed that the rock material is linearly elastic, and the fracturing crack opening degree w and the injected fluid pressure p(r,t) have the following relationship:

[0077]

[0078] Let , so that , , wherein

[0079]

[0080] When , there is:

[0081]

[0082] When , there is:

[0083]

[0084] Therefore, the expression of the fracture opening w is written as:

[0085]

[0086] According to the integral relationship,

[0087]

[0088] Thus, we have:

[0089]

[0090]

[0091]

[0092]

[0093]

[0094] Substituting the above integral results into w, we have:

[0095]

[0096] When R1 approaches R, the above formula has:

[0097]

[0098] Let i.e. , then:

[0099]

[0100] When approaches 0, the above formula is approximately:

[0101]

[0102] Finally, the relationship between the fracture opening w and the fluid pressure P in the fracture is:

[0103]

[0104] The total fluid mass in the fracture:

[0105]

[0106] where, is the fluid density, and the density of clear water is taken in the calculation process.

[0107] By combining the above formula, the elastic modulus and the Poisson's ratio​ The relationship between the degree of influence on reservoir compressibility is as follows:

[0108]

[0109]

[0110] In the formula, This represents the total mass of fluid within the fracturing fracture. This indicates the fluid pressure within the fracturing fracture. Indicates geostress, Indicates the length of the coin-shaped crack. represents the wellbore radius, Indicates fluid density, This represents the distance from any point within the fracture to the center point of the wellbore.

[0111] In step S100 of this embodiment of the invention, the process of analyzing the relationship between fracture toughness and reservoir fracturing difficulty is as follows:

[0112] When the stress intensity factor at the crack tip Achieving fracture toughness At this point, the crack begins to initiate and propagate. Based on the above analysis, when the fluid flow rate is q0:

[0113]

[0114] The fracture toughness was obtained The relationship between the degree of influence on reservoir compressibility is as follows:

[0115]

[0116]

[0117] In the formula, Indicates the length of the coin-shaped crack. represents the wellbore radius, Indicates the fluid flow rate. Indicates the stress intensity factor at the crack tip. Indicates fluid density, Indicates the elastic modulus. Represents Poisson's ratio. Indicates the time.

[0118] In step S200 of this embodiment of the invention, under the same conditions of other construction parameters, the magnitude of the injection pressure p (i.e., fracturing pressure) corresponding to the fracturing of reservoir rock reflects the degree of fracturing difficulty; in engineering, the smaller the required fracturing pressure, the easier the reservoir is to fracture.

[0119] Based on this, this embodiment uses the control variable method to analyze the influence of normalized in-situ stress, elastic modulus, Poisson's ratio, and fracture toughness on water injection pressure, based on the relationship between the influence of each main control factor on the reservoir compressibility. The reservoir stimulation volume under different natural fracture network densities is calculated to characterize the influence of natural fractures on the reservoir compressibility.

[0120] Analysis of the influencing data revealed that: elastic modulus, Poisson's ratio, and natural fractures are positive indicators of reservoir compressibility; while in-situ stress and fracture toughness are negative indicators of reservoir compressibility.

[0121] In this embodiment, by analyzing the impact data obtained from the above process, the degree of influence of the main controlling factors on reservoir compressibility is obtained as follows: natural fractures > fracture toughness > geostress > elastic modulus > Poisson's ratio.

[0122] In a specific example of this invention, taking a shale gas well in the Dingshan area as the target layer, the quantitative characterization process of the above-mentioned main controlling factors is further described. The reservoir of this well is a shale gas layer located at a burial depth of 0-3800m, with E corresponding to a depth of approximately 20-100 GPa; v approximately 0.1-0.35; K IC Approximately ; Approximately 0-90 MPa.

[0123] In this embodiment, the process of quantitatively characterizing the influence of elastic modulus and Poisson's ratio on compressibility is as follows:

[0124] like Figure 3 and Figure 4 The figure shows the calculated results of the influence of different elastic models E and Poisson's ratio ν on the injection pressure p, with other parameters remaining constant. To avoid the influence of dimensions and orders of magnitude on the interpretation of the results, the normalized value of the elastic modulus, E, is used to compare the calculated results. ND All other parameters are handled in the same way, and will not be elaborated further below. Due to the assumptions of the theoretical solution... The calculation conditions are satisfied only when the value approaches 0. However, at the beginning of water injection, the fracture length R is extremely small, failing to meet the assumptions, thus leading to abnormal calculation results. As the fractures expand, the assumptions are satisfied, and the injection pressure tends to converge. Under constant water injection duration, as the elastic modulus and Poisson's ratio of the reservoir rock gradually increase, the injection pressure p shows a decreasing trend, but the rate of decrease gradually diminishes. This phenomenon indicates that the larger the elastic modulus and Poisson's ratio, the easier the reservoir is to fracture. However, it is worth noting that when the elastic modulus increases to a certain extent, the increasing trend in the ease of rock fracture encounters a boundary effect, i.e., the rate of increase tends to level off. Therefore, an increase in the elastic modulus and Poisson's ratio indicates stronger reservoir compressibility; thus, these two are positive indicators of reservoir compressibility.

[0125] In the present embodiment, the process of quantitatively characterizing the degree of influence of fracture toughness on the fracturability is as follows:

[0126] The fracture toughness of reservoir rock represents the difficulty of fracture initiation and propagation of the fracturing fracture. Smaller fracture toughness reflects that the fracture in the rock is more easily extended after being driven by fluid pressure, and then better communicates with the natural fracture, thereby improving the complexity of the fracture network and making the formation have higher fracturability. Figure 5 For the same other conditions, different fracture toughness K IC The calculation result of the influence of the injection pressure p. With the increase of the fracture toughness, the injection pressure increases significantly, and the increasing rate has a slight increasing trend. That is, the increase of the fracture toughness rapidly increases the difficulty of reservoir fracturing, which is a negative index of the fracturability of the reservoir. This calculation result shows that the fracturability index without considering the fracture characteristics of the reservoir rock under deep in-situ environment may have a risk of serious underestimation of the difficulty of fracturability. In addition, since the fracture toughness has a great influence on the difficulty of reservoir fracturing, inaccurate measurement or calculation of the fracture toughness will also lead to the deviation of the final calculation result of the fracturability index from the engineering practice.

[0127] In the present embodiment, the process of quantitatively characterizing the degree of influence of in-situ stress on the fracturability is as follows:

[0128] Figure 6 For the same other conditions, different in-situ stresses The calculation result of the influence of the injection pressure p. The greater the in-situ stress, the greater the resistance to the formation of fractures in the reservoir rock, and therefore the greater the injection pressure required to form fractures in the rock, which is a negative index of the fracturability of the reservoir.

[0129] In the present embodiment, the process of quantitatively characterizing the degree of influence of natural fractures on the fracturability is as follows:

[0130] The 3DEC numerical simulation software is used to calculate the full coupling between the fracturing fracture and the natural fracture, to analyze the flow of fracturing fluid in the fracture and the connectivity between the fractures, so as to reflect the influence of the natural discrete fracture network (DFN) on the reservoir reconstruction volume (SRV) by hydraulic fracturing. By calculating the reservoir reconstruction volume under different natural fracture network density conditions, the influence of the natural fracture on the fracturing degree of the reservoir is characterized, as shown in Figure 7 The reservoir reconstruction volume SRV increases slowly with time, and the overall influence of the time step is not obvious. But SRV increases linearly with the normalized DFN density, indicating that the difficulty of reservoir reconstruction increases rapidly with the increase of the natural fracture density, which is a positive index of the fracturability of the reservoir.

[0131] In the present embodiment, the process of quantitatively characterizing the degree of influence of the fracture toughness on the fracturability is as follows: Figures 3-7The influence degree of each main control factor on the reservoir compressibility is summarized to integrally evaluate the relative size of the influence of each main control factor on the compressibility.

[0132] Because the theoretical solution assumes that The fracture length R is extremely small at the beginning of water injection, and the assumption condition is not satisfied, so the calculation result is abnormal. With the expansion of the fracture, the assumption condition is satisfied, and the injection pressure tends to converge. Take Figures 3-7 The calculation result approaches convergence, such as the pressure as the last result of each calculation, that is, the injection pressure p when the dimensionless time approaches 1 is the last calculation result, according to the formula (X-X min ) / (X max -X min ) for the elastic modulus, Poisson's ratio, ground stress, fracture toughness and fracture density, and take them as the horizontal coordinates; according to the formula (X-X min ) / (X max -X min ) for the injection pressure p, take it as the vertical coordinate, and draw as Figure 8 shown.

[0133] The calculation result shows that the increase of the elastic modulus reduces the injection pressure by about 1.5 times, the increase of Poisson's ratio reduces the injection pressure by about 1.07 times, the increase of the fracture toughness increases the injection pressure by about 7 times, the increase of the ground stress increases the injection pressure by about 6 times, and the increase of the natural fracture density increases the reservoir reconstruction volume by about 8.8 times. In summary, the elastic modulus, Poisson's ratio and natural fracture are positive indicators of the reservoir compressibility, the fracture toughness and the ground stress are negative indicators of the reservoir compressibility, and the influence degree of each main control factor on the reservoir compressibility is in turn: natural fracture>fracture toughness>ground stress>elastic modulus>Poisson's ratio.

[0134] In step S300 of the embodiment of the present application, the analytic hierarchy process is used to evaluate the reservoir compressibility. The analytic hierarchy process (AHP) can efficiently and conveniently determine the weight value of multiple factors in the comprehensive evaluation problem. In combination with the results of the influence of the main control factors on the compressibility calculated in the foregoing, the AHP is used to quantitatively evaluate the weight coefficients of each main control factor in the compressibility index, so as to establish a compressibility evaluation model based on the comprehensive coefficient method.

[0135] Specifically, step S300 in the embodiment includes the following sub-steps:

[0136] S301, construct an AHP structure level, including rock inherent characteristics and geological environment;

[0137] S302. Based on the degree of influence of each main control factor on reservoir compressibility, compare each main control factor pairwise and construct a judgment matrix for AHP structure hierarchy, including a brittleness index judgment matrix and a compressibility index judgment matrix.

[0138] Among them, the factors in the brittleness index judgment matrix include elastic modulus and Poisson's ratio, while the factors in the compressibility index judgment matrix include brittleness, fracture toughness, geostress, and natural cracks.

[0139] S303. Based on the constructed judgment matrix, determine the weight coefficients of each controlling factor;

[0140] S304. Construct a compressibility assessment model based on the weight coefficients of each main control factor.

[0141] S305. Based on the constructed compressibility assessment model, calculate the compressibility index of rock reservoirs at different burial depths to achieve reservoir compressibility evaluation.

[0142] In step S301 of this embodiment, regarding reservoir compressibility evaluation, the core objective is to establish a rock reservoir compressibility evaluation strategy adapted to deep-seated characteristics. This objective includes two criteria: inherent rock characteristics and geological environment. The inherent rock characteristics are constrained by two influencing factors: brittleness (characterized by elastic modulus and Poisson's ratio) and fracture toughness. The geological environment is constrained by factors such as in-situ stress and natural fractures. A strategy is constructed such as... Figure 9 The aforementioned AHP hierarchical structure.

[0143] In step S302 of this embodiment, the results of the influence of various controlling factors on compressibility (injection pressure p and reservoir stimulation volume SRV) calculated by the previous numerical quantitative analysis of coin-shaped fractures, elastic modulus, Poisson's ratio, in-situ stress, fracture toughness, and natural fracture density are obtained (see details). Figure 8 Each factor is compared pairwise to establish a judgment matrix, as shown in Tables 1 and 2. This represents the scale by which controlling factor i contributes to the objective compared to controlling factor j.

[0144] Table 1: Brittleness Index Judgment Matrix

[0145]

[0146] Table 2: Compressibility Index Judgment Matrix

[0147]

[0148] In step S303 of this embodiment, the weight coefficient of each factor can be determined by the ranking equation of the asymptotically normalized coefficient (ANC) to obtain the weight coefficient. The calculation formula is:

[0149]

[0150] In the formula, The order of the brittleness index judgment matrix and the crushability index judgment matrix is 2 and 4 respectively, represents the scale of the contribution of the main control factor i to the target compared with the main control factor j.

[0151] Based on the calculation formula of the weight coefficient , the weight coefficient of each main control factor in the brittleness index judgment matrix is determined as , and the weight coefficient of each main control factor in the crushability index judgment matrix is determined as ; wherein, and represent the weight coefficients of the elastic modulus and the Poisson's ratio respectively, , , and represent the weight coefficients of brittleness, fracture toughness, ground stress and natural fracture respectively.

[0152] In step S304 of the embodiment, the expression of the crushability evaluation model is:

[0153]

[0154] In the formula, represents the crushability index, represents brittleness, represents fracture toughness, represents ground stress, represents natural fracture, and , represents the normalized elastic modulus, represents the normalized Poisson's ratio.

[0155] In one specific example of the embodiment, based on the case data of the influence of the main control factors on the reservoir crushability shown in Figure 8 , the above weight coefficients are 0.25, 0.75, 0.047, 0.369, 0.214 and 0.370 respectively.

[0156] In step S305 of the embodiment, the crushability index of the rock reservoir at different burial depths is calculated and sorted according to the above formula, so as to determine the high-quality rock reservoir for oil and gas exploitation, and to realize the evaluation of the crushability of the reservoir at different burial depths.

[0157] The principles and implementation manners of the present application are described by using specific examples in the present application, and the above examples are only used for helping to understand the method of the present application and its core idea; meanwhile, for the ordinary skilled in the art, according to the idea of the present application, the specific implementation manners and application ranges can be changed, and the above description should not be understood as the limitation of the present application.

[0158] Those skilled in the art will understand that the examples described herein are for the purpose of understanding the principles of the present application and should be understood as not limiting the scope of protection of the present application. Those skilled in the art can make various other specific modifications and combinations according to the technical inspiration disclosed in the present application without departing from the essence of the present application, and these modifications and combinations are still within the scope of protection of the present application.

Claims

1. A method for evaluating the compressibility of deep ultra-low permeability rock reservoirs, characterized in that, Includes the following steps: S100. Analyze the relationship between the main controlling factors of reservoir compressibility and the difficulty of reservoir fracturing, and determine the relationship formula that characterizes the degree of influence of the main controlling factors on reservoir compressibility; the main controlling factors include in-situ stress, elastic modulus, Poisson's ratio, fracture toughness and natural fractures. S200. Based on the relationship between the influence of each main control factor on reservoir compressibility, the influence of the main control factor on reservoir compressibility is quantitatively characterized. S300. Based on the degree of influence of different controlling factors, the analytic hierarchy process (AHP) is used to evaluate reservoir compressibility. In step S100, the characterization of geostress The relationship between the degree of influence on reservoir compressibility is as follows: In the formula, This indicates the fluid pressure within the fracturing fracture. Indicates the stress intensity factor at the crack tip. Indicates the length of the coin-shaped crack; In step S100, the elastic modulus Compared to Poisson The relationship between the degree of influence on reservoir compressibility is as follows: In the formula, This indicates the total mass of fluid within the fracturing fracture. This indicates the fluid pressure within the fracturing fracture. Indicates geostress, Indicates the length of the coin-shaped crack. represents the wellbore radius, Indicates fluid density, This represents the distance from any point within the crack to the center point of the wellbore. When the stress intensity factor at the crack tip Achieving fracture toughness At that time, the crack begins to initiate and propagate, thus obtaining the fracture toughness. The relationship between the degree of influence on reservoir compressibility is as follows: In the formula, Indicates the length of the coin-shaped crack. represents the wellbore radius, Indicates the fluid flow rate. Indicates fluid density, Indicates the elastic modulus. Represents Poisson's ratio. Indicates time; Step S300 includes the following sub-steps: S301. Construct the AHP structural hierarchy, including the inherent characteristics of the rock and the geological environment; S302. Based on the degree of influence of each main control factor on reservoir compressibility, compare each main control factor pairwise and construct a judgment matrix for AHP structure hierarchy, including a brittleness index judgment matrix and a compressibility index judgment matrix. Among them, the factors in the brittleness index judgment matrix include elastic modulus and Poisson's ratio, while the factors in the compressibility index judgment matrix include brittleness, fracture toughness, geostress, and natural cracks. S303. Based on the constructed judgment matrix, determine the weight coefficients of each controlling factor; S304. Construct a compressibility assessment model based on the weight coefficients of each main control factor. S305. Based on the constructed compressibility assessment model, calculate the compressibility index of rock reservoirs at different burial depths to achieve reservoir compressibility evaluation.

2. The method for evaluating the compressibility of deep ultra-low permeability rock reservoirs according to claim 1, characterized in that, In step S200, based on the relationship between the influence of each main control factor on reservoir compressibility, the control variable method is used to analyze the influence of normalized in-situ stress, elastic modulus, Poisson's ratio, and fracture toughness on water injection pressure; the reservoir stimulation volume under different natural fracture network densities is calculated to characterize the influence of natural fractures on the reservoir's compressibility. By analyzing the impact data, it can be determined that: The elastic modulus, Poisson's ratio, and natural fractures are positive indicators of reservoir compressibility. The in-situ stress and fracture toughness are negative indicators of reservoir compressibility.

3. The method for evaluating the compressibility of deep ultra-low permeability rock reservoirs according to claim 1, characterized in that, In step S200, the degree of influence of the main controlling factor on reservoir compressibility is as follows: Natural cracks > fracture toughness > geostress > elastic modulus > Poisson's ratio.

4. The method for evaluating the compressibility of deep ultra-low permeability rock energy storage according to claim 1, characterized in that, In step S303, the weighting coefficient The calculation formula is: In the formula, Let represent the order of the judgment matrices, and let the orders of the brittleness index judgment matrix and the compressibility index judgment matrix be 2 and 4, respectively. This represents the scale indicating the contribution of controlling factor i to the objective compared to controlling factor j; Based on the weighting coefficients The calculation formula is used to determine the weight coefficients of each controlling factor in the fragility index judgment matrix. The weight coefficients of each controlling factor in the compressibility index judgment matrix are determined as follows: ;in, and These represent the weighting coefficients for the elastic modulus and Poisson's ratio, respectively. , , and These represent the weighting coefficients for brittleness, fracture toughness, geostress, and natural cracks, respectively.

5. The method for evaluating the compressibility of deep ultra-low permeability rock energy storage according to claim 1, characterized in that, In step S304, the expression for the compressibility assessment model is: In the formula, Indicates the compressibility index. Indicates brittleness. Indicates fracture toughness. Indicates geostress, Indicates a natural crack, and , This represents the normalized elastic modulus. This represents the normalized Poisson's ratio.

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