A comprehensive compressibility assessment method for thin interbedded tight reservoirs

Abstract

The present invention discloses a comprehensive compressibility evaluation method for thin interlayer tight reservoirs, and its technical scheme includes the following steps: using logging data to identify core production layers, determine the number of thin layers and interlayers, and analyze and calculate the parameters of thin layers and interlayers; using logging data to establish a three-dimensional geological model, and establish a thin interlayer tight reservoir fracture expansion model; performing hydraulic fracturing simulation of thin interlayer tight reservoirs, with the dimensionless total fracture length as the output result; using the control variable method to change the input parameters, and obtaining a relationship diagram of each input parameter to the total fracture length; since each evaluation parameter has its own unit and dimension, the parameters need to be standardized before participating in the calculation; using the hierarchical analysis method to calculate the influence weight of each coefficient, and finally establishing a compressibility model. The beneficial effect is that the present invention overcomes the problem that the existing model cannot fully consider the influence of parameters such as interlayer rock mechanics and stress on fracture height expansion, and the relevant parameter indicators are easy to obtain, with high operability and accuracy.

Description

Technical Field

[0001] The invention relates to the field of tight oil and gas development, and in particular to a comprehensive compressibility evaluation method for thin interbedded tight reservoirs. Background Art

[0002] Tight reservoirs are widely distributed around the world, but due to the existence of multiple layers in the upper vertical section, thin reservoir thickness, strong heterogeneity, and the "many, scattered, and complex" problems of thin interlayer reservoirs, the effect of thin interlayer fracturing development is relatively limited, and it is difficult to achieve the mobilization of all production profiles in one construction. At present, for thin interlayer fracturing, the main methods formed on site are general fracturing, staged fracturing, and bridge plug staged fracturing, but the specific construction process on site faces problems such as weak natural energy, low production, and rapid decline, so the economic benefits are not very ideal.

[0003] Although thin interlayer reservoirs have the characteristics of low porosity and low permeability, there are usually differences in physical and mechanical properties between thin layers and interlayers. Therefore, the division of layers can be achieved through detailed logging interpretation, and similar layers can achieve better development results by using similar fracturing methods. However, the current division of layers in the development of thin interlayer reservoirs is mainly based on reservoir properties such as oil layer physics, ignoring the influence of the mechanical and stress properties of the thin interlayer itself on fracturing transformation. In order to effectively improve the efficiency of hydraulic fracturing in thin interlayer reservoirs and reduce the blindness of hydraulic fracturing construction, the formation of a fracturing process method that matches the reservoir has always been one of the research hotspots in the fracturing transformation of thin interlayers.

[0004] During the thin interlayer hydraulic fracturing process, due to the rock mechanics and stress constraints between the reservoir layers, the expansion of the fracture height is always extended. As the interlayer stress difference increases, the expansion and extension of hydraulic fracturing will be greatly restricted. At this time, the expansion of the hydraulic fracture height may be extremely limited, and the effective penetration of the fracture cannot be achieved. The number of production layers inside the entire thin interlayer is small, so the final fracture expansion volume is limited, resulting in unstable overall production. Therefore, it is extremely important to select the appropriate perforation layer for thin interlayer fracturing to achieve effective production of more production layers.

[0005] From the perspective of engineering compressibility, reservoir brittleness is one of the key parameters affecting the fracturing of thin interlayer reservoirs, among which Young's modulus and Poisson's ratio are the key parameters affecting the brittleness. The higher the Young's modulus of the rock and the lower the Poisson's ratio, the stronger the rock brittleness. In addition, the fracture toughness of the rock will also affect the expansion and extension of the cracks, which can characterize the ability of the rock cracks to expand forward. Generally, the smaller the fracture toughness parameter, the more conducive it is to the continuous expansion of the rock, and ultimately help to form a more complex fracture network.

[0006] Reservoir compressibility is extremely necessary for optimizing the fracturing layer of tight reservoirs and enhancing the pertinence and effectiveness of fracturing development. In the past, the compressibility evaluation model mainly modeled and evaluated the reservoir compressibility from the perspectives of brittleness index, fracture toughness parameter, and intralayer stress parameter. Among them, brittleness index and fracture toughness are generally positive indicators, and intralayer stress parameter is generally a negative indicator. However, most of the compressibility evaluation models are mainly for thicker reservoirs, and less consideration is given to the differences in mechanical and stress properties of thin interlayers. The mechanical and stress differences between layers will have a greater impact on the compressibility of thin interlayers. In addition, the same extreme difference processing method is used when processing each parameter, and it cannot effectively guide the optimization of thin interlayer fracturing layers on site.

[0007] In summary, there are three urgent problems that need to be solved in the modeling and evaluation of the compressibility of thin interlayer tight reservoirs: (1) It is necessary to take into account the mechanical and stress differences between the reservoir and interlayer in the entire reservoir compressibility evaluation model; (2) The extension of fracture height is a key indicator factor affecting the overall compressibility evaluation effect, and the interlayer stress factor is a key indicator affecting the extension of fracture height; (3) How to normalize the evaluation indicators in the comprehensive compressibility model of thin interlayers and scientifically determine the relevant weights is of great significance to the final compressibility results. Summary of the invention

[0008] The purpose of the present invention is to address the above-mentioned defects of the prior art and provide a comprehensive compressibility evaluation method for thin interbedded tight reservoirs, so as to overcome the problem that the existing model cannot fully consider the influence of parameters such as interlayer rock mechanics and stress on fracture height expansion. At the same time, the relevant parameter indicators are easy to obtain, with high operability and accuracy, and provide a new decision-making method for the optimization of fracturing layer selection in thin interbedded tight reservoirs.

[0009] The present invention provides a comprehensive compressibility assessment method for thin interbedded dense reservoirs, and the technical solution thereof is as follows:

[0010] Step 1: Use logging data to identify the core pay zone, determine the number of thin layers and interlayer units, analyze and calculate the parameters of thin layers and interlayers. The calculated parameters are cumulative thickness, brittleness index, fracture toughness, intralayer vertical stress, and interlayer vertical stress.

[0011] (1) Identify the core production layer based on the logging data, determine the number of thin layers in the core production layer, and determine the cumulative thickness of thin layers and interlayers in the fracturing stimulation interval M b and M z ;

[0012] (2) Calculate the brittleness index of thin layers and interlayers using logging data B band B z ;

[0013] (3) Calculation of fracture toughness index of thin layers and interlayers using logging data K b and K z ;

[0014] (4) Calculate the vertical stress profile of the entire thin layer and interlayer using logging data and ;

[0015] (5) Calculate the vertical stress profile of the entire thin layer and interlayer using logging data and ;

[0016] Step 2: Use the logging data to establish a 3D geological model, and use the five sets of parameters calculated in step 1 as the core data;

[0017] Step 3: Import the 3D geological model into commercial software to establish a fracture propagation model for thin interbedded tight reservoirs;

[0018] Step 4: Conduct hydraulic fracturing simulation of thin interbedded tight reservoirs, and use the dimensionless total fracture length as the output result;

[0019] Step 5: Using the control variable method to change 5 groups of input parameters, a relationship diagram of each input parameter versus the total length of the crack is obtained;

[0020] Step 6: Since each evaluation parameter has its own unit and dimension, the parameters need to be standardized before participating in the calculation; the geomechanical parameters are dimensionless; the evaluation parameters are divided into two indicators, positive and reverse. In order to facilitate calculation, the range change is used to unify the parameters, and the influence amplitude of each parameter is calculated;

[0021] Step 7: The influence weights of each coefficient are calculated using the hierarchical analysis method. Based on this, the overall reservoir is evaluated. The degree of compressibility is determined by the cumulative thickness, brittleness index, fracture toughness, interlayer vertical stress and intralayer vertical stress. The final compressibility model is:

[0022] .

[0023] Preferably, in step 2 of step 1: the brittleness index is mainly calculated comprehensively by the Young's modulus and Poisson's ratio calculated by well logging:

[0024] (1)

[0025] (2)

[0026] Where: E d is the dynamic elastic modulus, MPa; μ d is the dynamic Poisson’s ratio, dimensionless; is the formation density, g / cm 3 ; V p is the longitudinal wave velocity, km / s; V s is the shear wave velocity, km / s;

[0027] Among them, the thin layer brittleness index parameter B b The calculation formula is:

[0028] (3)

[0029] (4)

[0030] (5)

[0031] Among them, the interlayer brittleness index parameter B z The calculation formula is:

[0032] (6)

[0033] (7)

[0034] (8)

[0035] In the formula, E bmax , E bmin is the elastic modulus of all standard cylindrical cores E The maximum and minimum element values ​​in the vector. v bmax , v bmin is the Poisson's ratio of all standard cylindrical cores v Constructs the maximum and minimum element values ​​in a vector.

[0036] Preferably, in step 3 of step 1: the calculation of the fracture toughness index is: in the fracturing process of thin interlayer tight reservoirs, the fracture types are mainly type I and type II. Type I and type II fracture toughness experiments are the key means to provide fracture toughness values. The existing calculation formulas for type I and type II fracture toughness are mainly:

[0037] (9)

[0038] (10)

[0039] (11)

[0040] (12)

[0041] (13)

[0042] (14)

[0043] In the formula, K bImax , K bImin is the maximum and minimum Mode I fracture toughness value of the regional thin layer; K bⅡmax , K bⅡmin is the maximum and minimum value of mode II fracture toughness of the regional thin layer; K zImax , K zImin is the maximum and minimum Mode I fracture toughness value of the regional interlayer; K zⅡmax , K zⅡmin are the maximum and minimum mode II fracture toughness values ​​of the regional interlayer.

[0044] Preferably, in step 4 of step 1: vertical stress profile within the layer and ; The maximum and minimum horizontal ground stresses are calculated using the porous elastic model, and the calculation formula is as follows:

[0045] (15)

[0046] (16)

[0047] (17)

[0048] (18)

[0049] Among them, the calculation formula of the biaxial stress difference coefficient in the thin layer is as follows:

[0050] (19)

[0051] Among them, the calculation formula of the difference coefficient of the two-phase stress in the interlayer is as follows:

[0052] (20).

[0053] Preferably, in step 5 of step 1: interlaminar vertical stress and When calculating the interlayer vertical stress, it is necessary to add numerical simulation technology or field monitoring technology to determine the penetration threshold. The interlayer vertical stresses of thin layers and interlayers are:

[0054] Among them, the calculation formula of the biaxial stress difference coefficient in the thin layer is as follows:

[0055] (twenty one)

[0056] The calculation formula of the biaxial stress difference coefficient within the interlayer is as follows:

[0057] (twenty two).

[0058] Preferably, in step 2, a three-dimensional geological model is established using the logging data, and the five groups of parameters calculated in step 1 are used as core data. The steps for establishing the three-dimensional geological model are:

[0059] ① Establish a three-dimensional finite element network based on the three-dimensional geological model;

[0060] ② Using seismic inversion results and one-dimensional geomechanics research results to determine three-dimensional geomechanical parameters, including Young's modulus and Poisson's ratio;

[0061] ③Determine the three-dimensional pore pressure body;

[0062] ④ Use three-dimensional geomechanics software to determine the original field stress, including the minimum horizontal principal stress, the maximum horizontal principal stress and the overlying rock pressure;

[0063] Among them, the three-dimensional rock strength parameter model:

[0064] Dynamic elastic modulus E d :

[0065] (twenty three)

[0066] Dynamic Poisson's ratio μ d :

[0067] (twenty four)

[0068] In the formula, and are the P-wave and S-wave time differences of the stratum, μs / m;

[0069] Static Poisson's ratio μ s :

[0070] (25)

[0071] Static elastic modulus E s :

[0072] (26)

[0073] In the formula, A 1 , A 2 , B 1 , B 2 is an empirical constant, dimensionless;

[0074] Sedimentary rock cohesion c :

[0075] (27)

[0076] In the formula, A is a constant, dimensionless; V cl is the mud content, %;

[0077] Internal friction angle :

[0078] (28)

[0079] Compressive strength UCS :

[0080] (29)

[0081] tensile strength ST :

[0082] (30)

[0083] In the formula, a , b , A , B , K is an empirical constant and dimensionless.

[0084] Among them, the three-dimensional ground stress model:

[0085] Underground rock mass is a complex, heterogeneous, anisotropic and nonlinear elastic-plastic material. The finite element method can well solve the stress distribution state of this heterogeneous, porous and elastic-plastic body. The influence of additional pore fluid in deep rock on the simulation results of ground stress is as follows:

[0086] Total stress :

[0087] (31)

[0088] Biot coefficient :

[0089] (32)

[0090] Effective stress :

[0091] (33)

[0092] In the formula, K T is the bulk modulus of the rock skeleton and rock pores, MPa; K S is the bulk modulus of the rock skeleton, MPa; D is the model constitutive matrix, is the strain, m;

[0093] The constructed three-dimensional pore pressure model, three-dimensional elastic modulus model, and three-dimensional elastoplastic model are assembled to construct a three-dimensional pore-elastoplastic model of the study area. The three-dimensional geostress distribution of the study area is calculated using the finite element method, and finally a three-dimensional geological model is obtained.

[0094] Preferably, in step 3: establish a thin interbedded tight reservoir fracture propagation model equation:

[0095] The rock matrix seepage-stress coupling model uses the linear poroelastic equation to establish the stress and strain relationship:

[0096] (34)

[0097] (35)

[0098] In the formula, is stress, MPa; and G is the Lame parameter of porous material; C and M is the additional elastic modulus of the two-phase medium, MPa; is the volume strain; is the strain parameter of the fluid relative to the solid volume deformation; p is the pore pressure, MPa;

[0099] Fluid flow law in fractures, tangential flow calculation formula:

[0100] (36)

[0101] In the formula, q is the volume flow rate of the unit width in the crack, m 3 / (s·m); is the crack width, m; μ is the fluid viscosity, Pa·s; is the fluid pressure gradient along the fracture direction, Pa / m;

[0102] The normal flow equation of the fluid in the fracture is:

[0103] (37)

[0104] In the formula, q t and q b are the fluid loss at the top and bottom of the fracture, m 2 / s; c t and c b are the fluid loss coefficients at the top and bottom of the fracture, respectively, m 2 / (s·Pa); p i is the surface pressure in the middle of the unit, Pa; p t and p b are the surface pressures at the top and bottom of the crack, Pa, respectively;

[0105] The fluid mass conservation equation within the unit is:

[0106] (38)

[0107] In the formula, Q ( t ) is the injection rate of fracturing fluid, m 3 / s;

[0108] Crack propagation evolution rules, unit crack initiation criteria:

[0109] (39)

[0110] In the formula, t n , t s , t t is the stress in the three loading directions ( t n is the normal stress; t s and tt are the first and second shear stresses, respectively), MPa; , and are the tensile, first and second shear strengths of the undamaged unit, MPa, respectively; Macaulay brackets indicate that pure extrusion deformation or stress state will not cause unit damage;

[0111] Evolution of unit damage:

[0112] (40)

[0113] (41)

[0114] (42)

[0115] In the formula, , and is the stress in the three loading directions when the model is assumed to be not evolving and still in the linear elastic deformation process, MPa; t n , t s and t t is the true stress in the three loading directions, MPa; D is the dimensionless damage coefficient ( D =0, the material is not damaged, D =1, material is completely damaged);

[0116] The BK criterion describes the evolution of damage during fracture propagation:

[0117] (43)

[0118] In the formula, G c is the total critical energy release rate of the cohesive unit in the mixed mode, Pa·m; is the critical energy release rate in the element normal direction, Pa·m; is the critical energy release rate in the tangential direction of the unit, Pa·m; G n , G s and G t are the energy release rates in the normal direction, the first tangential direction, and the second tangential direction of the cohesive unit, Pa·s; It is a constant related to the material itself, and its value is 2.284.

[0119] Preferably, in step 6: the parameters are unified by using the range variation, and the influence amplitude of each parameter is calculated;

[0120] For the positive indicator, take:

[0121] (44)

[0122] For negative indicators, take:

[0123] (45)

[0124] In the formula, S is the standard parameter; X is the parameter value; X max is the maximum value of the parameter; X min is the minimum value of the parameter; after the range transformation, the index values ​​are all 0~1, the positive and reverse indicators are averaged into positive indicators, the best value is 1, and the worst value is 0;

[0125] Based on the geomechanical parameters after range transformation and the dimensionless total length of the hydraulic fractures, the normalized geomechanical parameters and the dimensionless total length of the hydraulic fractures are fitted to calculate the influence amplitude of each geomechanical parameter on the dimensionless total length of the hydraulic fractures. The calculation equation is as follows:

[0126] (46)

[0127] In the formula, R ( X nor ) is the normalized geomechanical parameter X nor For crack length S f increase in L f ( i ), L f ( j ) are data points i and data points j The total length of the hydraulic fracture is L f ; X nor ( i ), X nor ( j ) are data points i and data points j The geomechanical parameters below.

[0128] Preferably, in step 7, the compressibility model finally established is:

[0129] (47)

[0130] In the formula, D is the comprehensive compressibility index of the reservoir; M , B , K , and are the influence weights of cumulative thickness, brittleness index, fracture toughness, interlayer vertical stress and intralayer vertical stress coefficient on the total compressibility coefficient. The weight coefficient is determined by comprehensively analyzing the influence amplitude of the geomechanical parameters contained in each coefficient. The sum of the weights of the five influencing factors is 1, that is: a + b + c + d + e =1, and finally a comprehensive compressibility evaluation model is established.

[0131] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0132] The present invention can evaluate the compressibility of thin interlayer reservoirs from the perspectives of formation brittleness, fracture toughness and geostress. The method can be used to guide on-site fracturing engineers to optimize and evaluate the fracturing positions of dense thin interlayers, and guide the selection of perforation positions. In addition, the present invention overcomes the problem that the existing model cannot fully consider the influence of parameters such as interlayer rock mechanics and stress on fracture height expansion. At the same time, the relevant parameter indicators are easy to obtain, have high operability and accuracy, and provide a new decision-making method for the optimization of fracturing layer selection of thin interlayer dense reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0133] Attached Figure 1 It is a schematic diagram of the thin interbedded tight reservoir in the present invention;

[0134] Attached Figure 2 This is a flow chart of a comprehensive compressibility assessment method for thin interbedded tight reservoirs according to an embodiment of the present invention;

[0135] Attached Figure 3 A schematic diagram of a thin interlayer model constructed in the present invention;

[0136] Attached Figure 4 A schematic diagram of the relationship between the brittleness index, one of the input parameters in the present invention, and the dimensionless total length of the hydraulic fracture;

[0137] Attached Figure 5 is the impact amplitude of the dimensionless total length of the hydraulic fracture in the present invention;

[0138] Attached Figure 6 This is a schematic diagram of the feature importance scoring of the present invention;

[0139] Attached Figure 7 This is a numerical simulation diagram of crack extension in the present invention.

[0140] In the above picture: core production layer 1, interlayer 2, thin layer 3. DETAILED DESCRIPTION

[0141] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0142] Example 1, a comprehensive compressibility assessment method for thin interbedded tight reservoirs mentioned in the present invention comprises the following steps:

[0143] Step 1: Use logging data to identify the core production layer 1, determine the number of thin layers 3 and interlayer units, analyze and calculate the parameters of thin layers and interlayer 2. The calculated parameters are cumulative thickness, brittleness index, fracture toughness, intralayer vertical stress, and interlayer vertical stress.

[0144] (1) Identify the core production layer 1 based on the logging data, determine the number of thin layers in the core production layer 1, and determine the cumulative thickness of thin layer 3 and interlayer 2 in the fracturing stimulation interval M b and M z ;

[0145] (2) Calculate the brittleness index of thin layers and interlayers using logging data B b and B z Considering the continuity of data, the brittleness index can be calculated comprehensively through Young's modulus and Poisson's ratio calculated by well logging.

[0146] (1)

[0147] (2)

[0148] Where: E d is the dynamic elastic modulus, MPa; μ d is the dynamic Poisson’s ratio, dimensionless; is the formation density, g / cm 3 ; V p is the longitudinal wave velocity, km / s; V s is the shear wave velocity, km / s.

[0149] Among them, the thin layer brittleness index parameter Bb The calculation formula is:

[0150] (3)

[0151] (4)

[0152] (5)

[0153] Among them, the interlayer brittleness index parameter B z The calculation formula is:

[0154] (6)

[0155] (7)

[0156] (8)

[0157] In the formula, E bmax , E bmin is the elastic modulus of all standard cylindrical cores E The maximum and minimum element values ​​in the vector. v bmax , v bmin is the Poisson's ratio of all standard cylindrical cores v Constructs the maximum and minimum element values ​​in a vector.

[0158] (3) Calculation of fracture toughness index of thin layers and interlayers using logging data K b and K z In the fracturing process of thin interlayered tight reservoirs, the fracture types are mainly type I and type II. Type I and type II fracture toughness experiments are the key means to provide fracture toughness values. However, the existing measurement methods are relatively cumbersome and require high cores. Therefore, type I and type II logging data are important methods for calculating fracture toughness. The existing calculation formulas for type I and type II fracture toughness are mainly:

[0159] (9)

[0160] (10)

[0161] (11)

[0162] (12)

[0163] (13)

[0164] (14)

[0165] In the formula, K bImax , K bImin is the maximum and minimum Mode I fracture toughness value of the regional thin layer; K bⅡmax , K bⅡmin is the maximum and minimum value of mode II fracture toughness of the regional thin layer; K zImax , K zImin is the maximum and minimum Mode I fracture toughness value of the regional interlayer; K zⅡmax , K zⅡmin are the maximum and minimum mode II fracture toughness values ​​of the regional interlayer.

[0166] (4) Calculate the vertical stress profile of the entire thin layer and interlayer using logging data and ; The maximum and minimum horizontal ground stresses are calculated using the porous elastic model, and the calculation formula is as follows:

[0167] (15)

[0168] (16)

[0169] (17)

[0170] (18)

[0171] Among them, the calculation formula of the biaxial stress difference coefficient in the thin layer is as follows:

[0172] (19)

[0173] Among them, the calculation formula of the difference coefficient of the two-phase stress in the interlayer is as follows:

[0174] (20)

[0175] (5) Using logging data to calculate the vertical profile of the in-situ stress in the entire thin layer and interlayer and It should be noted that when calculating interlayer shielding stress, it is necessary to add numerical simulation technology or field monitoring technology to determine the threshold of penetration. The vertical profiles of interlayer stress between thin layers and interlayers are:

[0176] Among them, the calculation formula of the biaxial stress difference coefficient in the thin layer is as follows:

[0177] (twenty one)

[0178] The calculation formula of the biaxial stress difference coefficient within the interlayer is as follows:

[0179] (twenty two)

[0180] Step 2: Use the logging data to build a 3D geological model, and use the five sets of parameters calculated in step 1 as the core data. The steps to build a 3D geological model are:

[0181] ① Establish a three-dimensional finite element network based on the three-dimensional geological model;

[0182] ② Using seismic inversion results and one-dimensional geomechanics research results to determine three-dimensional geomechanical parameters, including Young's modulus, Poisson's ratio, etc.;

[0183] ③Determine the three-dimensional pore pressure body;

[0184] ④ Use three-dimensional geomechanics software to determine the original field stress, including the minimum horizontal principal stress, the maximum horizontal principal stress and the overlying rock pressure.

[0185] Among them, the three-dimensional rock strength parameter model:

[0186] Dynamic elastic modulus E d :

[0187] (twenty three)

[0188] Dynamic Poisson's ratio μ d :

[0189] (twenty four)

[0190] In the formula, and They are the time differences of longitudinal and transverse waves of the strata, μs / m.

[0191] Static Poisson's ratio μ s :

[0192] (25)

[0193] Static elastic modulus E s :

[0194] (26)

[0195] In the formula, A 1 , A 2 , B 1 , B 2 is an empirical constant and dimensionless.

[0196] Sedimentary rock cohesion c :

[0197] (27)

[0198] In the formula, A is a constant, dimensionless; V cl is the mud content, %.

[0199] Internal friction angle :

[0200] (28)

[0201] Compressive strength UCS :

[0202] (29)

[0203] tensile strength ST :

[0204] (30)

[0205] In the formula, a , b , A , B , K is an empirical constant and dimensionless.

[0206] Among them, the three-dimensional ground stress model:

[0207] Underground rock mass is a complex, heterogeneous, anisotropic and nonlinear elastic-plastic material. The finite element method can well solve the stress distribution state of this heterogeneous, porous and elastic-plastic body. The influence of additional pore fluid in deep rock on the simulation results of ground stress is as follows:

[0208] Total stress :

[0209] (31)

[0210] Biot coefficient :

[0211] (32)

[0212] Effective stress :

[0213] (33)

[0214] In the formula, K T is the bulk modulus of the rock skeleton and rock pores, MPa; K S is the bulk modulus of the rock skeleton, MPa; D is the model constitutive matrix, is the strain, m.

[0215] The three-dimensional non-homogeneous material properties such as the constructed three-dimensional pore pressure model, three-dimensional elastic modulus model, and three-dimensional elastoplastic model are assembled to construct a three-dimensional pore-elastoplastic model of the study area. The three-dimensional geostress distribution of the study area is calculated using the finite element method, and finally a three-dimensional geological model is obtained.

[0216] Step 3: Import the 3D geological model into commercial software to establish a fracture propagation model for thin interbedded tight reservoirs.

[0217] Among them, the basic model equation is:

[0218] The rock matrix seepage-stress coupling model uses the linear poroelastic equation to establish the stress and strain relationship:

[0219] (34)

[0220] (35)

[0221] In the formula, is stress, MPa; and G is the Lame parameter of porous material; C and M is the additional elastic modulus of the two-phase medium, MPa; is the volume strain; is the strain parameter of the fluid relative to the solid volume deformation; p is the pore pressure, MPa.

[0222] Fluid flow law in fractures, tangential flow calculation formula:

[0223] (36)

[0224] In the formula, q is the volume flow rate of the unit width in the crack, m3 / (s·m); is the crack width, m; μ is the fluid viscosity, Pa·s; is the fluid pressure gradient along the fracture direction, Pa / m.

[0225] The normal flow equation of the fluid in the fracture is:

[0226] (37)

[0227] In the formula, q t and q b are the fluid loss at the top and bottom of the fracture, m 2 / s; c t and c b are the fluid loss coefficients at the top and bottom of the fracture, respectively, m 2 / (s·Pa); p i is the surface pressure in the middle of the unit, Pa; p t and p b are the surface pressures at the top and bottom of the crack, Pa, respectively.

[0228] The fluid mass conservation equation within the unit is:

[0229] (38)

[0230] In the formula, Q ( t ) is the injection rate of fracturing fluid, m 3 / s.

[0231] Crack propagation evolution rules, unit crack initiation criteria:

[0232] (39)

[0233] In the formula, t n , t s , t t is the stress in the three loading directions ( t n is the normal stress; t s and t t are the first and second shear stresses, respectively), MPa; , and are the tensile, first and second shear strengths of the undamaged unit, MPa, respectively; Macaulay brackets indicate that pure extrusion deformation or stress state will not cause unit damage.

[0234] Evolution of unit damage:

[0235] (40)

[0236] (41)

[0237] (42)

[0238] In the formula, , and is the stress in the three loading directions when the model is assumed to be not evolving and still in the linear elastic deformation process, MPa; t n , t s and t t is the true stress in the three loading directions, MPa; D is the dimensionless damage coefficient ( D =0, the material is not damaged, D =1, material is completely damaged).

[0239] The BK criterion describes the evolution of damage during fracture propagation:

[0240] (43)

[0241] In the formula, G c is the total critical energy release rate of the cohesive unit in the mixed mode, Pa·m; is the critical energy release rate in the element normal direction, Pa·m; is the critical energy release rate in the tangential direction of the unit, Pa·m; G n , G s and G t are the energy release rates in the normal direction, the first tangential direction, and the second tangential direction of the cohesive unit, Pa·s; It is a constant related to the material itself and is usually taken as 2.284.

[0242] Step 4: Conduct hydraulic fracturing simulation of thin interbedded tight reservoirs, and use the dimensionless total fracture length as the output result.

[0243] Step 5: Use the control variable method to change 5 groups of input parameters and obtain the relationship diagram of each input parameter to the total length of the crack.

[0244] Step 6: Since each evaluation parameter has its own unit and dimension, the parameters need to be standardized before participating in the calculation. Each geomechanical parameter is dimensionless. The evaluation parameters are divided into two indicators: positive and negative. In order to facilitate calculation, the range change is used to unify the parameters, and the influence amplitude of each parameter is calculated.

[0245] For the positive indicator, take:

[0246] (44)

[0247] For negative indicators, take:

[0248] (45)

[0249] In the formula, S is the standard parameter; X is the parameter value; X max is the maximum value of the parameter; X min is the minimum value of the parameter. After the range transformation, the index values ​​are all 0~1, the positive and reverse indicators are averaged into positive indicators, the best value is 1, and the worst value is 0.

[0250] Based on the geomechanical parameters after range transformation and the dimensionless total length of the hydraulic fractures, the normalized geomechanical parameters and the dimensionless total length of the hydraulic fractures are fitted to calculate the influence amplitude of each geomechanical parameter on the dimensionless total length of the hydraulic fractures. The calculation equation is as follows:

[0251] (46)

[0252] In the formula, R ( X nor ) is the normalized geomechanical parameter X nor For crack length S f increase in L f ( i ), L f ( j ) are data points i and data points j The total length of the hydraulic fracture is L f ; X nor ( i ), Xnor ( j ) are data points i and data points j The geomechanical parameters below.

[0253] Step 7: Use the analytic hierarchy process to calculate the influence weights of each coefficient. Based on this, the overall reservoir is evaluated, and the degree of compressibility is determined by the cumulative thickness, brittleness index, fracture toughness, interlayer vertical stress and intralayer vertical stress. The final compressibility model is:

[0254] (47)

[0255] In the formula, D is the comprehensive compressibility index of the reservoir; M , B , K , and The weights of cumulative thickness, brittleness index, fracture toughness, interlayer vertical stress and intralayer vertical stress coefficient on the total compressibility coefficient are determined by comprehensively analyzing the influence of the geomechanical parameters contained in each coefficient. The sum of the weights of the five influencing factors is 1 (i.e.: a + b + c + d + e =1), and finally a comprehensive compressibility evaluation model was established.

[0256] The above are only some preferred embodiments of the present invention. Any person skilled in the art may modify the above technical solutions or modify them into equivalent technical solutions. Therefore, the corresponding simple modifications or equivalent transformations made according to the technical solutions of the present invention shall fall within the scope of protection claimed by the present invention.

Claims

1. A comprehensive compressibility assessment method for thin interbedded tight reservoirs, characterized by: The following steps are involved: Step 1: Use logging data to identify the core pay zone, determine the number of thin layers and interlayer units, analyze and calculate the parameters of thin layers and interlayers. The calculated parameters are cumulative thickness, brittleness index, fracture toughness, intralayer vertical stress, and interlayer vertical stress. (1) Identify the core production layer based on the logging data, determine the number of thin layers in the core production layer, and determine the cumulative thickness of thin layers and interlayers in the fracturing stimulation interval M b and M z ; (2) Calculate the brittleness index of thin layers and interlayers using logging data B b and B z ; The brittleness index is calculated by combining the Young's modulus and Poisson's ratio calculated by well logging: (1) (2) Where: E d is the dynamic elastic modulus, MPa; μ d is the dynamic Poisson’s ratio, dimensionless; is the formation density, g / cm 3 ; V p is the longitudinal wave velocity, km / s; V s is the shear wave velocity, km / s; Among them, the thin layer brittleness index parameter B b The calculation formula is: (3) (4) (5) Among them, the interlayer brittleness index parameter B z The calculation formula is: (6) (7) (8) In the formula, E bmax , E bmin is the elastic modulus of all standard cylindrical cores E The maximum and minimum element values ​​in the vector. v bmax , v bmin is the Poisson's ratio of all standard cylindrical cores v The maximum and minimum element values ​​in the vector; (3) Calculation of fracture toughness index of thin layers and interlayers using logging data K b and K z ; The calculation of fracture toughness index is: in the fracturing process of thin interlayer tight reservoir, the fracture types are type I and type II. Type I and type II fracture toughness experiments are the key means to provide fracture toughness values. The existing calculation formulas for type I and type II fracture toughness are: (9) (10) (11) (12) (13) (14) In the formula, K bImax , K bImin is the maximum and minimum Mode I fracture toughness value of the regional thin layer; K bⅡmax , K bⅡmin is the maximum and minimum value of mode II fracture toughness of the regional thin layer; K zImax , K zImin is the maximum and minimum Mode I fracture toughness value of the regional interlayer; K zⅡmax , K zⅡmin is the maximum and minimum value of mode II fracture toughness of the regional interlayer; (4) Calculate the vertical stress profile of the entire thin layer and interlayer using logging data and ; Vertical stress profile within the layer and , the maximum and minimum horizontal ground stresses are calculated using the porous elastic model, and the calculation formula is as follows: (15) (16) (17) (18) Among them, the calculation formula of the biaxial stress difference coefficient in the thin layer is as follows: (19) Among them, the calculation formula of the difference coefficient of the two-phase stress in the interlayer is as follows: (20)； (5) Calculate the vertical stress profile of the entire thin layer and interlayer using logging data and ; Interlaminar vertical stress and When calculating the interlayer shielding stress, it is necessary to add numerical simulation technology or field monitoring technology to determine the threshold of penetration. The vertical stresses between thin layers and interlayers are: Among them, the calculation formula of the biaxial stress difference coefficient in the thin layer is as follows: (21) The calculation formula of the biaxial stress difference coefficient within the interlayer is as follows: (22)； Step 2: Use the logging data to establish a 3D geological model, and use the five sets of parameters calculated in step 1 as the core data; Step 3: Import the 3D geological model into commercial software to establish a fracture propagation model for thin interbedded tight reservoirs; Step 4: Conduct hydraulic fracturing simulation of thin interbedded tight reservoirs, and use the dimensionless total fracture length as the output result; Step 5: Using the control variable method to change 5 groups of input parameters, a relationship diagram of each input parameter versus the total length of the crack is obtained; Step 6: Since each evaluation parameter has its own unit and dimension, the parameters need to be standardized before participating in the calculation; the geomechanical parameters are dimensionless; the evaluation parameters are divided into two indicators, positive and reverse. In order to facilitate calculation, the range change is used to unify the parameters, and the influence amplitude of each parameter is calculated; Step 7: The influence weights of each coefficient are calculated using the hierarchical analysis method. Based on this, the overall reservoir is evaluated. The degree of compressibility is determined by the cumulative thickness, brittleness index, fracture toughness, interlayer vertical stress and intralayer vertical stress. The final compressibility model is: ； In the formula, D is the comprehensive compressibility index of the reservoir; M , B , K , and are the influence weights of cumulative thickness, brittleness index, fracture toughness, interlayer vertical stress and intralayer vertical stress coefficient on the total compressibility coefficient. The weight coefficient is determined by comprehensively analyzing the influence amplitude of the geomechanical parameters contained in each coefficient. The sum of the weights of the five influencing factors is 1, that is: a + b + c + d + e =1, and finally a comprehensive compressibility evaluation model is established.

2. The comprehensive compressibility assessment method for thin interbedded tight reservoirs according to claim 1, characterized in that: In step 2, the three-dimensional geological model is established using the logging data. The five sets of parameters calculated in step 1 are used as core data. The steps for establishing the three-dimensional geological model are as follows: ① Establish a three-dimensional finite element network based on the three-dimensional geological model; ② Using seismic inversion results and one-dimensional geomechanics research results to determine three-dimensional geomechanical parameters, including Young's modulus and Poisson's ratio; ③Determine the three-dimensional pore pressure body; ④ Use three-dimensional geomechanics software to determine the original field stress, including the minimum horizontal principal stress, the maximum horizontal principal stress and the overlying rock pressure; Among them, the three-dimensional rock strength parameter model: Dynamic elastic modulus E d : (23) Dynamic Poisson's ratio μ d : (24) In the formula, and are the P-wave and S-wave time differences of the stratum, μs / m; Static Poisson's ratio μ s : (25) Static elastic modulus E s : （26） In the formula, A 1, A 2, B 1, B 2 is an empirical constant, dimensionless; Sedimentary rock cohesion c : (27) In the formula, A is a constant, dimensionless; V cl is the mud content, %; Internal friction angle : (28) Compressive strength UCS : (29) tensile strength ST : (30) In the formula, a , b , A , B , K is an empirical constant, dimensionless; Among them, the three-dimensional ground stress model: Underground rock mass is a complex, heterogeneous, anisotropic and nonlinear elastic-plastic material. The finite element method can well solve the stress distribution state of this heterogeneous, porous and elastic-plastic body. The influence of additional pore fluid in deep rock on the simulation results of ground stress is as follows: Total stress : (31) Biot coefficient : （32） Effective stress : (33) In the formula, K T is the bulk modulus of the rock skeleton and rock pores, MPa; K S is the bulk modulus of the rock skeleton, MPa; D is the model constitutive matrix, is the strain, m; The constructed three-dimensional pore pressure model, three-dimensional elastic modulus model, and three-dimensional elastoplastic model are assembled to construct a three-dimensional pore-elastoplastic model of the study area. The three-dimensional geostress distribution of the study area is calculated using the finite element method, and finally a three-dimensional geological model is obtained.

3. The comprehensive compressibility assessment method for thin interbedded tight reservoirs according to claim 2, characterized in that: In step 3: establish the fracture propagation model equation of thin interbedded tight reservoir: The rock matrix seepage-stress coupling model uses the linear poroelastic equation to establish the stress and strain relationship: （34） （35） In the formula, is stress, MPa; and G is the Lame parameter of porous material; C and M is the additional elastic modulus of the two-phase medium, MPa; is the volume strain; is the strain parameter of the fluid relative to the solid volume deformation; p is the pore pressure, MPa; Fluid flow law in fractures, tangential flow calculation formula: (36) In the formula, q is the volume flow rate of the unit width in the crack, m 3 / (s·m); is the crack width, m; μ is the fluid viscosity, Pa·s; is the fluid pressure gradient along the fracture direction, Pa / m; The normal flow equation of the fluid in the fracture is: (37) In the formula, q t and q b are the fluid loss at the top and bottom of the fracture, m 2 / s; c t and c b are the fluid loss coefficients at the top and bottom of the fracture, respectively, m 2 / (s·Pa); p i is the surface pressure in the middle of the unit, Pa; p t and p b are the surface pressures at the top and bottom of the crack, Pa, respectively; The fluid mass conservation equation within the unit is: (38) In the formula, Q ( t ) is the injection rate of fracturing fluid, m 3 / s; Crack propagation evolution rules, unit crack initiation criteria: (39) In the formula, t n , t s , t t are the stresses in the three loading directions, t n is the normal stress; t s and t t are the first and second shear stresses, MPa, respectively; , and are the tensile, first and second shear strengths of the undamaged unit, MPa, respectively; Macaulay brackets indicate that pure extrusion deformation or stress state will not cause unit damage; Evolution of unit damage: (40) (41) (42) In the formula, , and is the stress in the three loading directions when the model is assumed to be not evolving and still in the linear elastic deformation process, MPa; t n , t s and t t is the true stress in the three loading directions, MPa; D is the dimensionless damage coefficient, D =0, the material is not damaged, D =1, the material is completely damaged; The BK criterion describes the evolution of damage during fracture propagation: (43) In the formula, G c is the total critical energy release rate of the cohesive unit in the mixed mode, Pa·m; is the critical energy release rate in the element normal direction, Pa·m; is the critical energy release rate in the tangential direction of the unit, Pa·m; G n , G s and G t are the energy release rates in the normal direction, the first tangential direction, and the second tangential direction of the cohesive unit, Pa·s; It is a constant related to the material itself, and its value is 2.

284.

4. The comprehensive compressibility assessment method for thin interbedded tight reservoirs according to claim 3 is characterized by: In step 6: the parameters are unified by using the range variation, and the impact amplitude of each parameter is calculated; For the positive indicator, take: (44) For negative indicators, take: (45) In the formula, S is the standard parameter; X is the parameter value; X max is the maximum value of the parameter; X min is the minimum value of the parameter; after the range transformation, the index values ​​are all 0~1, the positive and reverse indicators are averaged into positive indicators, the best value is 1, and the worst value is 0; Based on the geomechanical parameters after range transformation and the dimensionless total length of the hydraulic fractures, the normalized geomechanical parameters and the dimensionless total length of the hydraulic fractures are fitted to calculate the influence amplitude of each geomechanical parameter on the dimensionless total length of the hydraulic fractures. The calculation equation is as follows: (46) In the formula, R ( X nor ) is the normalized geomechanical parameter X nor For crack length S f increase in L f ( i ), L f ( j ) are data points i and data points j The total length of the hydraulic fracture is L f ; X nor ( i ), X nor ( j ) are data points i and data points j The geomechanical parameters below.

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