A method for determining fracture connectivity of carbonate reservoirs in a well area

Abstract

The present invention provides a method for determining the connectivity of fractures in a carbonate reservoir in a well area. By taking into account the types of carbonate rocks in the well area and the lithology of different types of carbonate rocks in the well area, the method can accurately determine the connectivity of fractures in the carbonate reservoir in the well area and the interlayer permeability of fractures in the carbonate reservoir in the well area, thereby overcoming the bottleneck effect of existing modeling and prediction of fractures in carbonate reservoirs in low-density well areas and providing a basis for the exploration and development of low-permeability oil and gas reservoirs.

Description

Technical Field

[0001] The invention relates to a method for determining the connectivity of carbonate reservoir fractures in a well area, belonging to the technical field of petroleum drilling. Background Art

[0002] As the exploration and development of oil and gas resources gradually shifts from the east to the west and from conventional reservoirs to unconventional reservoirs, the search for fractured oil and gas reservoirs has become a hot topic. How to predict the spatial distribution of fractures and quantitatively characterize fracture parameters is a frontier issue in petroleum geology research. Compared with other reservoirs, carbonate reservoirs are often formed in marine environments. The lateral distribution of the formations is stable, but the changes in interlayer muddy barriers and lithology often cause spatial differences in mechanical properties. The fracture distribution is highly heterogeneous due to tectonic movements, and the drilling density is relatively low compared to other types of oil and gas fields, which brings great difficulties to the development of carbonate oil and gas reservoirs.

[0003] The current prediction method of low-density fracture space based on geomechanics theory ignores the complexity of mechanical properties and stress distribution at the lithology interface, which leads to bottlenecks in fracture modeling and spatial connectivity determination of carbonate reservoirs in low-density well areas. Summary of the invention

[0004] The present invention provides a method for determining the connectivity of fractures in a carbonate reservoir in a well area. The method selects corresponding fracture criteria according to the type of carbonate rock, and can accurately determine the spatial connectivity of fractures in a carbonate reservoir in a well area.

[0005] The present invention provides a method for determining fracture connectivity of carbonate reservoirs in a well area, comprising the following steps:

[0006] 1) According to the fracture formation period of the carbonate reservoir fracture in the well area, the stress parameters of the carbonate rock in the well area during the fracture formation period are obtained;

[0007] 2) establishing a layered geomechanical model of carbonate rock in the well area according to the stress parameters;

[0008] 3) obtaining the corrected mechanical parameters of the carbonate rock in the well area during the fracture formation period according to the layered geomechanical model;

[0009] 4) determining the fracture criterion of the carbonate rock in the well area according to the stress parameter and the type of the carbonate rock in the well area;

[0010] 5) obtaining the fracture stress of the carbonate rock in the well area according to the corrected mechanical parameters and the fracture criterion;

[0011] 6) obtaining the theoretical inclination angle of the carbonate reservoir fracture in the well area according to the fracture stress;

[0012] 7) Determining the theoretical connectivity of the carbonate reservoir fractures in the well area according to the theoretical inclination.

[0013] The determination method as described above, wherein step 7) further includes:

[0014] Establishing a three-dimensional paleostress field distribution model according to the layered geomechanical model and the corrected mechanical parameters;

[0015] According to the three-dimensional paleostress field distribution model, the law of conservation of energy and the maximum strain energy density theory, the actual density and actual aperture of the carbonate reservoir fractures in the well area are obtained;

[0016] Acquire the actual length of the carbonate reservoir fracture in the well area according to the actual density and the actual aperture;

[0017] Determine the actual connectivity of the carbonate reservoir fractures in the well area according to the actual density, the actual aperture and the actual length;

[0018] When the ratio of the theoretical connectivity to the actual connectivity is greater than 90%, the theoretical connectivity meets the first requirement.

[0019] In the determination method as described above, step 4) further comprises:

[0020] Under the triaxial stress state, if the angle between the horizontal tensile stress σ3 of the layered strata in the carbonate rock in the well area and the lithology interface is 45°-75°, the fracture criterion of the layered strata is a tension-shear composite fracture criterion.

[0021] As described above, in the determination method, under the triaxial stress state, if the angle between the horizontal compressive stress σ1 of the layered strata in the carbonate rock in the well area and the lithological interface is 45°-75°, the fracture criterion of the layered strata is the Mohr-Coulomb shear criterion.

[0022] In the determination method as described above, step 4) further comprises:

[0023] Under the triaxial stress state, if the state of the bedrock in the carbonate rock in the well area is a fractured state, and the horizontal tensile stress σ3 of the bedrock is less than 0, and the angle between σ3 and the lithology interface is 0-45°, then the fracture criterion of the brittle rock in the bedrock is the Griffith fracture criterion.

[0024] In the determination method as described above, step 4) further comprises:

[0025] Under the triaxial stress state, if the state of the bedrock in the carbonate rock in the well area is a fractured state, and the horizontal tensile stress σ3 of the bedrock is less than 0, and the angle between σ3 and the lithology interface is 0-45°, then the fracture criterion of the weak rock in the bedrock is the Griffith fracture criterion modified by McClintock and Valls.

[0026] In the determination method as described above, step 4) further comprises:

[0027] Under the triaxial stress state, if the state of the bedrock in the carbonate rock in the well area is a fractured state, and the angle between the horizontal compressive stress σ1 of the bedrock and the lithology interface is 0-45° and 75°-90°, then the fracture criterion of the brittle rock in the bedrock is the two-stage Mohr-Coulomb fracture criterion.

[0028] In the determination method as described above, step 4) further comprises:

[0029] The Drucker-Prager yield criterion is used to determine the state of the bedrock.

[0030] The determination method as described above, wherein before step 1), it also includes:

[0031] Seismic interpretation profiles in the study area are collected, and the fault throw and two-dimensional fold morphology are restored using the structural equilibrium profile method to determine the fracture formation period of the carbonate reservoir fractures in the well area.

[0032] In the determination method as described above, the step of obtaining the stress parameters of carbonate rock in the well area during the fracture formation period includes the following:

[0033] The structural trace method is used to count the azimuth of the conjugate fracture angle bisector, and determine the maximum principal stress direction of the carbonate rock in the well area at different layers in the carbonate reservoir in the well area during the fracture formation period;

[0034] The carbonate rocks in different layers of the carbonate reservoir in the well area are made into a variety of standard plug samples, and the standard plug samples are tested for Kessel points by rock fracture acoustic emission method to obtain the ancient stress values ​​of the carbonate rocks in different layers of the well area during the fracture formation period.

[0035] The method for determining the connectivity of fractures in a carbonate reservoir in a well area of ​​the present invention can accurately determine the connectivity of fractures in a carbonate reservoir in a well area and the interlayer permeability of fractures in a carbonate reservoir in a well area, because it takes into account the types of carbonate rocks in the well area and the lithology of different types of carbonate rocks in the well area. It overcomes the bottleneck effect of existing modeling and prediction of fractures in carbonate reservoirs in low-density well areas and provides a basis for the exploration and development of low-permeability oil and gas reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or related technologies, the following briefly introduces the drawings required for use in the embodiments of the present invention or related technologies. Obviously, the drawings described below are only some embodiments of the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0037] Figure 1 The present invention is a flowchart of a method for determining fracture connectivity of a carbonate reservoir in a well area according to some embodiments of the present invention. DETAILED DESCRIPTION

[0038] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] At present, the research on reservoir fracture prediction mainly focuses on the following aspects:

[0040] (1) Based on geological analysis, natural fracture parameters are estimated. Previous researchers have described and counted fracture parameters through field outcrops, drilling cores, and thin section analysis, and proposed qualitative calculation methods for fracture porosity, permeability, fracture density, and fracture strength. Currently, industrial CT scanning technology has been successfully introduced into the field of petroleum engineering. It can achieve high-resolution three-dimensional imaging of fractures and accurate characterization of fracture parameters, and can intuitively display the destruction process of microcracks from initiation and expansion to penetration, which is a good supplement to conventional fracture identification and characterization methods.

[0041] (2) The relationship between the principal curvature of the structure and the development of cracks is explored from the perspective of structural characteristics, a mechanical model of fractured rock mass is established, and the relationship between folds and crack parameters is explored. However, the curvature method is only applicable to tensile cracks in brittle rocks and does not consider the plastic deformation of weak rocks, which has certain limitations.

[0042] (3) Starting from the tectonic stress field, the rock fracture criterion and numerical simulation methods are applied to establish a quantitative prediction model between strain energy, fracture rate, fracture frequency and fracture body density. This method began in the 1990s, represented by the binary method of quantitative modeling of brittle sandstone structural fractures proposed by Ding Zhongyi, Qian Xianglin, etc., which showed relatively good application and practical results; Zhou Xingui, Dai Junsheng, etc. established a quantitative calculation model of fracture parameters in unidirectional and tridirectional compression stress states of brittle sandstone. Under the premise that the strain energy released when the rock fractures is equal to the surface energy of the newly added fractures, the three-dimensional spatial prediction of single-period fracture density, opening and other parameters is realized, and the friction energy consumption and plastic deformation energy of the fracture surface are simplified. However, the geological model and mechanical model used are based on homogeneous bodies, without considering the influence of early fractures and rock strength heterogeneity. Zeng Lianbo, Tong Hengmao, Zhao Wentao, etc. also proposed a comprehensive reservoir characterization method based on fracture genesis analysis, and discussed the comprehensive factors affecting the development of low-permeability tight sandstone fractures and the heterogeneity of mechanical strength.

[0043] (4) A discrete fracture network model (DFN) is established using multivariate statistical methods and random interpolation methods. This model can fully integrate seismic, logging, geological, drilling, production and other data to provide a realistic and detailed description of the fracture system from geometric morphology to seepage behavior. However, the accuracy of the model depends on the reliability of the basic data and the analysis of the main controlling factors of the fractures.

[0044] (5) Application of geophysical data for fracture identification and characterization. For example, based on logging and seismic data, advanced technical methods are used to characterize the length, density, width, porosity, permeability, and plane fracture orientation and density of single well fractures. This is currently a common means of fracture identification and prediction. However, this method is still in the qualitative (or semi-quantitative) research stage, and the identification accuracy is often affected by multiple factors.

[0045] Based on the above, the present invention provides a method for determining the connectivity of fractures in a carbonate reservoir in a well area. Figure 1 Flow chart of a method for determining fracture connectivity of carbonate reservoirs in a well area according to some embodiments of the present invention. Figure 1 As shown, the determination method of the present invention comprises the following steps:

[0046] S1: According to the fracture formation period of the carbonate reservoir fracture in the well area, the stress parameters of the carbonate rock in the well area during the fracture formation period are obtained;

[0047] S2: Establish a layered geomechanical model of carbonate rocks in the well area based on stress parameters;

[0048] S3: Obtain the corrected mechanical parameters of carbonate rock in the well area during the fracture formation period according to the layered geomechanical model;

[0049] S4: Determine the fracture criterion of carbonate rock in the well area according to stress parameters and the type of carbonate rock in the well area;

[0050] S5: Obtain the fracture stress of carbonate rock in the well area according to the corrected mechanical parameters and fracture criteria;

[0051] S6: Obtain theoretical inclination angle of carbonate reservoir fractures in the well area according to fracture stress;

[0052] S7: Determine the theoretical connectivity of carbonate reservoir fractures in the well area based on the theoretical inclination.

[0053] In the present invention, before S1, it also includes: collecting seismic interpretation profiles in the study area, using the structural equilibrium profile method to restore the fault throw and two-dimensional fold morphology, and determining the fracture formation period of the carbonate reservoir fractures in the well area.

[0054] Specifically, the above-mentioned section is as perpendicular to the main structural trend as possible, and the above-mentioned section includes the target layer and the fault interpretation result. The above-mentioned fracture formation period is a period of intense structural activity or a key fracture formation period.

[0055] The stress parameters of carbonate rock in the well area obtained in S1 during the fracture formation period include the following:

[0056] The structural trace method is used to count the azimuth of the conjugate fracture angle bisector, and determine the maximum principal stress direction of the carbonate rock in the well area at different layers in the carbonate reservoir in the well area during the fracture formation period;

[0057] The carbonate rocks in different layers of the carbonate reservoir in the well area are made into a variety of standard plug samples, and the standard plug samples are tested for Kessel point by rock fracture acoustic emission method to obtain the paleostress values ​​of carbonate rocks in different layers of the well area during the fracture formation period.

[0058] The above-mentioned paleostress values ​​are obtained specifically by sampling the carbonate reservoir in the well area according to different layers, taking 4 carbonate samples from each layer, the long axis direction of the carbonate samples is basically the same as the long axis direction of the carbonate reservoir in the well area, and processing the carbonate samples in different layers into standard plunger samples of 50×25 mm in the laboratory;

[0059] The standard plunger sample was tested for Kessel point by rock fracture acoustic emission method. The corresponding paleostress value was calculated based on the Kessel point. The maximum value was selected to represent the maximum principal stress value during the fracture formation period, and the average stress value of carbonate rock samples in each layer was obtained.

[0060] S2 specifically includes: establishing a geological model of the target layer structure in the study area based on the depth domain seismic interpretation results collected above.

[0061] S3 is specifically as follows: a. Horizontally sample the carbonate reservoirs in the well area according to different lithologies, with an interval of 45°. The carbonate reservoirs in the well area include sandstone and mudstone according to lithology. Generally, 6 carbonate samples of each lithology are taken from the well area. The long axis direction of the carbonate samples in the well area is perpendicular to the long axis direction of the carbonate reservoirs in the well area. The carbonate samples in different layers of the well area are processed into standard plunger samples of 50mm×25mm in the laboratory;

[0062] b. One carbonate rock sample of each lithology is selected to conduct uniaxial loading test on MTS true triaxial rock mechanics instrument to obtain the uniaxial compressive strength, yield strength, Poisson's ratio and elastic modulus of the carbonate rock sample in the well area;

[0063] c. One carbonate rock sample of each lithology is selected to conduct a direct shear test on a direct shear rock mechanics instrument to obtain the uniaxial shear strength of the carbonate rock sample in the well area;

[0064] d. One carbonate rock sample from each lithology is selected for Brazilian splitting test to test the uniaxial tensile or tensile strength of the rock;

[0065] e. According to the key fracture formation period, combined with the burial history map of the formation in the study area, the ancient burial depth of the carbonate rock samples in the well area is obtained, and the confining pressure range of the overlying formations of the carbonate rock samples in the well area is calculated. The confining pressure level is set every 5-10Mpa. Three carbonate rock samples of each lithology in the well area are selected for true triaxial loading experiments on the MTS true triaxial rock mechanics instrument to test the triaxial compressive strength, yield strength, Poisson's ratio, elastic modulus, internal friction angle, and cohesion of the carbonate rock samples in the well area;

[0066] f. Select the logging interpretation model, conduct vertical continuous interpretation of dynamic rock mechanics parameters such as Young's modulus, Poisson's ratio, density, etc., and conduct dynamic-static correction with the rock mechanics experimental results to obtain corrected mechanical parameters, which are more realistic static mechanical strength parameters of the formation;

[0067] g. According to step a, drill carbonate rock samples from the well area near the lithologic interface of the carbonate reservoir in the well area, process them into standard plunger samples, conduct Brazilian splitting tests and direct shear tests along the lithologic interface direction, and obtain the tensile strength, friction coefficient, and shear strength of the carbonate rock samples in the well area at the lithologic interface.

[0068] The carbonate reservoir in the well area is generally composed of limestone, dolomite, argillaceous limestone and mudstone, showing the characteristics of layered composite rocks. Under deep conditions, argillaceous limestone and mudstone have certain plastic deformation characteristics, which will lead to the overall destruction of the carbonate reservoir in the well area with strong heterogeneity. The present invention determines the corresponding yield or fracture criteria for the carbonate rocks of different lithologies in the carbonate reservoir in the well area.

[0069] S4 is specifically as follows: a. Determine the damage of the lithologic interface under different inclination conditions. Under the triaxial stress state, if the angle between the horizontal tensile stress σ3 of the layered strata in the carbonate rock in the well area and the lithologic interface is 45°-75°, the fracture criterion of the layered strata is the tension-shear composite fracture criterion.

[0070] In this case, the layered strata are located within the possible range of tensile fracture, tension-shear fracture or shear fracture, so it is necessary to use the tensile-shear composite fracture criterion. The tensile-shear composite fracture criterion can be understood as first using the tensile fracture criterion to determine whether the layered strata will undergo tensile fracture. If the tensile fracture condition of the layered strata cannot be met, then using the Mohr-Coulomb criterion to determine whether the layered strata will undergo shear fracture. Formula (1) is the tensile-fracture criterion:

[0071]

[0072] Where: F t is the tensile fracture stress of the layered strata (MPa); F s is the shear fracture stress of the layered strata (MPa); is the internal friction angle of the layer (°); σ t is the tensile strength of the layer (MPa). Many experiments have shown that the tensile strength of layered strata increases with the increase of confining pressure P. t and P conform to the following linear relationship: σ tc =σ t0 +2μP, where σ tc is the tensile strength under confining pressure (MPa), σ t0 is the tensile strength when the confining pressure is zero (MPa), μ is the Poisson's ratio of the rock, dimensionless, and P is the confining pressure (MPa);

[0073] Under the triaxial stress state, if the angle between the horizontal compressive stress σ1 of the layered strata in the carbonate rock in the well area and the lithology interface is 45°-75°, the rupture criterion of the layered strata is the Mohr-Coulomb shear criterion.

[0074] In this case, the layered strata are located within the possible range of interface shear fracture, and the Mohr-Coulomb shear criterion is used to determine whether the interface is shear fractured due to slip. The Mohr-Coulomb shear criterion is shown in formula (2):

[0075]

[0076] Where: c j , are the cohesion and internal friction angle of the interface (GPa, °); F sis the shear fracture stress of the layered strata (MPa), and θ is the angle between the horizontal compressive stress σ1 and the lithological interface (°).

[0077] b. Carbonate rocks in the well area include layered strata and bedrock outside the lithology interface. Bedrock includes weak rock and brittle rock. The present invention uses the Drucker-Prager yield criterion to determine the state of bedrock.

[0078] The Drucker-Prager yield criterion states that when the maximum shape deformation energy (also known as distortion energy) of a material reaches a certain constant, the material yields and enters a plastic state. The Drucker-Prager yield criterion is shown in formula (3):

[0079]

[0080] Among them, I1=σ1+σ2+σ3 is the first stress invariant; is the second stress invariant; a and K are experimental constants related only to the internal friction angle and cohesion of the bedrock. is the internal friction angle of bedrock (°).

[0081] In the present invention, if the bedrock is in a yield state, it indicates that it will not break and will not produce cracks. If the bedrock is in a broken state, its breaking criterion is further determined. Furthermore, if the weak rock does not reach the critical yield state, it is considered to be broken and can be regarded as brittle rock to determine its breaking criterion.

[0082] If the state of the bedrock in the carbonate rock in the well area is a fractured state, under a triaxial stress state, and the horizontal tensile stress of the bedrock σ3 is less than 0, and the angle between σ3 and the lithology interface is 0-45°, then the fracture criterion of the brittle rock in the bedrock is the Griffith fracture criterion.

[0083] Under the triaxial stress state, when σ3 < 0, and the angle between σ3 and the lithology interface is 0-45°, the bedrock is within the possible range of tensile fracture and tension-shear fracture. For brittle rocks, such as limestone and dolomite, the Griffith fracture criterion is used. When (σ1+3σ3) > 0, the fracture criterion is as shown in formula (4):

[0084]

[0085] When (σ1+3σ3)≤0, the rupture criterion is as shown in formula (5):

[0086] θ=0,σ3=-σ T (5)

[0087] Where: Tis the tensile strength of the bedrock (MPa); θ is the tensile fracture angle (°). For weak rocks that undergo brittle failure, such as mudstone and argillaceous limestone, their “critical fracture stress value” can be obtained according to the Griffith brittle failure criterion modified by McClintock and Valls. The Griffith brittle failure criterion modified by McClintock and Valls is shown in formula (6):

[0088]

[0089] Where: III is the triaxial compressive strength of bedrock (MPa); f is the bedrock strength parameter, K is the bedrock weakness coefficient under triaxial compression, where the K of limestone and dolomite is between 1-4.5, and that of mudstone is between 4.5-7.

[0090] It also includes: under the triaxial stress state, if the state of the bedrock in the carbonate rock in the well area is a fractured state, and the angle between the horizontal compressive stress σ1 of the bedrock and the rock interface is 0-45° and 75°-90°, then the fracture criterion of the brittle rock in the bedrock is a two-stage Mohr-Coulomb fracture criterion.

[0091] The two-stage Mohr-Coulomb fracture criterion is shown in equation (7):

[0092]

[0093] In the formula, the Mohr-Coulomb rupture envelope is divided into two sections, left and right, with a confining pressure of σ0 = 5 MPa. When the confining pressure is less than this value, the internal friction angle The rupture angle is smaller, and the tensile rupture is dominant. When the confining pressure is greater than a certain value, the internal friction angle The fracture angle increases, and the fracture of the bedrock gradually changes from tensile fracture to tensile shear fracture to compressive shear fracture. They are the internal friction angles of the left and right segments of the rupture envelope, respectively; k1 and k2 are the slopes of the straight line segments of the left and right segments of the rupture envelope, also known as the internal friction coefficient.

[0094] In S5, the corrected mechanical parameters obtained in S3 are substituted into the fracture criterion determined in S4, so as to obtain the fracture stress of the carbonate rock in the well area.

[0095] S6 is specifically as follows: Calculate the occurrence of the cracks and judge the vertical expansion or extension. Based on the above multi-level composite fracture criteria, in three-dimensional space, the inclination and direction of the cracks are determined by projection calculation. The X-axis of the Ansys coordinate system coincides with the X-axis (east) of the geodetic coordinates, the Z-axis coincides with the negative direction (south) of the Y-axis, and the Y and Z-axes coincide. Therefore, if the direction of the normal direction vector of the crack surface in the global coordinate system has been determined as Will Projected onto the XOZ plane, the angle between the projection line and the negative direction of the Z axis is αz , then α z =arctan(-l / n), the strike angle α can be determined according to equations (8) and (9),

[0096] If 0≤α z <90°, α=90°-α z (8)

[0097] If -90°<α z <0, α=(-90°-α z )+360° (9)

[0098] From a geological perspective, the fracture inclination should be the angle between the fracture rupture surface and the XOZ plane, that is, the angle α between the plane lx+my+nz=0 and the plane y=0. dip (0°≤α dip ≤90°), while in the Ansys three-dimensional coordinate system, the crack inclination is the angle between the crack surface and the XY plane, that is, the angle α between the plane lx+my+nz=0 and the plane y=0 dip (0°≤α dip ≤90°), the calculation formula is as follows:

[0099]

[0100] S7 is specifically as follows: a. The theoretical trend and inclination of the carbonate reservoir fractures in the well area are calculated by S6. When α dip When the angle is >15°, it is considered that the carbonate reservoir fractures in the well area are not horizontal fractures, so the fractures may extend to the upper and lower sides and penetrate the layers into the matrix. According to the fracture mechanics theory, the fractures formed by the expansion in the underground layered strata are mainly type I (opening type), type II (sliding type) or type I-II composite. After the shape change specific energy density criterion is corrected, the expansion or penetration of the fracture tip can be effectively judged in the cylindrical coordinate system or polar coordinate system. d It represents the intensity of the specific energy density field caused by the change in the crack tip shape, S d According to formula (11), we can obtain:

[0101] S d =C 11 K I 2 +2C 12 K I K II +C 22 K II 2 (11)

[0102] in,

[0103]

[0104]

[0105]

[0106]

[0107] In the formula, K I , K II is the stress intensity factor ( ), the larger its value, the greater the tendency of crack instability; θ is the polar coordinate angle of the crack tip (rad); β is the angle between the existing crack and the principal stress; G is the shear elastic modulus (GPa); μ is the Poisson's ratio, the specific energy density factor S of the crack along the shape change d The smallest crack is at θ=θ0. dmin Reaching the critical value S dc When , the crack begins to expand, and the criterion is formula (12):

[0108]

[0109] Where a is the half length of the crack (m), calculated by Ansys software, and S dc It can be regarded as a fracture toughness parameter that resists crack extension, and is expressed by the fracture toughness K IC It is calculated that θ0 is the rupture angle (°) at which the crack begins to expand;

[0110] b. Based on the judgment in step a on whether the vertical cracks continue to expand and penetrate the lithology interface and the stress field simulation, in the Ansys finite element simulation platform, the nodes N1 near the lithology interface are first selected, and the multi-level composite fracture criteria are used to determine whether these points are broken. If they are broken, the fracture properties and inclination angle α of the carbonate rock fractures in the well area are determined. dip At the same time, search for nearby nodes N2, N3, N4, N5, and N6, and also determine the fracture properties and inclination of carbonate rock fractures in the well area. If the situation is similar to that of node N1, it is considered to belong to the same fracture, and the fracture length is simplified to the distance a between the two nodes. Secondly, read out the stress component of node N1 in polar coordinates in the Ansys stress field, and calculate the stress intensity factor K at the fracture tip. I , K II and shape change specific energy density factor S d, and determine the extended fracture angle θ0 of the point, find the next node N7 in the lithological interface in the deflection direction of the crack or in the rock on the other side, extract the stress component again and judge whether it is cracked, so as to solve and determine the continuous nodes of the same crack by using the node stress extrapolation method until the node is no longer cracked, and finally complete the quantitative characterization of the connectivity of the crack space.

[0111] In some embodiments of the present invention, step 7) further includes: establishing a three-dimensional paleostress field distribution model based on the layered geomechanical model and the corrected mechanical parameters;

[0112] According to the three-dimensional paleostress field distribution model, the law of conservation of energy and the maximum strain energy density theory, the actual density and actual aperture of the carbonate reservoir fractures in the well area are obtained;

[0113] Obtain the actual length of carbonate reservoir fractures in the well area based on actual density and actual aperture;

[0114] Determine the actual connectivity of carbonate reservoir fractures in the well area based on actual density, actual aperture and actual length;

[0115] When the consistency between the actual connectivity and the theoretical connectivity is greater than 90%, the theoretical connectivity meets the first requirement.

[0116] Specifically, the three-dimensional paleostress field distribution model is established based on the layered geomechanical model and the corrected mechanical parameters, including:

[0117] a. Based on the finite element simulation platform, the corrected mechanical parameters of the fracture formation period obtained in step 3) are assigned to the layered geomechanical model established in step 2), including assigning corrected mechanical parameters to limestone or dolomite, mudstone or mud limestone, lithology interface, and fault, respectively. The corrected mechanical parameters include: density, elastic modulus, Poisson's ratio, and fault mechanical parameters can refer to the rock mechanics tutorial. Generally, the elastic modulus is 0.65 times that of limestone or dolomite;

[0118] b. Grid the geomechanical model after the assignment. According to the actual situation, the grid density of the stratum is generally large, and the grid density at the fault and lithology interface is encrypted;

[0119] c. Set mechanical boundary conditions according to the results of structural evolution analysis, apply force to the model according to the results of paleo-stress state test analysis, and repeatedly adjust the boundary conditions and force range until the stress difference (σ1-σ3) or stress intensity conforms to the structural distribution characteristics of the study area, that is, the formation mechanism of faults and folds can be reasonably explained.

[0120] According to the three-dimensional paleostress field distribution model, the law of conservation of energy and the maximum strain energy density theory, the actual density and actual aperture of the carbonate reservoir fractures in the well area are obtained, including the following:

[0121] a. Based on the finite element grid and unit division of the carbonate reservoir in the well area, a micro-parallel hexahedron is selected as the characterization unit (REV) with the node as the center. The unit side length along the maximum principal stress σ1 direction is L1, the unit side length along the intermediate principal stress σ2 direction is L2, and the unit side length along the minimum principal stress σ3 direction is L3. In addition, L1>L2>L3, the fracture surface normal is located in the σ1-σ3 principal plane, and the main direction of the fracture (long axis direction) forms an angle θ with the maximum principal stress, where is the internal friction angle, parallel to the intermediate principal stress σ2;

[0122] b. A large number of rock mechanics experimental results have shown that elastic strain energy is the fundamental cause of rock failure, and no matter it is brittle rock or elastic-plastic rock, when the loading stress reaches the peak stress σ c When the strain energy is 0.85 times of that at this moment, precursory microcracks begin to gather in large numbers and generate new cracks. Therefore, the strain energy at this moment is called the maximum elastic strain energy that must be overcome to generate cracks. According to the law of conservation of energy and the maximum strain energy density theory, the energy required for the newly added crack surface area is equal to the difference between the strain energy in the current unit body and the maximum elastic strain energy that must be overcome to generate cracks, that is, formula (13):

[0123]

[0124]

[0125]

[0126]

[0127] Where D vf It is defined as the volume density of cracks in the REV unit body, that is, the ratio of the total surface area of ​​the crack body to the unit body volume, m 2 / m 3 ; σ c is the uniaxial peak strength of carbonate rock in the well area, MPa; E is 0.85σ c The average elastic modulus of the stress point during unloading is approximately equal to the elastic modulus of the elastic stage during loading, GPa, μ is the Poisson's ratio, is the coefficient; σ p is the fracture stress obtained when the carbonate rock in the well area meets the Mohr-Coulomb fracture condition, MPa; w is the density of stored releasable elastic strain energy, J / m 3 ;w e is the elastic strain energy density that must be overcome to generate cracks, J / m 3 ;w f is the strain energy density for the newly added crack surface area, J / m 3 , S fis the surface area of ​​the newly added cracks, m 2 ; J is the energy required to produce a crack per unit area, that is, the crack surface energy, J / m 2 ;

[0128] c. Calculate the actual density and actual aperture of carbonate reservoir fractures in the well area under different stress states. Under the triaxial compressive stress state without tensile stress, the above multi-level composite fracture criteria are used to judge by nodes. When the carbonate rock in the well area does not reach the fracture condition, the fracture parameters are all 0. When the carbonate rock in the well area reaches the fracture condition, the actual density, actual linear density and actual aperture are calculated as follows:

[0129]

[0130] Where J0 is the crack surface energy when the confining pressure is zero, J / m 2 ; ε0 is the maximum elastic tensile strain of carbonate rock in the well area; E0 is the proportional coefficient, which is related to the lithology, and J0 for limestone and dolomite is 1087.35 J / m 2 , E0=112.6, mudstone, mudstone limestone J0=1391.44J / m 2 , E0 = 101.4; other parameters w are strain energy density, w f is the crack strain energy density, w e is the strain energy that must be overcome per unit crack surface area, σ p is the fracture stress of carbonate rock in the well area, D vf is the density of the fracture volume, D lf is the minimum tensile strain, b is the crack opening, E is the elastic modulus, C0 is the cohesive force, is the internal friction angle, μ is the Poisson's ratio, J is the crack surface energy, and θ is the rupture angle;

[0131] d. Under the triaxial compressive stress state with tensile stress, the above multi-level composite fracture criteria are used to judge by node. When the carbonate rock in the well area does not reach the fracture condition, the fracture parameters are all 0. When the carbonate rock in the well area reaches the fracture condition, the fracture body density, linear density and aperture are calculated using formula (15):

[0132]

[0133] Among them, ε0 is the maximum elastic tensile strain of carbonate rock in the well area, and J0 for limestone and dolomite is 1087.35 J / m 2 , ε0=-4.9×10 -4 , mudstone, mudstone limestone J0=1391.44J / m 2 , ε0=-7.2×10 -4 ; σ tis the uniaxial tensile strength at zero confining pressure, in MPa; other parameters w, w f 、w e , D vf , D lf , ε3, b, J, and θ are the same as those in the case of triaxial compressive stress;

[0134] e. Based on parameters such as actual density and actual opening, the average length of the cracks in the unit can be obtained after simplification. According to the theory of structural geology, a group of cracks with roughly the same direction and scale are often generated under the same stress in the same period. Here, a group of cracks in the unit is equivalent to a number of rectangular blocks, whose length, width and height are L, b and H respectively, and the cracks are equivalent to parallel and equidistantly arranged. These cracks do not exceed the boundary of the unit. From the above, it can be seen that θ is the crack rupture angle, which is defined as the angle between the crack surface and σ1. Therefore, the number of cracks along the direction with an angle of θ with L3 can be expressed as: The volume density of cracks in the unit cell is expressed as: A large number of experiments have shown that the length and height parameters of a single crack are related to the maximum principal stresses σ1 and σ3 at the node, that is, Finally, the average length of a set of cracks is:

[0135] The actual connectivity of carbonate reservoir fractures in the well area is determined based on actual density, actual aperture and actual length, including:

[0136] Based on the simulation results of the paleostress field, the quantitative expressions of the fracture body density, line density, length, opening, inclination, and strike parameters were written into an APDL program using the secondary development interface in the finite element simulation platform to predict the spatial distribution characteristics of the fracture parameters and obtain the actual connectivity of the fractures.

[0137] When the ratio of theoretical connectivity to actual connectivity is greater than 90%, the theoretical connectivity meets the first requirement.

[0138] In the present invention, the first requirement can be considered to be a requirement within the error range, and it can be determined that the obtained theoretical connectivity has high reliability.

[0139] In a specific implementation manner, if the ratio of theoretical connectivity to actual connectivity is ≤90%, the geomechanical model and the fracture parameter calculation model may be revised.

[0140] In the present invention, the spatial longitudinal connectivity of the cracks and the effectiveness of the cracks can be predicted by combining the crack theoretical density, crack theoretical length, and crack theoretical inclination parameters. If the crack theoretical length is generally less than the interlayer thickness under normal distribution, the crack is a non-penetrating type, that is, the interlayer effectiveness is good. If the crack theoretical length is generally greater than the interlayer thickness, it is a penetrating type, that is, the interlayer effectiveness is poor. If the crack theoretical length is partially greater than the interlayer thickness, it is a semi-penetrating type. Then, the crack theoretical inclination range is combined for evaluation. If the crack theoretical inclination is mainly in the range of 0-45°, it is a low-angle crack with good interlayer effectiveness. If the crack theoretical inclination is mainly in the range of 45-75°, it is an oblique cross crack with average interlayer effectiveness. If the crack theoretical inclination is mainly in the range of 75-90°, it is a high-angle crack with the worst interlayer effectiveness.

[0141] The present invention can also evaluate the lateral connectivity of spatial fractures in combination with the results of simulation calculations of the theoretical density, theoretical aperture and trend of the fractures. For example, under normal distribution, if the theoretical density and theoretical aperture of the fractures are both large and the trend is stable, the lateral connectivity of the fractures is good. If the theoretical density and theoretical aperture of the fractures are both large, but the trend distribution is messy or mainly low-angle fractures, the lateral connectivity of the fractures is average. If the theoretical density and theoretical aperture of the fractures are both low and the trend is stable, the lateral connectivity of the fractures is average. If the theoretical density and theoretical aperture of the fractures are both low and the trend is stable, the lateral connectivity of the fractures is average. If the theoretical density and theoretical aperture of the fractures are both low and the trend is messy, the lateral connectivity of the fractures is the worst.

[0142] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for determining fracture connectivity of carbonate reservoirs in a well area, characterized in that: The following steps are involved: 1) According to the fracture formation period of the carbonate reservoir fracture in the well area, the stress parameters of the carbonate rock in the well area during the fracture formation period are obtained; 2) establishing a layered geomechanical model of carbonate rock in the well area according to the stress parameters; 3) obtaining the corrected mechanical parameters of the carbonate rock in the well area during the fracture formation period according to the layered geomechanical model; 4) determining the fracture criterion of the carbonate rock in the well area according to the stress parameter and the type of the carbonate rock in the well area; 5) obtaining the fracture stress of the carbonate rock in the well area according to the corrected mechanical parameters and the fracture criterion; 6) obtaining the theoretical inclination angle of the carbonate reservoir fracture in the well area according to the fracture stress; 7) determining the theoretical connectivity of the carbonate reservoir fractures in the well area according to the theoretical inclination; Step 4) also includes: Under the triaxial stress state, if the angle between the horizontal tensile stress σ3 of the layered strata in the carbonate rock in the well area and the lithology interface is 45°-75°, the fracture criterion of the layered strata is the tension-shear composite fracture criterion; Under the triaxial stress state, if the angle between the horizontal compressive stress σ1 of the layered strata in the carbonate rock in the well area and the lithology interface is 45°-75°, the rupture criterion of the layered strata is the Mohr-Coulomb shear criterion; Under the triaxial stress state, if the state of the bedrock in the carbonate rock in the well area is a fractured state, and the horizontal tensile stress σ3 of the bedrock is less than 0, and the angle between σ3 and the lithology interface is 0-45°, then the fracture criterion of the brittle rock in the bedrock is the Griffith fracture criterion; Under the triaxial stress state, if the state of the bedrock in the carbonate rock in the well area is a fractured state, and the horizontal tensile stress σ3 of the bedrock is less than 0, and the angle between σ3 and the lithology interface is 0-45°, then the fracture criterion of the weak rock in the bedrock is the Griffith fracture criterion modified by McClintock and Valls; Under the triaxial stress state, if the state of the bedrock in the carbonate rock in the well area is a fractured state, and the angle between the horizontal compressive stress σ1 of the bedrock and the lithology interface is 0-45° and 75°-90°, then the fracture criterion of the brittle rock in the bedrock is a two-stage Mohr-Coulomb fracture criterion; Step 4) also includes: The Drucker-Prager yield criterion is used to determine the state of the bedrock.

2. The determination method according to claim 1, characterized in that: Step 7) also includes: Establishing a three-dimensional paleostress field distribution model according to the layered geomechanical model and the corrected mechanical parameters; According to the three-dimensional paleostress field distribution model, the law of conservation of energy and the maximum strain energy density theory, the actual density and actual aperture of the carbonate reservoir fractures in the well area are obtained; Acquire the actual length of the carbonate reservoir fracture in the well area according to the actual density and the actual aperture; Determine the actual connectivity of the carbonate reservoir fractures in the well area according to the actual density, the actual aperture and the actual length; When the ratio of the theoretical connectivity to the actual connectivity is greater than 90%, the theoretical connectivity meets the first requirement.

3. The determination method according to claim 1, characterized in that: Before step 1), it also includes: Seismic interpretation profiles in the study area are collected, and the fault throw and two-dimensional fold morphology are restored using the structural equilibrium profile method to determine the fracture formation period of the carbonate reservoir fractures in the well area.

4. The determination method according to claim 1, characterized in that: The method of obtaining the stress parameters of carbonate rock in the well area during the fracture formation period includes: the following: The structural trace method is used to count the azimuth of the conjugate fracture angle bisector, and determine the maximum principal stress direction of the carbonate rock in the well area at different layers in the carbonate reservoir in the well area during the fracture formation period; The carbonate rocks in different layers of the carbonate reservoir in the well area are made into a variety of standard plug samples, and the standard plug samples are tested for Kessel points by rock fracture acoustic emission method to obtain the ancient stress values ​​of the carbonate rocks in different layers of the well area during the fracture formation period.

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