Complex lithofacies shale reservoir structure weak plane mechanical parameter determination method and system

Abstract

The invention belongs to a complex lithofacies shale reservoir structure weak plane mechanical parameter determination method and system, and belongs to the field of unconventional shale oil and gas reservoir transformation. The method comprises the following steps: carrying out a nanoindentation experiment test on a rock core sample containing a structural weak plane to obtain a macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak plane; constructing a particle flow discrete element numerical model; on the basis of construction of the particle flow discrete element numerical model and the macroscopic uniaxial elastic modulus and the macroscopic fracture toughness of the structure weak plane, particle flow micromechanical parameter calibration is carried out; and obtaining the mechanical parameters of the weak plane of the structure based on particle flow microscopic mechanical parameter calibration. According to the method, the compressive strength, the elastic modulus, the Poisson's ratio, the cohesion, the internal friction angle, the fracture toughness and the tensile strength of the structure weak plane can be rapidly and accurately predicted, the defect that an existing indoor physical experiment device cannot measure mechanical parameters of a non-standard sample is effectively overcome, and input parameters are provided for fine simulation of hydraulic fracturing of a complex lithofacies shale reservoir.

Description

Technical Field

[0001] The present invention belongs to the field of unconventional shale oil and gas reservoir transformation, and in particular relates to a method and system for determining mechanical parameters of weak planes in complex lithofacies shale reservoir structures. Background Art

[0002] Complex lithofacies shale reservoirs generally develop a large number of structural weak planes, mainly including bedding, natural fractures and lithological interfaces, and are characterized by strong heterogeneity and anisotropy. The physical properties of the shale matrix are extremely poor, and hydraulic fractures must be generated through hydraulic fracturing, and structural weak planes must be activated and communicated to form a complex fracture network in order to obtain industrial production capacity. Structural weak planes are generally filled with minerals such as carbonate rocks and silicates. Generally speaking, the mechanical properties of structural weak planes are quite different from those of the matrix. When hydraulic fractures extend from the matrix to the structural weak planes, the fracture extension behavior will be affected to a certain extent, making the fracture morphology more complex. Therefore, obtaining various mechanical parameters of structural weak planes is particularly important for accurately studying the influence of structural weak planes on the extension morphology of fracturing cracks.

[0003] At present, the main mechanical parameters of rocks (including compressive strength, elastic modulus, Poisson's ratio, cohesion, and internal friction angle) are directly measured mainly through triaxial mechanical experimental instruments. The experiment requires the sample shape to be processed into a standard plunger shape (diameter is 25mm, length is about 50mm). However, the thickness of the cement filling layer of the structural weak surface is generally millimeter-level, which cannot be processed into the required size of the standard sample. Therefore, it is impossible to use the current triaxial mechanical experimental instruments to directly measure the compressive strength, elastic modulus, Poisson's ratio, cohesion, and internal friction angle of the structural weak surface. Similarly, to measure other mechanical parameters of the structural weak surface, such as conducting experimental tests on fracture toughness and tensile strength, it is also necessary to process the core into the required size of the corresponding sample, and the structural weak surface cannot meet the conditions for direct measurement. Summary of the invention

[0004] In response to the above problems, the present invention provides a method and system for measuring the mechanical properties of structural weak surfaces, which are constructed based on nanoindentation experiments and discrete element simulations. The method and system can quickly and accurately predict the compressive strength, elastic modulus, Poisson's ratio, cohesion, internal friction angle, fracture toughness and tensile strength of structural weak surfaces, effectively overcoming the defect that current indoor physical experimental equipment cannot measure the mechanical parameters of non-standard samples, and provides input parameters for the fine simulation of hydraulic fracturing in complex lithofacies shale reservoirs, which can guide fracturing well selection and layer selection as well as process parameter optimization.

[0005] The first object of the present invention is to provide a method for determining mechanical parameters of weak planes in complex lithofacies shale reservoir structures, the method comprising:

[0006] Nanoindentation tests were performed on core samples containing structural weak planes to obtain the macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak planes.

[0007] Construct discrete element numerical model of particle flow;

[0008] Based on the construction of the particle flow discrete element numerical model and the macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak surface, the particle flow micromechanical parameters are calibrated;

[0009] Based on the calibration of particle flow micromechanical parameters, the mechanical parameters of the structural weak surface are obtained.

[0010] In the embodiment of the present invention, the core sample containing the structural weak surface is obtained by radial or axial drilling along the full-diameter core of the well, and the structural weak surface should intersect with the end face of the core.

[0011] In the embodiment of the present invention, the nanoindentation test is performed on the core sample containing the structural weak surface to obtain the macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak surface, including:

[0012] Nanoindentation tests were performed on core samples containing structural weak planes to obtain the mesoscopic elastic modulus and mesoscopic fracture toughness.

[0013] Based on the conversion relationship between meso elastic modulus, meso fracture toughness and macro-micro mechanical parameters, the macroscopic uniaxial elastic modulus and macroscopic fracture toughness are converted.

[0014] In an embodiment of the present invention, the acquisition of the conversion relationship between macroscopic and microscopic mechanical parameters includes:

[0015] Uniaxial compression test and nanoindentation test were carried out on two groups of matrix core samples with the same sampling position to obtain the macroscopic uniaxial elastic modulus and microscopic elastic modulus of the matrix.

[0016] Based on the macroscopic uniaxial elastic modulus and microscopic elastic modulus of the matrix, a conversion relationship between the macroscopic and microscopic elastic moduli is obtained;

[0017] Fracture toughness test and nanoindentation test were carried out on two groups of matrix core samples with the same sampling position to obtain the macroscopic and microscopic fracture toughness of the matrix;

[0018] Based on the macroscopic and microscopic fracture toughness of the matrix, the conversion relationship between the macroscopic and microscopic fracture toughness is obtained.

[0019] In an embodiment of the present invention, the step of obtaining mechanical parameters of structural weak surfaces based on particle flow micromechanical parameter calibration includes:

[0020] Based on the calibration of particle flow micromechanical parameters, discrete element numerical simulation of particle flow is carried out;

[0021] Based on the particle flow discrete element numerical simulation, the mechanical parameters of the structural weak surface are obtained.

[0022] In an embodiment of the present invention, the particle flow discrete element numerical simulation based on the particle flow micromechanical parameter calibration includes:

[0023] The triaxial compression model is subjected to triaxial compression numerical simulation to obtain the stress-strain curve;

[0024] The Brazilian splitting numerical simulation was carried out on the Brazilian disk model, and the load size and specimen displacement change curve were obtained.

[0025] In an embodiment of the present invention, the method of obtaining mechanical parameters of structural weak surfaces based on particle flow discrete element numerical simulation includes:

[0026] Based on the stress-strain curve, the triaxial compressive strength, triaxial elastic modulus, triaxial Poisson's ratio, cohesion and internal friction angle of the structural weak surface are obtained;

[0027] Based on the load size and the specimen displacement change curve, the tensile strength of the weak surface of the structure is obtained.

[0028] The second object of the present invention is to provide a system for determining mechanical parameters of weak planes in complex lithofacies shale reservoir structures, the system comprising an experimental module, a model building module, a calibration module and an acquisition module;

[0029] The experimental module is used to perform nanoindentation test on a core sample containing a structural weak surface to obtain the macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak surface;

[0030] The model building module is used to build a particle flow discrete element numerical model;

[0031] The calibration module is used to calibrate the particle flow micromechanical parameters based on the construction of a particle flow discrete element numerical model and the macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak surface;

[0032] The acquisition module is used to obtain the mechanical parameters of the structural weak surface based on the calibration of the particle flow micromechanical parameters.

[0033] The third object of the present invention is to provide a computer-readable storage medium having a computer program stored thereon, and when the computer program is executed by a processor, the method for determining the weak plane mechanical parameters of a complex lithofacies shale reservoir structure as described above is implemented.

[0034] Beneficial effects of the present invention:

[0035] The present invention provides a method and system for determining the mechanical parameters of the weak surface of a complex lithofacies shale reservoir structure. The method and system are based on nanoindentation experiments and discrete element simulations, and can quickly and accurately predict the compressive strength, elastic modulus, Poisson's ratio, cohesion, internal friction angle, fracture toughness and tensile strength of the weak surface of the structure. The method effectively overcomes the defect that the current indoor physical experimental equipment cannot measure the mechanical parameters of non-standard samples, and provides input parameters for the fine simulation of hydraulic fracturing of complex lithofacies shale reservoirs, which can guide the selection of wells and layers for fracturing and the optimization of process parameters.

[0036] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0038] Figure 1 A flow chart showing a method for determining mechanical parameters of weak planes in complex lithofacies shale reservoir structures according to an embodiment of the present invention is shown;

[0039] Figure 2 A picture showing a core sample containing a structural weak plane according to an embodiment of the present invention is shown;

[0040] Figure 3 The indentation load displacement curve obtained by the nanoindentation test according to the embodiment of the present invention is shown;

[0041] Figure 4 A diagram showing a triaxial compression numerical simulation result according to an embodiment of the present invention is shown;

[0042] Figure 5 A diagram showing numerical simulation results of Brazilian splitting according to an embodiment of the present invention is shown;

[0043] Figure 6 A framework diagram of a system for determining mechanical parameters of weak planes in complex lithofacies shale reservoir structures according to an embodiment of the present invention is shown;

[0044] In the figure: 1. Experimental module model; 2. Construction module; 3. Calibration module; 4. Acquisition module. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are 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.

[0046] like Figure 1 As shown, a method for determining mechanical parameters of weak planes in a complex lithofacies shale reservoir structure according to an embodiment of the present invention includes:

[0047] Step S1, performing a nanoindentation test on a core sample containing a structural weak surface to obtain a macroscopic uniaxial elastic modulus and a macroscopic fracture toughness of the structural weak surface;

[0048] Step S2, constructing a particle flow discrete element numerical model;

[0049] Step S3, calibrating the particle flow micromechanical parameters based on the constructed particle flow discrete element numerical model and the macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak surface;

[0050] Step S4: based on the particle flow micromechanical parameter calibration, obtain the mechanical parameters of the structural weak surface.

[0051] Specifically, in step S1, the nanoindentation test is performed on the core sample containing the structural weak surface to obtain the macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak surface, including:

[0052] Step A1, performing nanoindentation test on a core sample containing a structural weak surface to obtain a mesoscopic elastic modulus and a mesoscopic fracture toughness;

[0053] Step A2: based on the conversion relationship between the meso elastic modulus, the meso fracture toughness and the macro-micro mechanical parameters, the macro uniaxial elastic modulus and the macro fracture toughness are converted.

[0054] Specifically, in step A1, the core sample containing the structural weak surface is taken from the core containing the structural weak surface drilled radially or axially along the full-diameter core (diameter of about 100 mm) downhole, and the structural weak surface should intersect with the core end face, the core diameter is 25 mm, and the length is 10 mm, wherein the mechanical properties of the low-angle structural weak surface, such as bedding and lithology interface, can be obtained by coring along the radial direction; the mechanical properties of the high-angle structural weak surface, such as natural fractures, can be obtained by coring along the axial direction;

[0055] After sampling, the surface of the drilled core was mechanically polished with 60-2000 mesh sandpaper, and then polished again with a broad beam argon ion device. Figure 2 ;

[0056] Specifically, in step A1, the nanoindentation test is performed on the core sample containing the structural weak surface to obtain the meso elastic modulus and meso fracture toughness, including:

[0057] Nano The G200 nanoindenter was used to perform nanoindentation tests on core samples with structural weak surfaces. The indenter loading speed was 10 nm / s, the number of indentation points was 8, the spacing between indentation points was greater than 300 μm, and the holding time was 15 s.

[0058] Among them, nanoindentation test is an important technology that has developed rapidly in the testing of surface engineering mechanical properties in recent years. It has the advantages of non-destructive, high resolution, and simple operation. The principle of nanoindentation is to apply a small-scale load to the surface of the material through a diamond indenter and produce an inward-concave tetrahedral pit on the surface. The cross-section of the indenter is a triangle. The experiment can obtain the load and displacement change curve (Ph curve), which includes the loading stage, the load holding stage and the unloading stage, which correspond to the loading curve, the load holding curve and the unloading curve respectively.

[0059] The specific experimental steps are as follows: the nanoindenter gradually approaches the sample surface at a certain descending rate. When the test system shows that the load suddenly increases, it means that the indenter has contacted the sample surface. At this time, the system starts loading according to the setting method and automatically records the load and the corresponding indentation depth. It is loaded to the maximum load and maintained for a certain time, and then unloaded. The experiment is stopped when the load becomes zero. The load and displacement change curve is formed according to the full load and displacement data;

[0060] In the nanoindentation test, the sample surface roughness, indentation depth, loading and unloading system, and indentation spacing will affect the mechanical properties test results; the deeper the indentation depth, the larger the contact area between the indenter and the sample, and the mechanical properties of the rock material will be affected by different mineral phases, as well as the microcracks and micro-nanopores developed inside; the experiment uses grid indentation technology, and inappropriate indentation spacing will lead to mutual interference between indentation points; in the indentation experiment, creep will affect the measurement of the test accuracy, so it is generally maintained at the maximum load for a period of time; in the embodiment of the present invention, the test standard of homogeneous materials can be used as a reference to select appropriate test conditions to ensure the accuracy and reliability of the nanoindentation test results. The present invention does not limit the test conditions of the nanoindentation test;

[0061] Based on the results obtained from the nanoindentation test (i.e. the load and displacement curves as shown in Figure 3 As shown, Figure 3 In the figure, the horizontal axis is displacement in nanometers, and the vertical axis is load in mN. Figure 3The relationship between load size and displacement can be known from the figure). The meso elastic modulus and meso fracture toughness can be obtained by following the steps below:

[0062] Micro elastic modulus:

[0063] The indentation load-displacement curve generally presents a power function relationship. The power function is used to fit its change trend, and the expression is:

[0064] p u =η(h u -h f ) b (1)

[0065] In formula (1), p u is the load of the unloading curve; h u is the displacement of the unloading curve; η is the power function coefficient obtained by fitting; h f is the displacement at the end of unloading; b is the power function exponent obtained by fitting.

[0066] By taking the derivative of equation (1) and substituting the maximum displacement as the horizontal coordinate, the unloading contact stiffness at the maximum penetration depth can be obtained:

[0067] k=ηb(h max -h f ) b-1 (2)

[0068] In formula (2), k is the unloading contact stiffness at the maximum penetration depth; h max is the maximum penetration depth.

[0069] The contact area between the indenter and the material can be calculated by the following formula:

[0070]

[0071] In formula (3), A c is the contact area between the indenter and the material; a is the half length of the indentation; h c is the contact depth of the indenter; r is the radius of the indenter tip; α is the angle between the load direction and the indenter contact edge, that is, half of the indenter cone angle.

[0072] The indenter contact depth can be calculated by the following formula:

[0073]

[0074] In formula (4), p max is the maximum indentation load; γ is the indentation head constant.

[0075] The reduced elastic modulus is a parameter that comprehensively characterizes the elastic deformation of the indenter and the sample at the microscopic scale, and can be calculated by the following formula:

[0076]

[0077] In formula (5), E r is the reduced elastic modulus; β is a constant related to the geometry of the indenter.

[0078] The relationship between the reduced elastic modulus and the rock mechanical parameters of rock and indenter is as follows:

[0079]

[0080] In formula (6), E s is the micro-elastic modulus of the sample; s E is the microscopic Poisson's ratio of the sample; i is the elastic modulus of the indenter; i is the pressure head Poisson’s ratio.

[0081] The microscopic elastic modulus of the sample can be calculated according to formula (6). It is necessary to assume the microscopic Poisson's ratio of the sample. Previous studies have shown that a change in the Poisson's ratio of ±0.1 will only cause a change in the elastic modulus of less than 5%, and the Poisson's ratio of rock is generally between 0.1 and 0.4, that is, the effect of the Poisson's ratio on the elastic modulus is very small. In general, it can be assumed that the microscopic Poisson's ratio of the sample is 0.25. The material of the indenter is diamond, the elastic modulus is 1140Gpa, and the Poisson's ratio is 0.070;

[0082] Micro fracture toughness:

[0083] The total energy consumed by material deformation is:

[0084]

[0085] In formula (7), U t is the total energy consumed by material deformation; p c is the load corresponding to the contact depth of the indenter.

[0086] The energy required for material fracture can be calculated by the following formula:

[0087]

[0088] In formula (8), U c The energy required for the material to break.

[0089] Critical energy release rate for material fracture:

[0090]

[0091] In formula (9), G c A is the critical energy release rate of material fracture; max is the maximum contact projection area under the corresponding load.

[0092] The microscopic fracture toughness of the material can be calculated by the following formula:

[0093]

[0094] In formula (10), K c is the microscopic fracture toughness of the material;

[0095] In the embodiment of the present invention, the micro elastic modulus of the structural weak surface is calculated according to equations 6 and 10 to be 33.24 GPa, and the micro fracture toughness is 1.481 MPa·m 0.5 .

[0096] In fact, the mechanical parameters used in engineering are macroscopic, while the mechanical parameters obtained through indentation experiments are microscopic, and there is a certain difference in the values ​​between the two. Therefore, it is necessary to convert the microscopic elastic modulus and microscopic fracture toughness into the corresponding macroscopic values, that is, to perform step A2;

[0097] Specifically, in step A2, the macroscopic uniaxial elastic modulus and macroscopic fracture toughness are converted based on the conversion relationship between the microscopic elastic modulus, the microscopic fracture toughness and the macroscopic and microscopic mechanical parameters.

[0098] Among them, the acquisition of the conversion relationship between macroscopic and microscopic mechanical parameters includes:

[0099] Since the shale matrix is ​​widely distributed and has the conditions for conducting triaxial mechanical experiments and nanoindentation experiments, the macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak surface are generally converted by referring to the macroscopic and microscopic parameter conversion formula of the shale matrix. Specifically, the acquisition of the macroscopic and microscopic parameter conversion formula includes:

[0100] The core samples of two groups of matrices with the same sampling position were subjected to uniaxial compression test and nanoindentation test to obtain the macroscopic uniaxial elastic modulus and microscopic elastic modulus of the matrix.

[0101] That is, multiple columnar cores are drilled along the radial direction and at a distance from the surface of the full-diameter core matrix, and processed into samples required for uniaxial compression test (diameter 25mm, length 50mm) and nanoindentation test (diameter 25mm, length 10mm), respectively, and the macroscopic uniaxial elastic modulus (confining pressure is 0) and microscopic elastic modulus of the matrix are measured respectively. Among them, the number of samples in the two groups is not less than 5, that is, core samples are taken from at least 5 positions;

[0102] Based on the macroscopic uniaxial elastic modulus and microscopic elastic modulus of the matrix, the conversion relationship between the macroscopic and microscopic elastic moduli is obtained.

[0103] That is, according to the data of the macroscopic uniaxial elastic modulus and the mesoscopic elastic modulus of the above-mentioned at least 5 samples, a scatter plot of the macroscopic elastic modulus and the mesoscopic elastic modulus of the matrix is ​​drawn, and then the mathematical relationship between the two is fitted to obtain the conversion relationship between the macroscopic and mesoscopic elastic moduli. In an embodiment of the present invention, the conversion relationship between the macroscopic and mesoscopic elastic moduli is shown in formula (11):

[0104] E m =0.825E s -17.016 (11)

[0105] In formula (11), E m is the macroscopic uniaxial elastic modulus; E s is the indentation micro-elastic modulus;

[0106] Fracture toughness test and nanoindentation test were carried out on the matrix core samples with the same sampling position to obtain the macroscopic and microscopic fracture toughness of the matrix.

[0107] That is, multiple columnar cores are drilled along the radial direction and at a distance from the surface of the full-diameter core matrix, and processed into samples required for the fracture toughness test (disc diameter 50mm, thickness 25mm, preset straight seam length 33mm) and the nanoindentation test (diameter 25mm, length 10mm), respectively, to measure the macroscopic fracture toughness and microscopic fracture toughness of the matrix, wherein the number of samples in the two groups is not less than 5, that is, core samples are taken from at least 5 positions;

[0108] Based on the macroscopic and microscopic fracture toughness of the matrix, the conversion relationship between macroscopic and microscopic fracture toughness is obtained.

[0109] That is, according to the data of the macroscopic fracture toughness and the mesoscopic fracture toughness of the above-mentioned at least 5 samples, a scatter plot of the macroscopic fracture toughness and the mesoscopic fracture toughness of the matrix is ​​drawn, and then the mathematical relationship between the two is fitted. In the embodiment of the present invention, the conversion relationship between the macroscopic and mesoscopic fracture toughness is shown in formula (12):

[0110] K cm =0.459K cs +0.218 (12)

[0111] In formula (12), K cm is the macro fracture toughness; K cs is the indentation mesoscopic fracture toughness.

[0112] Specifically, in step A2, the macro elastic modulus and macro fracture toughness are converted, that is, the micro elastic modulus and micro fracture toughness obtained in step A1 are substituted into equations (11) and (12), respectively, to obtain a macro uniaxial elastic modulus of 10.40 GPa and a macro fracture toughness of 0.898 MPa·m of the structural weak surface. 0.5 .

[0113] Since the nanoindentation experiment can only convert the macroscopic uniaxial elastic modulus and fracture toughness, but cannot obtain other macroscopic mechanical parameters, it is necessary to construct a particle flow discrete element numerical model in step S2. The particle flow discrete element is a simulation method based on particle aggregates, which can more accurately simulate the deformation and failure process of rocks.

[0114] Specifically, in step S2, constructing a particle flow discrete element numerical model includes:

[0115] The model assumptions are as follows: the model is two-dimensional, the research object is composed of thousands of round particles, and the contact between particles is characterized by a parallel bonding model; the particles are assumed to be rigid bodies, and the particles do not deform but only displace; the contact range between particles is very small, the contact between particles is flexible, and a certain overlap between particles is allowed during the movement, and the amount of overlap represents the magnitude of the contact force; the deformation of most particle systems is caused by the translation and rotation of particles, rather than the deformation of a single particle; there is no bonding between the wall and the particles, the particles all satisfy the motion equation, but the wall part does not satisfy the motion equation, and the movement of the wall is artificially set;

[0116] In the parallel bonding model, the motion, force, and torque of the particles satisfy Newton's laws of motion. There is contact between particles, and the force and displacement relationship of each particle contact can be represented by equations (13) to (18). The contact force vector acting between two adjacent particles (particle A and particle B) can be decomposed into normal contact force and tangential contact force:

[0117] F i =F n n i +F s n j (13)

[0118] In formula (13), F i is the contact force between particle A and particle B; F n is the normal contact force between particle A and particle B; F s is the tangential contact force between particle A and particle B; n i is the unit normal vector; n j is the unit tangent vector.

[0119] The normal force can be calculated as follows:

[0120] F n =k n U n (14)

[0121] In formula (14), k n —Normal contact stiffness; U n —Overlap displacement.

[0122] Among them, the normal contact stiffness is:

[0123]

[0124] In formula (15), is the normal contact stiffness of particle A; is the normal contact stiffness of particle B.

[0125] The shear force can be expressed in terms of increments:

[0126] ΔF s =k s ΔU s (16)

[0127] In formula (16), ΔF s is the shear force increment between particle A and particle B; k s is the tangential contact stiffness; ΔU s is the tangential displacement increment.

[0128] The tangential stiffness can be calculated as follows:

[0129]

[0130] In formula (17), is the tangential stiffness of particle A; is the tangential stiffness of particle B.

[0131] The relationship between the normal contact force and the tangential contact force can be expressed as follows:

[0132] F s ≤μF n (18)

[0133] In formula (18), μ is the particle friction coefficient.

[0134] For the particle bonding part, the relationship between the force, moment and displacement of each parallel bond can be expressed by equations (19) to (24). The resultant force and moment vector of the parallel bond acting on particle B can also be decomposed into normal and tangential components:

[0135]

[0136]

[0137] In formulas (19)-(20), is the total force acting on particle B in parallel bonding; is the normal force acting on particle B in the parallel bonding direction; is the tangential force acting on particle B in parallel bonding; is the total moment of parallel bonding acting on particle B; is the normal moment acting on particle B due to parallel bonding; is the tangential moment acting on particle B in parallel bonding.

[0138] The increments of force and moment can be obtained from the following equations (21) to (24):

[0139]

[0140]

[0141]

[0142]

[0143] In formulas (21)-(24), is the normal force increment acting on particle B in the parallel bonding direction; is the increment of the tangential force acting on particle B due to parallel bonding; is the normal moment increment of the parallel bonding acting on particle B; is the increment of the tangential moment acting on particle B due to parallel bonding; is the parallel bond normal stiffness; is the parallel bond tangential stiffness; A is the cross-sectional area of ​​the parallel bond; I is the moment of inertia; J is the polar moment of inertia.

[0144] The maximum normal stress and tangential stress acting on the parallel bonded cross section can be calculated using equations (25)-(26):

[0145]

[0146]

[0147] in:

[0148]

[0149]

[0150]

[0151] In formulas (25)-(29), σ max is the maximum normal stress acting on the parallel bond; τ max is the maximum tangential stress acting on the parallel bond; R is the parallel bond radius; t is the particle disk thickness; R is the expansion multiple of the parallel bonding radius; [A] is the radius of particle A; R [B] is the radius of particle B.

[0152] When the maximum normal force acting on the parallel bond reaches or exceeds the normal strength of the parallel bond, or the maximum shear force acting on the parallel bond reaches or exceeds the tangential strength of the parallel bond, the bond will undergo tensile or shear failure, and the parallel bond will be removed from between the corresponding two particles and generate a microcrack. In the particle flow model, hydraulic fractures are composed of hundreds or thousands of microcracks.

[0153] The criteria for judging tensile failure and shear failure of parallel bonding are:

[0154]

[0155]

[0156] In formulas (30) and (31), σ max is the maximum normal force acting on the parallel bond; τ max is the maximum shear force acting on parallel bonding; is the parallel bond normal strength; is the parallel bond tangential strength.

[0157] In step S2, a particle flow discrete element numerical model is constructed. The particle flow discrete element numerical simulation method can be used to simulate various macroscopic mechanical parameters. First, the microscopic mechanical parameters in the discrete element model need to be determined. There are 8 types in total, including effective modulus, normal to tangential stiffness ratio, friction coefficient, normal bond strength, bond force, friction angle, softening factor and normal strength softening factor. The above parameters cannot be directly obtained through experiments, but need to be obtained through numerical simulation correction. The particle flow discrete element numerical model needs to be calibrated with particle flow microscopic mechanical parameters, and the "trial and error method" is used to obtain the size of the material microscopic mechanical parameters. Specifically, the macroscopic mechanical parameters obtained by discrete element simulation are highly consistent with the results of indoor rock mechanics experiments by adjusting the microscopic mechanical parameters one by one. At this time, the corresponding microscopic mechanical parameters are the basic input parameters of discrete element numerical simulation. Then, based on the input parameters, the mechanical parameters that cannot be measured by traditional indoor experiments can be simulated, that is, the particle flow microscopic mechanical parameters are calibrated and the particle flow discrete element numerical simulation is carried out, that is, step S3 is entered;

[0158] Specifically, in step S3, the particle flow micromechanical parameter calibration is performed based on the construction of the particle flow discrete element numerical model and the macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak surface, including:

[0159] Step B1, calibrating the uniaxial compression model parameters based on the particle flow discrete element numerical model and the macroscopic uniaxial elastic modulus of the structural weak surface;

[0160] Step B2: Calibrate the parameters of the Brazilian disk model based on the particle flow discrete element numerical model and the macroscopic fracture toughness of the structural weak surface.

[0161] Specifically, in step B1, the uniaxial compression model parameters are calibrated based on the particle flow discrete element numerical model and the macroscopic uniaxial elastic modulus of the structural weak surface, including:

[0162] Based on the particle flow discrete element numerical model and the macroscopic uniaxial elastic modulus of the structural weak surface, the parameters of the uniaxial compression model are calibrated, that is, a uniaxial compression model is established based on the particle flow discrete element method, and the model size is 50 mm×25 mm. In the process of parameter calibration, the microscopic effective modulus, normal to tangential stiffness ratio, cohesion, softening factor, normal strength softening factor, and normal cohesion strength of the particles are changed so that the simulated uniaxial elastic modulus is close to the macroscopic uniaxial elastic modulus of the structural weak surface converted based on the indentation experiment (obtained in step A2 in step S1), that is, the values ​​of the microscopic effective modulus, normal to tangential stiffness ratio, cohesion, softening factor, normal strength softening factor, and normal cohesion strength at this time are determined as calibration parameters (that is, uniaxial compression model parameters);

[0163] In step B2, the parameters of the Brazilian disk model are calibrated based on the particle flow discrete element numerical model and the macroscopic fracture toughness of the structural weak surface, that is, a Brazilian disk model is established based on the particle flow discrete element method, the disk diameter is 50 mm, and the preset straight seam length is 33 mm; by changing the normal bonding strength and bonding force of the particles, the fracture toughness obtained by the numerical simulation of the Brazilian splitting of the core model (preset straight seam) is close to the macroscopic fracture toughness of the structural weak surface converted based on the indentation experiment (obtained in step A2 in step S1), and the values ​​of the normal bonding strength and bonding force at this time are determined as calibration parameters (i.e., Brazilian disk model parameters);

[0164] In an embodiment of the present invention, based on the above-mentioned particle flow discrete element numerical model, the particle flow micromechanical parameters are calibrated to obtain the most matching micromechanical parameters, so that the error rate between the simulated macroscopic uniaxial elastic modulus and macroscopic fracture toughness and the conversion result in step A2 does not exceed 6%. The results are shown in Table 1.

[0165] Table 1

[0166]

[0167] The calibrated microscopic mechanical parameters are used as input parameters to simulate the mechanical parameters that cannot be measured by traditional indoor experiments. In order to determine the triaxial mechanical parameters (elastic modulus, Poisson's ratio, compressive strength, cohesion and internal friction angle) and tensile strength of the structural weak surface, it is necessary to carry out triaxial compression numerical simulation and Brazilian splitting numerical simulation (without preset straight seams), that is, enter step S4;

[0168] Specifically, in step S4, the step of obtaining the mechanical parameters of the structural weak surface based on the particle flow micromechanical parameter calibration includes:

[0169] Step C1, based on the calibration of particle flow micromechanical parameters, carry out particle flow discrete element numerical simulation;

[0170] Step C2: Based on the particle flow discrete element numerical simulation, the mechanical parameters of the structural weak surface are obtained.

[0171] Specifically, in step C1, based on the calibration of particle flow micromechanical parameters, discrete element numerical simulation of particle flow is carried out, including:

[0172] Step D1, performing triaxial compression numerical simulation on the triaxial compression model to obtain a stress-strain curve;

[0173] Step D2, carry out Brazilian splitting numerical simulation on the Brazilian disk model to obtain the load magnitude and specimen displacement variation curve.

[0174] Specifically, in step D1, the triaxial compression model is subjected to triaxial compression numerical simulation to obtain a stress-strain curve. That is, in the numerical experiment, the model size of the triaxial compression specimen is 50 mm × 25 mm. The above two models are composed of a number of particles, and the particle sizes are not completely consistent. It is generally believed that the particle sizes in the rock model follow a random uniform distribution, and the maximum particle radius is in a certain ratio to the minimum particle radius. Then, the microscopic mechanical parameters of each particle are assigned according to the calibration results of step S3;

[0175] For the triaxial compression model, the model boundary consists of four walls, which are located at the four sides of a rectangle with a geometric size of 50mm×25mm. Constraint stress is applied through the walls at the model boundary to simulate the confining pressure. The wall located at one short side is fixed, and the other three walls are movable; first, the two walls located at the two long sides are used to approach the sample at a certain speed and press the sample (simulating radial pressure) until the applied stress reaches the formation confining pressure value, and then the loading speed of the model wall is adjusted through the servo mechanism to keep the confining pressure constant;

[0176] The boundary constraint velocity of the wall is:

[0177] v=GΔσ (32)

[0178] In formula (32), v is the boundary constraint velocity of the wall; G is the incremental parameter estimated based on the Fish function; Δσ is the constraint stress increment.

[0179] in,

[0180]

[0181] In formula (33), η is the magnification factor, which is generally taken as 0.5; A is the contact area between the wall and the particles; k n is the average stiffness of the contact surface; N c is the number of contact surfaces between the wall and the particles; Δt is the time step.

[0182] After stress is applied to the two sides of the specimen, the movable wall on the other short side is used to approach the specimen at the same speed and compact it (simulating axial loading). The stress (axial pressure) applied by the wall and the strain of the specimen are recorded until the specimen is fully destroyed and the wall movement is stopped. The stress and strain data are recorded during the experiment.

[0183] In the embodiment of the present invention, the ratio of the maximum and minimum particle sizes of the particles is set to 1.66, the average particle radius is 0.24 mm, and the particle density is 2600 kg / m 3 The micromechanical parameters obtained by correction are used to assign values ​​to the particles. The radial confining pressure applied by the triaxial compression model is 20 MPa. After the model is stable, the particle flow discrete element numerical simulation is performed. The triaxial compression simulation results are shown in Figure 4 ;

[0184] Specifically, in step D2, the Brazilian splitting numerical simulation is carried out on the Brazilian disk model to obtain the load size and the sample displacement change curve, that is, for the Brazilian splitting model, the model is a disk with a diameter of 25 mm, and the distribution of the particles in the model is consistent with that described in step D1, and the microscopic mechanical parameters of each particle are assigned according to the calibration result of step S3. The model boundary consists of two upper and lower walls, which are two parallel tangents of the disk, one of which is fixed and the other is movable; first, the wall is moved toward the sample at a certain speed, and the wall movement rate is calculated by formula (32); in this process, the sample will be compacted and damaged, and the wall movement will be stopped after the sample is fully damaged, and the load size acting on the wall and the sample displacement change curve will be recorded;

[0185] In the embodiment of the present invention, the numerical simulation results of Brazilian splitting are shown in Figure 5 shown.

[0186] Specifically, in step C2, the step of obtaining mechanical parameters of the structural weak surface based on the particle flow discrete element numerical simulation includes:

[0187] Step E1, based on the stress-strain curve, obtain the triaxial compressive strength, triaxial elastic modulus, triaxial Poisson's ratio, cohesion and internal friction angle of the structural weak surface;

[0188] Step E2: Based on the load size and the specimen displacement curve, the tensile strength of the weak surface of the structure is obtained.

[0189] Specifically, in step E1, the triaxial compressive strength, triaxial elastic modulus, triaxial Poisson's ratio, cohesion and internal friction angle of the structural weak surface are obtained based on the stress-strain curve, that is, the triaxial compressive strength, triaxial elastic modulus, triaxial Poisson's ratio, cohesion and internal friction angle of the structural weak surface are calculated according to equations (34) to (38) based on the stress-strain curve obtained by the triaxial compression model in step D1:

[0190] The axial pressure at rock failure is calculated by the following formula:

[0191]

[0192] In formula (34), σ c is the axial pressure when the sample is damaged; p c is the load when the sample rock is compressed and destroyed; A is the cross-sectional area of ​​the sample specimen.

[0193] In the rock triaxial compression test, the triaxial compressive strength of the sample is expressed by the peak value of the differential stress and is calculated by the following formula:

[0194] S d =σ c -P c (35)

[0195] In formula (35), S d is the triaxial compressive strength, P c is the confining pressure applied.

[0196] The elastic modulus can be expressed as:

[0197]

[0198] In formula (36), L is the core length; ΔL is the axial strain; A is the core cross-sectional area; and F is the axial load.

[0199] Poisson's ratio can be calculated as follows:

[0200]

[0201] In formula (37), d is the core diameter; Δd is the radial strain.

[0202] By setting the confining pressure to zero, the uniaxial compressive strength can be simulated. According to the Mohr-Coulomb criterion, the cohesion and internal friction angle can be obtained by combining the triaxial compressive strength and the uniaxial compressive strength. The Mohr-Coulomb linear envelope is usually used to obtain the rock cohesion and internal friction angle. The curve equation is shown in formula (38). In the plane coordinate system τoσ, the angle between the envelope line and the σ axis is the internal friction angle, and the intercept of the curve on the τ axis is the cohesion.

[0203]

[0204] In formula (38), τ is the shear stress of sample failure; σ is the normal stress of sample failure; is the internal friction angle of the sample; C is the internal cohesion of the sample.

[0205] Specifically, in step E2, the tensile strength of the structural weak surface is obtained based on the load size and the sample displacement change curve, that is, based on the load size and the sample displacement change curve obtained by the Brazilian splitting numerical simulation in step D2, the tensile strength of the sample is calculated according to formula (39), that is, the tensile strength of the structural weak surface, which is expressed by the following formula:

[0206]

[0207] In formula (39), σ t is the tensile strength of the sample; P is the maximum load when the sample breaks; d is the diameter of the sample; L is the length of the sample.

[0208] Based on the triaxial compression numerical simulation and Brazilian splitting numerical simulation in step C1, and the calculation formulas (34)-(39) in step C2, the triaxial mechanical parameters of the structural weak surface are obtained, specifically: the triaxial elastic modulus is 32.62GPa, the triaxial Poisson's ratio is 0.172, the triaxial compressive strength is 173.8MPa, the cohesion is 9.2MPa, the internal friction angle is 25.6°, and the tensile strength of the structural weak surface is 2.62MPa. Combined with step A2 in step S1, the uniaxial elastic modulus of the structural weak surface is converted to 14.12GPa, and the macroscopic fracture toughness is 0.898MPa·m 0.5 , that is, the mechanical parameters of the weak surface of the structure are determined.

[0209] In some other embodiments of the present invention, a system for determining mechanical parameters of weak planes in complex lithofacies shale reservoir structures is provided, the mechanical parameter determination system comprising an experimental module 1, a model building module 2, a calibration module 3 and an acquisition module 4;

[0210] The experimental module 1 is used to perform a nano-indentation test on a core sample containing a structural weak surface to obtain a macroscopic uniaxial elastic modulus and a macroscopic fracture toughness of the structural weak surface;

[0211] The model building module 2 is used to build a particle flow discrete element numerical model;

[0212] The calibration module 3 is used to calibrate the particle flow micromechanical parameters based on the construction of the particle flow discrete element numerical model and the macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak surface;

[0213] The acquisition module 4 is used to obtain the mechanical parameters of the structural weak surface based on the calibration of the particle flow micromechanical parameters.

[0214] In some other embodiments of the present invention, a computer-readable storage medium is provided, on which a computer program is stored, characterized in that when the computer program is executed by a processor, the method for determining the weak plane mechanical parameters of the complex lithofacies shale reservoir structure described in the above embodiments is implemented.

[0215] According to an embodiment of the present invention, the computer-readable storage medium may be a non-volatile computer-readable storage medium, for example, may include but is not limited to: a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), a portable compact disk read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination thereof. In the present invention, a computer-readable storage medium may be any tangible medium containing or storing a program, which may be used by or in combination with an instruction execution system, apparatus, or device.

[0216] 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 make equivalent substitutions for some of the technical features therein; and these modifications or substitutions do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for determining mechanical parameters of weak planes in complex lithofacies shale reservoir structures, characterized in that: include: Nanoindentation tests were performed on core samples containing structural weak planes to obtain the macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak planes. Construct discrete element numerical model of particle flow; Based on the construction of the particle flow discrete element numerical model and the macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak surface, the particle flow micromechanical parameters are calibrated; Based on the calibration of particle flow micromechanical parameters, the mechanical parameters of the structural weak surface are obtained.

2. The method for determining mechanical parameters of weak planes in complex lithofacies shale reservoir structures according to claim 1, characterized in that: The core sample containing the structural weak surface is obtained by radial or axial drilling along the full-diameter core of the well, and the structural weak surface should intersect with the end face of the core.

3. The method for determining mechanical parameters of weak planes in complex lithofacies shale reservoir structures according to claim 1, characterized in that: The nanoindentation test is performed on the core sample containing the structural weak surface to obtain the macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak surface, including: Nanoindentation tests were performed on core samples containing structural weak planes to obtain the mesoscopic elastic modulus and mesoscopic fracture toughness. Based on the conversion relationship between meso elastic modulus, meso fracture toughness and macro-micro mechanical parameters, the macroscopic uniaxial elastic modulus and macroscopic fracture toughness are converted.

4. The method for determining mechanical parameters of weak planes in complex lithofacies shale reservoir structures according to claim 3, characterized in that: The acquisition of the conversion relationship between macroscopic and microscopic mechanical parameters includes: Uniaxial compression test and nanoindentation test were carried out on two groups of matrix core samples with the same sampling position to obtain the macroscopic uniaxial elastic modulus and microscopic elastic modulus of the matrix. Based on the macroscopic uniaxial elastic modulus and microscopic elastic modulus of the matrix, a conversion relationship between the macroscopic and microscopic elastic moduli is obtained; Fracture toughness test and nanoindentation test were carried out on two groups of matrix core samples with the same sampling position to obtain the macroscopic and microscopic fracture toughness of the matrix; Based on the macroscopic and microscopic fracture toughness of the matrix, the conversion relationship between the macroscopic and microscopic fracture toughness is obtained.

5. The method for determining mechanical parameters of weak planes in complex lithofacies shale reservoir structures according to any one of claims 1 to 4, characterized in that: The particle flow micromechanical parameter calibration is performed based on the construction of a particle flow discrete element numerical model and the macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak surface, including: Based on the particle flow discrete element numerical model and the macroscopic uniaxial elastic modulus of the structural weak surface, the uniaxial compression model parameters are calibrated; The parameters of the Brazilian disk model are calibrated based on the particle flow discrete element numerical model and the macroscopic fracture toughness of the structural weak surface.

6. The method for determining mechanical parameters of weak planes in complex lithofacies shale reservoir structures according to claim 5, characterized in that: The method of obtaining the mechanical parameters of the structural weak surface based on the particle flow micromechanical parameter calibration includes: Based on the calibration of particle flow micromechanical parameters, discrete element numerical simulation of particle flow is carried out; Based on the particle flow discrete element numerical simulation, the mechanical parameters of the structural weak surface are obtained.

7. The method for determining mechanical parameters of weak planes in complex lithofacies shale reservoir structures according to claim 6, characterized in that: The particle flow discrete element numerical simulation based on the particle flow micromechanical parameter calibration is carried out, including: The triaxial compression model is subjected to triaxial compression numerical simulation to obtain the stress-strain curve; The Brazilian splitting numerical simulation was carried out on the Brazilian disk model, and the load size and specimen displacement change curve were obtained.

8. The method for determining mechanical parameters of weak planes in complex lithofacies shale reservoir structures according to claim 6, characterized in that: The method of obtaining mechanical parameters of the weak surface of the structure based on the particle flow discrete element numerical simulation includes: Based on the stress-strain curve, the triaxial compressive strength, triaxial elastic modulus, triaxial Poisson's ratio, cohesion and internal friction angle of the structural weak surface are obtained; Based on the load size and the specimen displacement change curve, the tensile strength of the weak surface of the structure is obtained.

9. A system for determining mechanical parameters of weak planes in complex lithofacies shale reservoir structures, characterized in that: It includes experimental module, model building module, calibration module and acquisition module; The experimental module is used to perform nanoindentation test on a core sample containing a structural weak surface to obtain the macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak surface; The model building module is used to build a particle flow discrete element numerical model; The calibration module is used to calibrate the particle flow micromechanical parameters based on the construction of a particle flow discrete element numerical model and the macroscopic uniaxial elastic modulus and macroscopic fracture toughness of the structural weak surface; The acquisition module is used to obtain the mechanical parameters of the structural weak surface based on the calibration of the particle flow micromechanical parameters.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the method for determining the weak plane mechanical parameters of a complex lithofacies shale reservoir structure according to any one of claims 1 to 8 is implemented.

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