A method and system for optimizing infinite-stage fracturing technology for continental shale reservoirs

Abstract

The present invention relates to a method and system for optimizing an infinite-stage fracturing process for a continental shale reservoir, comprising the steps of: determining the lithology of the continental shale in a research block and the weak plane attributes of the bedding plane of the continental shale reservoir; establishing a continental shale reservoir model based on the lithology of the continental shale and the weak plane attributes of the bedding plane of the continental shale reservoir; applying different fracturing process operation parameters to the continental shale reservoir model, and then performing perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation using a three-dimensional discrete lattice method, respectively, to obtain perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process operation parameters; and comparing the perforation fracturing numerical simulation results and the infinite-stage fracturing numerical simulation results under different fracturing process operation parameters to determine an optimal infinite-stage fracturing process solution, thereby improving the transformation effect of the continental shale reservoir and thereby improving the effect of oil and gas development.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas development, and in particular to a new infinite-stage fracturing process optimization method and system for improving the transformation effect of continental shale reservoirs. Background Art

[0002] Multi-stage fracturing technology offers advantages such as short operation time, high fracturing efficiency, cost savings, and reduced environmental pollution. However, traditional bridge plug perforation fracturing techniques suffer from inaccurate fracturing layer positioning, long cycle times, multiple wellbore entries, and difficulty ensuring perforation and fracturing efficiency. Therefore, openhole staged sliding sleeve fracturing is now widely used in China for multi-stage fracturing operations. This technique uses casing to inject fracturing fluid, cement sheaths and plugs to achieve interlayer isolation, and balls are used to open the fracturing sleeves for fracturing, resulting in highly uniform reservoir stimulation. However, the size of the casing and balls used in this technique limits the number of fracturing stages that can be achieved, and layer positioning remains unclear, making it impossible to measure pressure in each layer. The continental shale reservoirs in the Kong 2 Member of the Cangdong Sag in the Bohai Bay Basin exhibit complex and diverse lithology, highly developed stratification, and strong reservoir stress heterogeneity. In the early stages of production, horizontal wells with multi-cluster perforation completion and dense cutting fracturing were used to stimulate the continental shale reservoirs through volumetric fracturing. After on-site fracturing, acoustic emission monitoring results show low perforation initiation efficiency, large variations in crack propagation, and difficulty in forming a complex fracture network. To address this issue, the present invention provides a novel, infinite-stage fracturing process optimization method and system for improving the transformation of continental shale reservoirs. Summary of the Invention

[0003] The present invention aims to provide a method and system for optimizing infinite-stage fracturing in continental shale reservoirs. Using a three-dimensional discrete lattice method, numerical simulations of perforation and infinite-stage fracturing are performed under different construction parameters. The simulations are then compared under different construction parameters. Combined with the simulation results, an optimal infinite-stage fracturing process solution is derived. This approach addresses the low perforation initiation efficiency, large variations in fracture propagation, and difficulty in forming complex fracture networks that are common in conventional fracturing processes.

[0004] To achieve the above object, the present invention provides the following solutions:

[0005] A method for optimizing an infinite-stage fracturing process for a continental shale reservoir, comprising:

[0006] Determine the lithology of the continental shale in the study area and the weak plane properties of the continental shale reservoir; the lithology of the continental shale includes laminated felsic shale and layered lime-dolomite shale; the weak plane properties of the continental shale reservoir include the tensile strength and shear strength of the bedding fractures;

[0007] Establishing a continental shale reservoir model based on the lithology of the continental shale and the weak plane attributes of the bedding plane of the continental shale reservoir;

[0008] After applying different fracturing process operation parameters to the continental shale reservoir model, perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation are respectively performed using a three-dimensional discrete lattice method, and perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process operation parameters are obtained; the fracturing process operation parameters include the initial crack aperture, the number of fracturing construction clusters and the cluster spacing, the fracturing fluid viscosity, and the fracturing pump injection and discharge rate; the perforation fracturing numerical simulation results and the infinite-stage fracturing numerical simulation results both include the average fracture pressure of each cluster, the induced stress of each cluster, the effective proportion of fracturing cracks, the uniformity of the stimulation volume of each cluster, and the average reservoir stimulation volume;

[0009] The optimal solution for the infinite-stage fracturing process is determined by comparing the numerical simulation results of the perforation fracturing and the numerical simulation results of the infinite-stage fracturing under different fracturing process operation parameters.

[0010] Optionally, the three-dimensional discrete lattice method is used to perform perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation respectively, and the perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process operation parameters are obtained, specifically including:

[0011] A three-dimensional discrete lattice model is constructed using rock particles as nodes and the contacts between the rock particles as springs between the nodes. Fluid units are located at the center of the springs where tensile-shear failure occurs, and the fluid units are connected by fluid pipes.

[0012] The three-dimensional discrete lattice model is used to perform perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation, respectively, to obtain perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process construction parameters; the three-dimensional discrete lattice model includes a solid mechanics model and a fluid flow model; a fluid-solid coupling process exists between the solid mechanics model and the fluid flow model.

[0013] Optionally, the three-dimensional discrete lattice model is used to perform perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation respectively, to obtain perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process operation parameters, specifically including:

[0014] For perforation fracturing or infinite-stage fracturing, the fluid pressure of each cluster is calculated using the fluid flow model within the preset time step, and a pumping curve for each cluster is constructed; the abscissa of the pumping curve is the preset time step, and the ordinate is the fluid pressure corresponding to each cluster at each time step;

[0015] Determine the highest point of each pumping curve, and calculate the average bursting pressure of each cluster based on each highest point and the number of clusters; the highest point of the pumping curve is the bursting pressure of the corresponding cluster;

[0016] For the perforation fracturing or the infinite-stage fracturing, within the preset time step, respectively calculate the final fracture width of each cluster under the perforation fracturing and the infinite-stage fracturing according to the fluid flow model and the fluid-solid coupling process;

[0017] Calculating the fracture volume according to the final fracture width of each cluster, and calculating the effective fracture ratio of the fracture according to the fracture volume;

[0018] Calculating the reservoir stimulation volume of each cluster according to the fracture volume of each cluster;

[0019] Calculating an average reservoir stimulation volume according to the reservoir stimulation volumes of each cluster;

[0020] Calculating the uniformity of the transformed volume of each cluster according to the reservoir transformed volume of each cluster and the mean value of the reservoir transformed volume;

[0021] For the perforation fracturing or the infinite-stage fracturing, the induced stress of each cluster is calculated based on the unit normal vectors of the nodes at both ends of the spring where tensile-shear failure occurs and the initial stress of the reservoir.

[0022] Optionally, the calculating of the final fracture width of each cluster under perforation fracturing and infinite-stage fracturing according to the fluid flow model and the fluid-solid coupling process within the preset time step specifically includes:

[0023] At the nth time step, the crack width corresponding to the nth time step is calculated using the fluid flow model; n=1, 2, ..., H;

[0024] adjusting the fluid-solid coupling process of n time steps according to the crack width corresponding to the n time steps, and updating the fluid flow model according to the fluid-solid coupling process of n time steps;

[0025] Calculate the fracture width corresponding to the n+1 time step using the updated fluid flow model;

[0026] When n+1 is the preset time step value, the crack width corresponding to the n+1 time step is the final crack width;

[0027] When n+1 is less than the preset time step value H, n=n+1 is set, and the process returns to step “at the nth time step, using the fluid flow model to calculate the crack width corresponding to the nth time step”.

[0028] Optionally, the expression for calculating the crack width using the fluid flow model is:

[0029]

[0030] k r =s 2 (3-2s)

[0031] Where q represents the fluid flow rate between two fluid units; β is the dimensionless coefficient; k r represents relative permeability; a represents crack width; μ represents fluid viscosity; P A and P B Represent the fluid pressure at fluid unit A and fluid unit B respectively; ρ w represents the fluid density; g represents the acceleration due to gravity; z A and z B represent the elevations of fluid unit A and fluid unit B respectively; s represents water saturation.

[0032] Optionally, the calculation expression for the effective fracture ratio is:

[0033]

[0034] Where, is the fracture volume of the jth fracture cluster; when the hydraulic fracture volume is greater than 70% of the total fracture volume divided by the number of fracture clusters N, the fracture is an effective fracture.

[0035] Optionally, the expression for the reservoir transformation volume is:

[0036]

[0037] Where, represents the reservoir transformation volume of the jth cluster.

[0038] Optionally, the expression for the transformed volume uniformity is:

[0039]

[0040] Where, is the mean reservoir transformation volume of each cluster of fractures.

[0041] The present invention also provides a continental shale reservoir infinite-stage fracturing process optimization system, comprising:

[0042] A lithology and bedding weak plane attribute determination module is used to determine the lithology and bedding weak plane attributes of the continental shale in the study area; the lithology of the continental shale includes laminated felsic shale and layered lime-dolomite shale; the bedding weak plane attributes of the continental shale reservoir include bedding fracture tensile strength and shear strength;

[0043] a continental shale reservoir model establishment module, configured to establish a continental shale reservoir model based on the lithology of the continental shale and the weak plane attributes of the bedding plane of the continental shale reservoir;

[0044] a fracturing numerical simulation module for applying different fracturing process operation parameters to the continental shale reservoir model, and then performing perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation respectively using a three-dimensional discrete lattice method to obtain perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process operation parameters; the fracturing process operation parameters include the initial fracture aperture, the number of fracturing construction clusters and the cluster spacing, the fracturing fluid viscosity, and the fracturing pump injection and discharge rate; the perforation fracturing numerical simulation results and the infinite-stage fracturing numerical simulation results both include the average fracture pressure of each cluster, the induced stress of each cluster, the effective proportion of fracturing fractures, the uniformity of the stimulated volume of each cluster, and the average reservoir stimulated volume;

[0045] The module for determining the optimal fracturing solution is used to compare the numerical simulation results of the perforation fracturing and the numerical simulation results of the infinite-stage fracturing under different fracturing process operation parameters to determine the optimal solution of the infinite-stage fracturing process.

[0046] Optionally, the fracturing numerical simulation module specifically includes:

[0047] A three-dimensional discrete lattice model is constructed using rock particles as nodes and the contacts between the rock particles as springs between the nodes. Fluid units are located at the center of the springs where tensile-shear failure occurs, and the fluid units are connected by fluid pipes.

[0048] The three-dimensional discrete lattice model is used to perform perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation, respectively, to obtain perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process construction parameters; the three-dimensional discrete lattice model includes a solid mechanics model and a fluid flow model; a fluid-solid coupling process exists between the solid mechanics model and the fluid flow model.

[0049] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0050] The present invention relates to a method and system for optimizing infinite-stage fracturing technology for continental shale reservoirs, comprising the following steps: determining the lithology of the continental shale in the research block and the weak plane attributes of the bedding plane of the continental shale reservoir; establishing a continental shale reservoir model according to the lithology of the continental shale and the weak plane attributes of the bedding plane of the continental shale reservoir; applying different fracturing process operation parameters to the continental shale reservoir model, and performing perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation respectively using a three-dimensional discrete lattice method, and obtaining the perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process operation parameters. The results are as follows: the numerical simulation results of the perforation fracturing and the numerical simulation results of the infinite-stage fracturing under different fracturing process construction parameters are compared to determine the optimal infinite-stage fracturing process scheme; by using the three-dimensional discrete lattice method to perform perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation respectively under different construction parameters, comparing the infinite-stage fracturing numerical simulation under different construction parameters, and combining the numerical simulation results of perforation fracturing to obtain the optimal infinite-stage fracturing process scheme, which can solve the problems of low perforation initiation efficiency, large crack expansion differences, and difficulty in forming a complex fracture network in conventional fracturing processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0052] Figure 1 The infinite-stage fracturing process construction flow chart provided by the present invention;

[0053] Figure 2 A flow chart of a method for optimizing an infinite-stage fracturing process for continental shale reservoirs provided in Example 1 of the present invention;

[0054] Figure 3 Modeling of different lithologies of continental shale provided in Example 1 of the present invention;

[0055] Figure 4 The field-scale perforation and infinite-stage clustered fracturing modeling provided in Example 1 of the present invention;

[0056] Figure 5 A diagram of a three-dimensional discrete lattice method fluid flow model provided in Example 1 of the present invention;

[0057] Figure 6 A comparison chart of average fracture pressure and reservoir transformation volume uniformity under two different fracturing processes and different lithologies and construction conditions after the pump injection is completed provided in Example 1 of the present invention;

[0058] Figure 7This is a comparison chart of the effective fracture ratio and average reservoir stimulation volume under two different fracturing processes and different lithologies and construction conditions after the pumping is completed provided in Example 1 of the present invention. DETAILED DESCRIPTION

[0059] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. 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 making creative efforts are within the scope of protection of the present invention.

[0060] like Figure 1 As shown, unlimited-stage fracturing technology involves gradually deploying soluble plugs to initiate unlimited-stage sliding sleeve fracturing. Simultaneously, pressure is transmitted to the next stage via a pressure-conducting pipeline. A hydraulic cylinder pushes the hydraulic plugs to form a closed ball seat, accurately fracturing the formation and achieving an unlimited number of fracturing stages from bottom to top. Field technicians aim to optimize the unlimited-stage fracturing process for the continental shale reservoirs in this block, effectively reducing the fracture initiation pressure in shales of varying lithologies and improving the uniformity of reservoir stimulation at each stage to form a complex fracture network and maximize the reservoir stimulation effect.

[0061] The present invention aims to provide a method and system for optimizing infinite-stage fracturing in continental shale reservoirs. Using this method and system, numerical simulations of perforation fracturing and infinite-stage fracturing are performed using a three-dimensional discrete lattice method under different construction parameters. The infinite-stage fracturing simulations under different construction parameters are compared, and the optimal infinite-stage fracturing process solution is derived based on the perforation fracturing simulation results. This solution addresses the low perforation initiation efficiency, large variations in crack propagation, and difficulty in forming complex fracture networks that are common in conventional fracturing processes.

[0062] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0063] Example 1

[0064] like Figure 2 As shown, this embodiment provides a method for optimizing an infinite-stage fracturing process for a continental shale reservoir, comprising:

[0065] S1: Determine the lithology of the continental shale and the weak plane attributes of the continental shale reservoir in the study area. Figure 3 As shown, the lithology of the continental shale includes laminated felsic shale and layered lime-dolomite shale; the bedding plane properties of the continental shale reservoir include bedding fracture tensile strength and shear strength.

[0066] The typical lithology of the continental shale reservoir in Kong2 Member was selected for example analysis, among which, Figure 4 As shown, the model has geometric dimensions of 300m × 300m × 18m. The reservoir lithologic layers were identified and statistically analyzed using a pre-existing field core library and lithologic distribution tables. The rock mechanical parameters of each layer were obtained using pre-existing geological exploration data combined with deep learning training, providing parameters for modeling complex lithologic reservoirs in continental shale. The tensile and shear strengths of bedding fractures were obtained based on existing literature data, providing parameters for modeling reservoirs in continental shale with developed bedding.

[0067] S2: Establishing a continental shale reservoir model according to the lithology of the continental shale and the weak plane attributes of the bedding plane of the continental shale reservoir.

[0068] S3: After applying different fracturing process construction parameters to the continental shale reservoir model, the three-dimensional discrete lattice method is used to perform perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation, respectively, to obtain the perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process construction parameters; the fracturing process construction parameters include the initial crack aperture, the number of fracturing construction clusters and the cluster spacing, the fracturing fluid viscosity, and the fracturing pump injection and discharge rate; the perforation fracturing numerical simulation results and the infinite-stage fracturing numerical simulation results both include the average fracture pressure of each cluster, the induced stress of each cluster, the effective proportion of fracturing cracks, the uniformity of the transformation volume of each cluster, and the average reservoir transformation volume.

[0069] S3 specifically includes:

[0070] S31: Rock particles are used as nodes, and the contacts between the rock particles are used as springs between the nodes; fluid units are located at the center of the springs where tensile-shear failure occurs, and the fluid units are connected by fluid pipes to construct a three-dimensional discrete lattice model.

[0071] The three-dimensional discrete lattice method uses a bonded particle model to simplify rock particles into nodes, springs to represent rock contact surfaces with elastic characteristics, and a smooth joint model to simulate the initial clusters formed by perforation or sliding sleeves and pre-existing discontinuous weak surfaces in the rock mass. The lattice points are connected to springs with normal and shear stiffness. The tension and shear of the springs correspond to the tensile and shear failure of the rock. The coin-shaped fluid units at the center of the ruptured springs can form a pipe network for fluid flow, such as Figure 5 As shown in Figure 2, a lattice spring network is composed of numerous quasi-randomly distributed nodes connected by springs, in which joints can be placed in any orientation, which can be used to accurately and efficiently characterize crack fractures.

[0072] The three-dimensional discrete lattice method mainly includes numerical models such as mechanical model and fluid flow model. The mechanical model and fluid flow model are fully coupled models, that is, there is a fluid-solid coupling process.

[0073] About the mechanical model:

[0074] The velocity at which the spring grid points are displaced is given by the following central difference equation:

[0075]

[0076] Where: and They represent the velocity and displacement of the i-th component (i=1, 2, 3) of the node at time t; ∑F i represents the resultant force of all i components acting on the node; △t is the time step; m is the node mass.

[0077] The shear or tensile failure of the spring corresponds to the shear or tensile failure of the rock. The corresponding relationship between the tensile shear strength of the micro spring and the macro rock mass is:

[0078]

[0079] Where: F Nmax With F Smax Respectively represent the breaking tension and breaking shear force of the spring; a t is the tensile strength correction factor; T and C represent the macroscopic rock mass tensile strength and shear strength, respectively; R represents the grid unit size; μ represents the friction coefficient; a s is the shear strength correction factor.

[0080] When the spring normal stress is greater than the tensile strength (F N >F Nmax ), or the spring tangential force is greater than the shear strength (F S >F Smax ), the spring undergoes tensile failure or shear failure. Microcracks are formed after the spring is damaged. At this time, the normal force and tangential force of the corresponding damaged spring are both 0, that is, F N =0 and F S =0.

[0081] For fluid flow models:

[0082] Assuming that the pipe width is equal to the pipe length, the flow rate formula of the fluid from fluid unit A to unit B along the pipe is:

[0083]

[0084] k r =s 2 (3-2s)

[0085] Where: q represents the fluid flow rate; β is the dimensionless coefficient; k r represents relative permeability; a represents crack width; μ represents fluid viscosity; P A and P BRepresent the fluid pressure at fluid unit A and fluid unit B respectively; ρ w represents the fluid density; g represents the acceleration due to gravity; z A and z B Represent the elevations of nodes A and B respectively; k r represents relative permeability; s represents water saturation.

[0086] The display calculation method is used to solve the flow evolution model that changes with time during the flow process. f The calculation formula of the flow pressure increment △P is:

[0087]

[0088] Where: △P is the flow pressure increment, Pa; Indicates the elastic modulus of the fluid; V is the volume of the node; q i Represents the flow rate of the fluid pipe connected to node i.

[0089] For the fluid-solid coupling process, the mechanically incompressible fluid fluid-solid coupling method proposed by Peter Cundall is used to couple fluid injection stress-induced fractures or pre-existing joints in the rock mass with rock deformation. This method uses rock deformation and initial fracture width to determine fracture permeability. Influenced by permeability, fluid pressure acts on the fracture surface, influencing rock deformation. Rock deformation, in turn, causes changes in fracture width and fluid pressure, which in turn lead to changes in fracture permeability.

[0090] S32: Using the three-dimensional discrete lattice model, perform perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation respectively to obtain perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process construction parameters; the three-dimensional discrete lattice model includes a solid mechanics model and a fluid flow model; there is a fluid-solid coupling process between the solid mechanics model and the fluid flow model.

[0091] In this example, a three-dimensional discrete lattice model was used to perform numerical simulations for two different fracturing processes. This yielded numerical simulation results (average fracture pressure per cluster, induced stress per cluster, effective fraction of fractured cracks, uniformity of stimulated volume per cluster, and average reservoir stimulated volume) for different fracturing processes and different operation parameters. It should be noted that the numerical simulations differed only in the fracturing processes; the method for calculating the numerical simulation results remained the same.

[0092] Specifically, step S32 includes:

[0093] S321: For perforation fracturing and infinite-stage fracturing, respectively, within the preset time step, the fluid pressure of each cluster is calculated using the fluid flow model, and a pumping curve for each cluster is constructed; the abscissa of the pumping curve is the preset time step, and the ordinate is the fluid pressure corresponding to each cluster at each time step.

[0094] like Figure 4 As shown in the figure, for perforation fracturing technology, calculations can be performed in segments and clusters due to the nature of the technology, but the data corresponding to each cluster within each segment is the same. For infinite-stage fracturing technology, the calculation is performed cluster by cluster for each stage (cluster), and the data corresponding to each cluster is generally different.

[0095] S322: Determine the highest point of each pumping curve, and calculate the average bursting pressure of each cluster based on each highest point and the number of clusters; the highest point of the pumping curve is the bursting pressure of the corresponding cluster.

[0096] S323: calculating the final fracture width of each cluster under the perforation fracturing and the infinite stage fracturing respectively within the preset time step according to the fluid flow model and the fluid-solid coupling process.

[0097] Since the mechanical model and the fluid flow model are fully coupled, the model is fully coupled. The flow of fluid in stress-induced fractures or pre-existing natural fractures is affected by permeability, and the fluid pressure acts on the surface of the rock fracture, affecting the deformation and strength of the rock. The deformation of the rock will cause the fluid pressure and width in the fracture to change, which in turn causes the permeability of the fracture to change. Therefore, due to the fluid-solid coupling process, during the pumping process, the fluid pressure and rock deformation affect each other, causing the crack width to change continuously. Therefore, it is necessary to use the initial crack width of the fluid flow model, and then combine the fluid-solid coupling process to continuously update the fluid flow model, and finally calculate the final crack width after the pumping is completed. Specifically, step S323 includes:

[0098] S323-1: At the nth time step, the fluid flow model is used to calculate the crack width corresponding to the nth time step; n=1, 2, ..., H.

[0099] S323-2: adjusting the fluid-solid coupling process for n time steps according to the crack width corresponding to the n time steps, and updating the fluid flow model according to the fluid-solid coupling process for the n time steps.

[0100] S323-3: Calculate the fracture width corresponding to the n+1 time step using the updated fluid flow model.

[0101] S323-4: When n+1 is a preset time step value, the crack width corresponding to the n+1 time step is the final crack width.

[0102] S323-5: When n+1 is less than the preset time step value H, set n=n+1 and return to step S323-1.

[0103] S34: Calculating the fracture volume according to the final fracture width of each cluster, and calculating the proportion of effective fractures according to the fracture volume.

[0104] The calculation expression for the effective fracture ratio of the fracturing is:

[0105]

[0106] Where, is the fracture volume of the jth fracture cluster; when the fracture volume is greater than 70% of the ideal fracture volume (the total fracture volume divided by the number of fracture clusters N), the fracture is an effective fracture.

[0107] After confirming whether the cracks are effective cracks, the proportion of effective fractures can be further determined.

[0108] S35: Calculating the reservoir transformation volume of each cluster according to the fracture volume of each cluster.

[0109] The expression of the reservoir transformation volume is:

[0110]

[0111] represents the reservoir transformation volume of the jth cluster.

[0112] S36: Calculating an average reservoir stimulation volume based on the reservoir stimulation volumes of each cluster.

[0113] The sum of the reservoir stimulation volumes of each cluster divided by the number of clusters is the average reservoir stimulation volume.

[0114] S37: Calculating the uniformity of the transformed volume of each cluster according to the reservoir transformed volume of each cluster and the mean value of the reservoir transformed volume.

[0115] The expression of the transformation volume uniformity is:

[0116]

[0117] Where, is the mean reservoir transformation volume of each cluster of fractures.

[0118] The average reservoir stimulation volume, fracturing efficiency (percentage), and uniformity of stimulation volume for each cluster are calculated based on the fracture geometry. The fracture geometry can be visualized using the three-dimensional displacement coordinate points calculated by coupling the mechanical model, the fluid model, and the calculation.

[0119] S38: For the perforation fracturing or the infinite-stage fracturing, the induced stress of each cluster is calculated according to the unit normal vectors of the nodes at both ends of the spring where tensile-shear failure occurs and the initial stress of the reservoir.

[0120] The specific expression is:

[0121]

[0122] Where: σ h,w Represents the three-dimensional coordinate system ground stress components of a node in the h direction component (h = X, Y, Z) and the w direction component (w = X, Y, Z); the unit normal vector n of the node h X =(u h X -u h Y ) / u h X -u h Y (i.e. the displacement vector u of the two nodes in the X, Y, Z directions h X ,u h Y ,u z Z The difference between the displacement vector in the corresponding tangential direction and its absolute value); represents the dot product (projection) of the unit normal vectors of the nodes P and Q at both ends of the spring in the X, Y, and Z directions respectively; σ X ,σ Y ,σ Z is the input initial triaxial stress of the formation.

[0123] According to the above expression, the stress component σ of each node in the fluid-solid coupling process can be calculated: h,w The fracturing-induced stress field of each cluster can be obtained based on the unit normal vectors of the nodes at both ends of the spring where tensile-shear failure occurs at the specific location where each cluster of cracks is formed and the initial stress of the reservoir.

[0124] S4: comparing the numerical simulation results of the perforation fracturing and the numerical simulation results of the infinite-stage fracturing under different fracturing process operation parameters to determine an optimal solution for the infinite-stage fracturing process.

[0125] In this embodiment, the average rupture pressure of each cluster under perforation fracturing, the distribution of induced stress of each cluster, the average reservoir transformation volume, the efficiency (percentage) of fracturing cracks and the uniformity of transformation volume of each cluster under different construction parameters, as well as the average rupture pressure of each cluster under infinite-stage fracturing, the distribution of induced stress of each cluster, the average reservoir transformation volume, the efficiency (percentage) of fracturing cracks and the uniformity of transformation volume of each cluster under infinite-stage fracturing are obtained. Not only can the numerical simulation results of infinite-stage fracturing under different construction parameters be compared, but also the numerical simulation results of perforation fracturing can be compared, so that the optimal infinite-stage fracturing process (optimal construction parameters) can be obtained to guide on-site infinite-stage fracturing technology. Figure 6 As shown in the figure, after the pumping is completed, the average fracture pressure and reservoir transformation volume uniformity of the two different fracturing processes and different lithology and construction conditions are compared. Figure 7 As shown in the figure, after the pumping is completed, the effective fracture ratio and average reservoir stimulation volume of the two different fracturing processes with different lithology and operation conditions are compared. These simulation results are combined to obtain an optimal infinite-stage fracturing process solution.

[0126] This embodiment also has the following advantages:

[0127] (1) Based on the characteristics of the continental shale reservoir in Kong II, the fracture propagation law under the conditions of different lithology and bedding weak plane properties of shale formations was studied.

[0128] (2) Based on the horizontal wells of the typical continental shale reservoir in Kong No. 2, we carried out research on the optimization scheme of infinite-stage fracturing. Through the numerical simulation method, we studied the influence of the initial fracture opening, the number of stages and the stage spacing, the pumping displacement and the viscosity of the fracturing fluid on the reservoir transformation effect, deepened the understanding of the expansion law of infinite-stage fracturing cracks, and provided guidance for the formulation of the optimization scheme of infinite-stage fracturing for the horizontal wells of the typical continental shale in Kong No. 2.

[0129] (3) For horizontal wells in the typical continental shale reservoir of Kong II, a field-scale numerical simulation of infinite-stage sliding sleeve fracturing crack expansion was carried out, and the reasons for the differences in volume uniformity and fracture pressure of each stage of reservoir transformation under different construction parameters (including: initial fracture aperture, number of stages and stage spacing, pump injection rate and fracturing fluid viscosity) were analyzed, providing a favorable basis for the optimization of infinite-stage fracturing construction parameters.

[0130] (4) The three-dimensional discrete lattice method builds a model by discretizing the simulated area into point masses and springs connecting the point masses. Its computational efficiency is greatly improved compared to the discrete element method. It can directly simulate the expansion of cracks under true three-dimensional conditions.

[0131] Example 2

[0132] This embodiment provides a continental shale reservoir infinite-stage fracturing process optimization system, comprising:

[0133] The lithology and bedding weak plane attribute determination module M1 is used to determine the lithology of the continental shale in the study area and the bedding weak plane attributes of the continental shale reservoir; the lithology of the continental shale includes laminated felsic shale and layered lime-dolomite shale; the bedding weak plane attributes of the continental shale reservoir include bedding fracture tensile strength and shear strength;

[0134] A continental shale reservoir model establishment module M2 is used to establish a continental shale reservoir model based on the lithology of the continental shale and the weak plane attributes of the bedding plane of the continental shale reservoir;

[0135] A fracturing numerical simulation module M3 is configured to apply different fracturing process operation parameters to the continental shale reservoir model, and then perform perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation respectively using a three-dimensional discrete lattice method to obtain perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process operation parameters; the fracturing process operation parameters include the initial fracture aperture, the number of fracturing construction clusters and the cluster spacing, the fracturing fluid viscosity, and the fracturing pump injection and discharge rate; the perforation fracturing numerical simulation results and the infinite-stage fracturing numerical simulation results both include the average fracture pressure of each cluster, the induced stress of each cluster, the effective proportion of fracturing fractures, the uniformity of the stimulation volume of each cluster, and the average reservoir stimulation volume;

[0136] The fracturing numerical simulation module M3 specifically includes:

[0137] A three-dimensional discrete lattice model is constructed using rock particles as nodes and the contacts between the rock particles as springs between the nodes. Fluid units are located at the center of the springs where tensile-shear failure occurs, and the fluid units are connected by fluid pipes.

[0138] The three-dimensional discrete lattice model is used to perform perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation, respectively, to obtain perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process construction parameters; the three-dimensional discrete lattice model includes a solid mechanics model and a fluid flow model; a fluid-solid coupling process exists between the solid mechanics model and the fluid flow model.

[0139] The optimal fracturing solution determination module M4 is used to compare the perforation fracturing numerical simulation results and the infinite-stage fracturing numerical simulation results under different fracturing process operation parameters to determine the optimal infinite-stage fracturing process solution.

[0140] Each embodiment in this specification focuses on the differences from other embodiments, and the same or similar parts between the embodiments can be referred to each other. For the system disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0141] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only intended to help understand the method and core concept of the present invention. At the same time, those skilled in the art will find that the specific implementation methods and application scopes may vary based on the concept of the present invention. In summary, the contents of this specification should not be construed as limiting the present invention.

Claims

1. A method for optimizing infinite-stage fracturing process for continental shale reservoirs, characterized in that: include: Determine the lithology of the continental shale and the weak plane properties of the continental shale reservoir in the study area; The lithology of the continental shale includes laminated felsic shale and layered dolomitic shale; The weak plane properties of the continental shale reservoir layer include bedding fracture tensile strength and shear strength; Establishing a continental shale reservoir model based on the lithology of the continental shale and the weak plane attributes of the bedding plane of the continental shale reservoir; After applying different fracturing process operation parameters to the continental shale reservoir model, perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation are respectively performed using a three-dimensional discrete lattice method, and perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process operation parameters are obtained; the fracturing process operation parameters include the initial crack aperture, the number of fracturing construction clusters and the cluster spacing, the fracturing fluid viscosity, and the fracturing pump injection and discharge rate; the perforation fracturing numerical simulation results and the infinite-stage fracturing numerical simulation results both include the average fracture pressure of each cluster, the induced stress of each cluster, the effective proportion of fracturing cracks, the uniformity of the stimulation volume of each cluster, and the average reservoir stimulation volume; Comparing the numerical simulation results of the perforation fracturing and the numerical simulation results of the infinite-stage fracturing under different fracturing process operation parameters to determine the optimal solution for the infinite-stage fracturing process; The three-dimensional discrete lattice method is used to perform perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation respectively, and the perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process operation parameters are obtained, specifically including: A three-dimensional discrete lattice model is constructed using rock particles as nodes and the contacts between the rock particles as springs between the nodes. Fluid units are located at the center of the springs where tensile-shear failure occurs, and the fluid units are connected by fluid pipes. The three-dimensional discrete lattice model is used to perform perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation, respectively, to obtain perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process operation parameters; the three-dimensional discrete lattice model includes a solid mechanics model and a fluid flow model; there is a fluid-solid coupling process between the solid mechanics model and the fluid flow model; The three-dimensional discrete lattice model is used to perform perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation respectively, and the perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process operation parameters are obtained, specifically including: For perforation fracturing or infinite-stage fracturing, the fluid pressure of each cluster is calculated using the fluid flow model within a preset time step, and a pumping curve for each cluster is constructed; the abscissa of the pumping curve is the preset time step, and the ordinate is the fluid pressure corresponding to each cluster at each time step; Determine the highest point of each pumping curve, and calculate the average bursting pressure of each cluster based on each highest point and the number of clusters; the highest point of the pumping curve is the bursting pressure of the corresponding cluster; For the perforation fracturing or the infinite-stage fracturing, within the preset time step, respectively calculate the final fracture width of each cluster under the perforation fracturing and the infinite-stage fracturing according to the fluid flow model and the fluid-solid coupling process; Calculating the fracture volume according to the final fracture width of each cluster, and calculating the effective fracture ratio of the fracture according to the fracture volume; Calculating the reservoir stimulation volume of each cluster according to the fracture volume of each cluster; Calculating an average reservoir stimulation volume according to the reservoir stimulation volumes of each cluster; Calculating the uniformity of the transformed volume of each cluster according to the reservoir transformed volume of each cluster and the mean value of the reservoir transformed volume; For the perforation fracturing or the infinite-stage fracturing, the induced stress of each cluster is calculated based on the unit normal vectors of the two end nodes of the spring where tensile-shear failure occurs and the initial stress of the reservoir.

2. The method according to claim 1, characterized in that The method of calculating the final fracture width of each cluster under perforation fracturing and infinite-stage fracturing according to the fluid flow model and the fluid-solid coupling process within the preset time step specifically includes: At the nth time step, the crack width corresponding to the nth time step is calculated using the fluid flow model; n=1, 2, ..., H; adjusting the fluid-solid coupling process of n time steps according to the crack width corresponding to the n time steps, and updating the fluid flow model according to the fluid-solid coupling process of n time steps; Calculate the fracture width corresponding to the n+1 time step using the updated fluid flow model; When n+1 is the preset time step value, the crack width corresponding to the n+1 time step is the final crack width; When n+1 is less than the preset time step value H, n=n+1 is set, and the process returns to step "at the nth time step, calculating the crack width corresponding to the nth time step using the fluid flow model".

3. The method according to claim 2, characterized in that The expression for calculating the crack width using the fluid flow model is: k r =s 2 (3-2s) Where q represents the fluid flow rate between two fluid units; β is the dimensionless coefficient; k r represents relative permeability; a represents crack width; μ represents fluid viscosity; P A and P B Represent the fluid pressure at fluid unit A and fluid unit B respectively; ρ w represents the fluid density; g represents the acceleration due to gravity; z A and z B represent the elevations of fluid unit A and fluid unit B respectively; s represents water saturation.

4. The method according to claim 3, characterized in that The calculation expression for the effective fracture ratio of the fracturing is: Where, is the fracture volume of the jth fracture cluster; when the hydraulic fracture volume is greater than 70% of the total fracture volume divided by the number of fracture clusters N, the fracture is an effective fracture.

5. The method according to claim 4, characterized in that The expression of the reservoir transformation volume is: Where, represents the reservoir transformation volume of the jth cluster.

6. The method according to claim 5, characterized in that The expression of the transformation volume uniformity is: Where, is the mean reservoir transformation volume of each cluster of fractures.

7. A system based on the method according to any one of claims 1 to 6, characterized in that: include: The lithology and bedding weak plane attribute determination module is used to determine the lithology and bedding weak plane attributes of the continental shale in the study area; The lithology of the continental shale includes laminated felsic shale and layered dolomitic shale; The weak plane properties of the continental shale reservoir layer include bedding fracture tensile strength and shear strength; a continental shale reservoir model establishment module, configured to establish a continental shale reservoir model based on the lithology of the continental shale and the weak plane attributes of the bedding plane of the continental shale reservoir; a fracturing numerical simulation module for applying different fracturing process operation parameters to the continental shale reservoir model, and then performing perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation respectively using a three-dimensional discrete lattice method to obtain perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process operation parameters; the fracturing process operation parameters include the initial fracture aperture, the number of fracturing construction clusters and the cluster spacing, the fracturing fluid viscosity, and the fracturing pump injection and discharge rate; the perforation fracturing numerical simulation results and the infinite-stage fracturing numerical simulation results both include the average fracture pressure of each cluster, the induced stress of each cluster, the effective proportion of fracturing fractures, the uniformity of the stimulated volume of each cluster, and the average reservoir stimulated volume; The three-dimensional discrete lattice method is used to perform perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation respectively, and the perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process operation parameters are obtained, specifically including: A three-dimensional discrete lattice model is constructed using rock particles as nodes and the contacts between the rock particles as springs between the nodes. Fluid units are located at the center of the springs where tensile-shear failure occurs, and the fluid units are connected by fluid pipes. The three-dimensional discrete lattice model is used to perform perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation, respectively, to obtain perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process operation parameters; the three-dimensional discrete lattice model includes a solid mechanics model and a fluid flow model; there is a fluid-solid coupling process between the solid mechanics model and the fluid flow model; The three-dimensional discrete lattice model is used to perform perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation respectively, and the perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process operation parameters are obtained, specifically including: For perforation fracturing or infinite-stage fracturing, the fluid pressure of each cluster is calculated using the fluid flow model within a preset time step, and a pumping curve for each cluster is constructed; the abscissa of the pumping curve is the preset time step, and the ordinate is the fluid pressure corresponding to each cluster at each time step; Determine the highest point of each pumping curve, and calculate the average bursting pressure of each cluster based on each highest point and the number of clusters; the highest point of the pumping curve is the bursting pressure of the corresponding cluster; For the perforation fracturing or the infinite-stage fracturing, within the preset time step, respectively calculate the final fracture width of each cluster under the perforation fracturing and the infinite-stage fracturing according to the fluid flow model and the fluid-solid coupling process; Calculating the fracture volume according to the final fracture width of each cluster, and calculating the effective fracture ratio of the fracture according to the fracture volume; Calculating the reservoir stimulation volume of each cluster according to the fracture volume of each cluster; Calculating an average reservoir stimulation volume according to the reservoir stimulation volumes of each cluster; Calculating the uniformity of the transformed volume of each cluster according to the reservoir transformed volume of each cluster and the mean value of the reservoir transformed volume; For the perforation fracturing or the infinite-stage fracturing, calculating the induced stress of each cluster according to the unit normal vectors of the two end nodes of the spring where tensile-shear failure occurs and the initial stress of the reservoir; The module for determining the optimal fracturing solution is used to compare the numerical simulation results of the perforation fracturing and the numerical simulation results of the infinite-stage fracturing under different fracturing process operation parameters to determine the optimal solution of the infinite-stage fracturing process.

8. The system according to claim 7, characterized in that The fracturing numerical simulation module specifically includes: A three-dimensional discrete lattice model is constructed using rock particles as nodes and the contacts between the rock particles as springs between the nodes. Fluid units are located at the center of the springs where tensile-shear failure occurs, and the fluid units are connected by fluid pipes. The three-dimensional discrete lattice model is used to perform perforation fracturing numerical simulation and infinite-stage fracturing numerical simulation, respectively, to obtain perforation fracturing numerical simulation results and infinite-stage fracturing numerical simulation results under different fracturing process construction parameters; the three-dimensional discrete lattice model includes a solid mechanics model and a fluid flow model; a fluid-solid coupling process exists between the solid mechanics model and the fluid flow model.

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