A method for determining a hydraulic fracture propagation pattern controlled by cemented natural fractures

Abstract

A kind of method for discriminating cementation natural fracture control hydraulic fracture propagation mode, the present application belongs to the field of unconventional oil and gas reservoir fracturing reconstruction;By obtaining hydraulic diffusion parameter and crack front dynamic motion parameter, local pore fluid pressure is obtained by diffusion analysis, and then effective normal stress is obtained by correcting macroscopic normal total stress;And it is compared with the crack threshold to generate damage accumulation factor, and the interface mechanical parameter is obtained by combining the tensile reference value;Then compared with local tip tensile stress, the propagation mode result is output, and the crack threshold is updated by using feedback optimization parameter;By constructing the causal coupling link of crack tip motion-forepoling seepage-damage accumulation-strength attenuation, the actual mechanical state of natural fracture interface can be dynamically characterized, so that the accuracy of propagation mode discrimination can be greatly improved.

Description

Technical Field

[0001] This application belongs to the field of unconventional oil and gas reservoir fracturing technology, specifically a method for identifying hydraulic fracture propagation modes under the control of cemented natural fractures. Background Technology

[0002] In the volumetric fracturing stimulation of unconventional oil and gas reservoirs, the interaction between hydraulic fractures and cemented natural fractures determines the complexity of the fracture network and the production rate of a single well. Accurately identifying the propagation mode when the two meet is a core aspect of engineering design and numerical simulation. The core assumption of existing mainstream identification methods is that the cemented natural fracture interface is regarded as an absolute impermeable boundary. It is believed that before the hydraulic fracture front contacts the interface, the pore fluid pressure at the interface maintains the original formation value, and mechanical parameters such as tensile strength maintain the initial static values ​​calibrated in the laboratory.

[0003] However, this static rigidity assumption deviates fundamentally from the actual physical process. During high-pressure fracturing, the fracturing fluid generates ultra-high driving pressure within the fracture. As the hydraulic fracture approaches the natural fracture, the fracturing fluid forms a pre-pressure permeation zone ahead of the mechanical fracture tip through the reservoir micropore network. This high-pressure seepage front reaches the cemented interface before the physical contact surface, causing a significant reduction in the local effective normal stress. This results in microcrack damage occurring inside the cementing minerals before direct impact, exhibiting a seepage-damage coupling phenomenon of pre-impact attenuation.

[0004] Existing discrimination methods completely ignore the aforementioned time-varying degradation mechanism, continuously using an initial high-intensity threshold to judge penetration capability. This leads to many interactions that should actually penetrate being misjudged as deflections or stagnation. This bias results in conservative displacement and fluid volume settings in fracturing engineering design, leading to material and energy waste and hindering the refined optimization of new processes such as variable displacement and pulsed pump injection. Therefore, it is urgent to introduce a quantitative description of the pre-pore pressure permeation characteristics and the dynamic weakening effect of interface strength, breaking through traditional static assumptions and achieving more accurate discrimination results for extended modes. Summary of the Invention

[0005] The purpose of this application is to provide a method for identifying hydraulic fracture propagation modes under the control of cemented natural fractures. By constructing a causal coupling link of fracture tip movement, pre-seepage, damage accumulation, and strength decay, the method can achieve dynamic characterization of the actual mechanical state of the natural fracture interface, thereby significantly improving the accuracy of propagation mode identification.

[0006] The objective of this application can be achieved through the following technical solution: Firstly, a method for determining the propagation mode of hydraulic fractures under the control of cemented natural fractures, comprising the following steps: Obtain hydraulic diffusion parameters characterizing the physical flow constraint boundary of fracturing fluid in the matrix pores of the target reservoir; Obtain the total macroscopic normal stress acting on the interface of the target natural fracture, and extract the dynamic motion parameters of the hydraulic fracture front at the current calculation time step; Based on the hydraulic diffusion parameters and dynamic motion parameters, diffusion analysis is performed on the hydraulic fracture front to obtain the local pore fluid pressure exerted by the hydraulic fracture front on the target natural fracture interface. The macroscopic normal total stress is effectively corrected based on the local pore fluid pressure to obtain the target effective normal stress characterizing the interface stress state. The target effective normal stress is compared with a preset crack initiation threshold to generate a damage accumulation factor characterizing the degree of interface weakening. The interface mechanical parameters are obtained based on the damage accumulation factor and the preset tensile reference value of the target natural crack interface. The local tip tensile stress induced from the hydraulic fracture front to the target natural fracture interface is obtained, and the local tip tensile stress is compared with the interface mechanical parameters to output the expansion mode result at the current calculation time step; Obtain feedback optimization parameters that characterize the accuracy of the extended mode results, and update the preset crack initiation threshold accordingly.

[0007] Secondly, a system for determining the propagation mode of hydraulic fractures under the control of cemented natural fractures includes the following modules: The data extraction module is used to obtain hydraulic diffusion parameters characterizing the physical flow constraint boundary of fracturing fluid in the target reservoir matrix pores, obtain the total macroscopic normal stress acting on the target natural fracture interface, and extract the dynamic motion parameters of the hydraulic fracture front at the current calculation time step. The data processing module is used to perform diffusion analysis processing on the hydraulic fracture front based on the hydraulic diffusion parameters and dynamic motion parameters to obtain the local pore fluid pressure of the hydraulic fracture front acting on the target natural fracture interface. The data correction module is used to perform effective stress correction on the total macroscopic normal stress based on the local pore fluid pressure, so as to obtain the target effective normal stress characterizing the stress state of the interface. The damage assessment module is used to compare the target effective normal stress with a preset crack initiation threshold to generate a damage accumulation factor that characterizes the degree of interface weakening, and to obtain the interface mechanical parameters based on the damage accumulation factor and the preset tensile benchmark value of the target natural crack interface. The result discrimination module is used to obtain the local tip tensile stress induced from the hydraulic fracture front to the target natural fracture interface, and compare the local tip tensile stress with the interface mechanical parameters to output the extended mode result at the current calculation time step. The feedback optimization module is used to obtain feedback optimization parameters that characterize the accuracy of the results of the extended mode, and update the preset crack initiation threshold according to them.

[0008] Thirdly, a computer storage medium storing computer-executable instructions, which, when executed, implement the method for determining the hydraulic fracture propagation mode under the control of cemented natural fractures as described in the first aspect.

[0009] Compared with the prior art, the beneficial effects of this application are: This application introduces dynamic motion parameters of hydraulic fractures and a one-dimensional moving point source diffusion model to reconstruct local pore fluid pressure in real time, eliminating the systematic overestimation of interfacial resistance and significantly reducing the misjudgment rate of penetration modes. Based on continuous damage mechanics, a causal coupling link between seepage, stress, and damage across physical fields is constructed, enabling the tensile strength to dynamically evolve with the fracture approach method, providing a physically complete resistance threshold. The introduction of an extension velocity parameter allows the model to adaptively respond to changes in process flow rates such as variable displacement and pulsed pumping, providing underlying algorithmic support for parameter optimization. Simultaneously, feedback optimization parameters are constructed based on microseismic monitoring data, dynamically updating the fracture initiation threshold with a negative feedback closed loop, achieving autonomous model correction and improving engineering applicability and reliability. Attached Figure Description

[0010] Figure 1 This is a schematic diagram illustrating the steps of a method for determining the propagation mode of hydraulic fractures under the control of cemented natural fractures, as described in this application. Figure 2 This is a schematic diagram of a module for a system for determining the propagation mode of hydraulic fractures under the control of cemented natural fractures, as described in this application. Detailed Implementation

[0011] The technical solutions of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. The components described and shown in the accompanying drawings can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of this application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but only to illustrate selected embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application. It should be noted that similar reference numerals and letters in the following figures indicate similar items. Therefore, once an item has been defined in one figure, it does not need to be further defined and explained in subsequent figures. The terms first, second, etc. are used only for distinguishing descriptions and should not be construed as indicating or implying relative importance.

[0012] In volumetric fracturing stimulation of unconventional oil and gas reservoirs, the interaction between hydraulic fractures and naturally occurring cemented fractures in the reservoir is the core mechanical process that determines the final fracture network complexity and single-well production. Accurately identifying the propagation mode that occurs when these two types of fractures meet—whether the hydraulic fracture directly penetrates the natural fracture interface to form a penetration propagation mode, deflects and extends along the natural fracture interface to form a deflection propagation mode, or is even captured and halted by the natural fracture—is a crucial aspect of fracturing engineering design and numerical simulation.

[0013] In the existing technical field, the mainstream method for judging the aforementioned interactive behavior is based on the Renshaw-Pollard criterion (RP criterion) and its series of improved versions. Its core logic is to output the propagation mode judgment result by comparing the local tensile stress induced at the tip of the hydraulic fracture with the tensile strength of the natural fracture interface, combined with the surrounding geostress boundary conditions. The practical application of this type of discrimination criterion in engineering numerical simulators is generally based on a core default assumption: The cemented natural fracture interface is regarded as an absolute physical boundary that is impermeable. It is assumed that before the hydraulic fracture front physically contacts the cemented interface, the pore fluid pressure at the interface remains the original formation pore pressure, and the interface mechanical parameters such as tensile strength also remain the initial static values ​​obtained from laboratory core calibration without any change.

[0014] However, the aforementioned static rigidity assumption deviates significantly and fundamentally from actual underground physical processes. Under unconventional high-pressure fracturing conditions in reservoirs, fracturing fluid can generate ultra-high driving pressures on the order of tens of megapascals within the fractures. When the hydraulic fracture approaches the natural fracture interface at a certain velocity, due to the naturally existing interconnected micropore network in the reservoir matrix, the fracturing fluid forms a pre-pore pressure permeation zone in front of the mechanical fracture tip, invisible to the naked eye but physically present. This high-pressure seepage front reaches the cemented natural fracture interface before the physical contact surface of the hydraulic fracture, causing a significant reduction in the effective normal stress at the interface. Consequently, microcrack damage occurs within the cementing minerals before they are actually impacted, resulting in the so-called pre-impact attenuation seepage damage coupling phenomenon.

[0015] Because existing discrimination methods completely ignore the time-varying degradation mechanism of damage induced by seepage, they continuously use the initial high-strength value calibrated in the laboratory as the tensile resistance threshold when judging penetration capability. This leads to a large number of interactive behaviors that should have penetrated under actual fracturing conditions being systematically misjudged as being forced to stop or deviated, resulting in a systematic and serious overestimation of the resistance capability of the cemented interface of natural fractures. Therefore, how to introduce a quantitative description of the pre-pore pressure seepage characteristics and the dynamic weakening effect of the interface strength induced during the approach of hydraulic fractures in the interactive discrimination process between hydraulic fractures and cemented natural fractures, fundamentally breaking through the traditional assumption of static and constant cemented interface strength, and achieving more accurate discrimination results for extended modes, is a core technical problem that urgently needs to be solved in this field.

[0016] Therefore, such as Figure 1 As shown, this application provides a method for determining the propagation mode of hydraulic fractures under the control of cemented natural fractures, including the following steps: Obtain hydraulic diffusion parameters characterizing the physical flow constraint boundary of fracturing fluid in the matrix pores of the target reservoir; Obtain the total macroscopic normal stress acting on the interface of the target natural fracture, and extract the dynamic motion parameters of the hydraulic fracture front at the current calculation time step; Based on the hydraulic diffusion parameters and dynamic motion parameters, diffusion analysis is performed on the hydraulic fracture front to obtain the local pore fluid pressure exerted by the hydraulic fracture front on the target natural fracture interface. The macroscopic normal total stress is effectively corrected based on the local pore fluid pressure to obtain the target effective normal stress characterizing the interface stress state. The target effective normal stress is compared with a preset crack initiation threshold to generate a damage accumulation factor characterizing the degree of interface weakening. The interface mechanical parameters are obtained based on the damage accumulation factor and the preset tensile reference value of the target natural crack interface. The local tip tensile stress induced from the hydraulic fracture front to the target natural fracture interface is obtained, and the local tip tensile stress is compared with the interface mechanical parameters to output the expansion mode result at the current calculation time step; Obtain feedback optimization parameters that characterize the accuracy of the extended mode results, and update the preset crack initiation threshold accordingly.

[0017] Before performing extended mode discrimination, it is first necessary to obtain the hydraulic diffusion parameters characterizing the physical flow constraint boundary of the fracturing fluid in the pores of the target reservoir matrix. This parameter is the core physical property benchmark used to quantify the advanced permeability of the fracturing fluid. Its physical meaning lies in describing the velocity limit of the pore pressure disturbance signal propagating from the fracture tip to the forward matrix under the current reservoir medium and fluid conditions.

[0018] The process of obtaining hydraulic diffusion parameters is as follows: For the target reservoir matrix, the matrix permeability, which characterizes its fluid conduction capacity, the matrix porosity, which characterizes its microscopic storage space, and the comprehensive compressibility coefficient, which characterizes its rock mass elastic deformation response, are obtained respectively; for the fracturing fluid, the dynamic viscosity, which characterizes its internal flow resistance, is obtained.

[0019] Matrix permeability physically represents the ability of reservoir rock to allow fluid to pass through, determining the resistance to fracturing fluid penetration into the rock's micropores. It is typically obtained through nuclear magnetic resonance (NMR) logging data inversion or laboratory overburden pressure porosity and permeability testing. Matrix porosity represents the proportion of the rock's internal micropore volume to its total volume, reflecting the amount of microscopic space the rock can hold for fluids. It can be obtained through density logging, neutron logging, sonic logging, or laboratory helium porosity testing.

[0020] Total compressibility describes the sensitivity of the combined elastic deformation of the rock skeleton volume and the fluid volume within the pores to changes in formation pressure. It is obtained by adding the fluid compressibility and the rock compressibility, and can be obtained through high-pressure physical property tests (PVT tests) and triaxial compression tests of rock mechanics. Dynamic viscosity characterizes the internal flow resistance of fracturing fluids and is directly determined by the fracturing fluid formulation parameters.

[0021] After obtaining the aforementioned basic parameters, the product of dynamic viscosity, matrix porosity, and overall compressibility coefficient is used as the denominator, and matrix permeability is used as the numerator. The ratio of these two is the hydraulic diffusion parameter. The calculation logic of the hydraulic diffusion parameter physically follows the pressure diffusion equation in the seepage theory of porous media. Its value directly determines the spatial decay rate of local pore fluid pressure in subsequent steps. The specific mathematical relationship is as follows: ; in, Characterizing hydraulic diffusion parameters, Characterizing matrix permeability, Characterizing dynamic viscosity, Characterizing matrix porosity, Characterizes the overall compression coefficient. The larger the value, the stronger the pre-permeation and diffusion capacity of the fracturing fluid in the matrix, and the more significant the weakening effect of the pre-permeation on the cementation interface. The smaller the value, the more limited the scope of advanced penetration and the weaker the interface weakening effect.

[0022] The obtained hydraulic diffusion parameters serve as rigid parameters characterizing the physical flow constraint boundary of fracturing fluid in the target reservoir matrix pores, and will be directly used in subsequent diffusion analysis of local pore fluid pressure.

[0023] After obtaining the hydraulic diffusion parameters, two data acquisition tasks need to be completed simultaneously: first, to obtain the total macroscopic normal stress acting on the interface of the target natural fracture; and second, to extract the dynamic motion parameters of the hydraulic fracture front at the current calculation time step.

[0024] The total normal stress characterizes the closed compression constraint generated by the background stratigraphic structure on the interface of the target natural fracture. It is a macroscopic mechanical quantity that reflects the geostress environment of the interface and can usually be extracted in real time from the global stress solution module of the numerical simulator.

[0025] The extraction process of dynamic motion parameters is as follows: Obtain the first three-dimensional coordinates of the hydraulic fracture front in the previous calculation time step and the second three-dimensional coordinates in the current calculation time step. Divide the Euclidean distance between the first and second three-dimensional coordinates by the time step length between adjacent calculation time steps to obtain the propagation velocity of the hydraulic fracture front. The mathematical relationship satisfies: ; in, Characterizing the propagation rate of the hydraulic fracture front, The second three-dimensional coordinates representing the current computation time step The first three-dimensional coordinates representing the previous computation time step. It represents the time step size between adjacent computation time steps.

[0026] Based on the second three-dimensional coordinates and the pre-obtained spatial plane equation of the target natural fracture interface, the shortest normal distance from the projection of the hydraulic fracture tip to the target natural fracture interface is obtained. Specifically, when the spatial plane equation of the target natural fracture interface is known (pre-obtained through geological modeling or three-dimensional fracture network fitting), the coordinates of the hydraulic fracture tip in the current calculation time step are... Shortest normal distance to the plane It can be directly calculated using the geometric relationship of the perpendicular projection from a point in space to a plane. The expansion velocity... Shortest normal distance The common output is the dynamic motion parameters at the current computation time step. .

[0027] Compared to traditional methods that only use static geometric distance for evaluation, dynamic motion parameters incorporate the time dimension into the description system—the shortest normal distance. At different expansion speeds The effects of different flow pretreatment methods on the cemented interface vary significantly. Higher flow rates result in shorter flow time for the fracturing fluid in the pretreatment zone, leading to a weaker pre-weakening effect; conversely, slower flow rates allow for more pre-flow time, resulting in a stronger weakening effect. This introduction of the time dimension is the core reason why this invention surpasses traditional methods in its physical description.

[0028] After obtaining the hydraulic diffusion parameters and dynamic motion parameters, diffusion analysis is performed on the hydraulic fracture front based on the hydraulic diffusion parameters and dynamic motion parameters to obtain the local pore fluid pressure of the hydraulic fracture front acting on the target natural fracture interface.

[0029] This step also requires the following two input data: first, the intra-fracturing fluid pressure, which characterizes the dynamic driving load of the fracturing fluid at the current calculation time step. This data is typically read directly in real time from the fracture fluid pressure solution module of the fracturing numerical simulator; secondly, it is the original formation pore pressure, which characterizes the static background energy baseline of the target reservoir matrix. This data can be obtained in advance from well logging data or geological modeling results.

[0030] In terms of physical modeling, this invention equates the high-speed advancing hydraulic fracture tip to a moving point source carrying high-pressure fluid. It employs a one-dimensional steady-state moving solution based on the transient convection-diffusion equation of porous media to analytically reconstruct the pore pressure distribution within the pre-seepage zone. Its core physical picture lies in: high-pressure fluid at the fracture tip (pressure is...) Driven by a huge pressure difference, it penetrates forward into the matrix ahead, and when the fluid front reaches the shortest normal distance... When targeting a natural fracture interface, the effective pore pressure it carries is relative to the original formation pore pressure. A natural exponential decay occurred. The rate of decay was determined by seepage characteristic variables. With hydraulic diffusion parameters The ratio of these factors determines the outcome.

[0031] Specifically, the expansion speed Shortest normal distance Perform a product operation to obtain the seepage characteristic variables, and then apply the hydraulic diffusion parameters. Fluid pressure inside the seam 1. Original formation pore pressure In addition to seepage characteristic variables, the local pore fluid pressure can be obtained by performing mapping calculations using the natural exponential decay function. Their mathematical relationship satisfies: ; in, Characterizing local pore fluid pressure, Characterizing the pore pressure of the original formation, Characterizing the fluid pressure within the crevice, Characterizing the propagation rate of the hydraulic fracture front, Characterized by the shortest normal distance, Characterizes parameters of hydraulic diffusion.

[0032] This formula contains abundant physical consistency: when the crack propagation speed Extremely fast or matrix diffusion coefficient When the value is extremely small, the exponential term approaches zero, and the local pore fluid pressure approaches the original formation pore pressure. This is consistent with the limiting case of the impermeable assumption; when the propulsion speed... Slowdown or diffusion coefficient As the pressure increases, the high-pressure leading edge can penetrate significantly into the cemented interface ahead, increasing the local pore fluid pressure. The interface experiences a significantly increased pre-weakening effect. Furthermore, this formula achieves an analytical approximation without complex numerical mesh partitioning, making the computation extremely lightweight and perfectly suited to the computational constraints of typical engineering projects.

[0033] The local pore fluid pressure obtained in this step This fundamentally breaks the core assumption in existing technologies that the interfacial pore pressure before the fractures make physical contact is equal to the original formation pressure, and is the first key data node for achieving dynamic discrimination in this invention.

[0034] After obtaining the local pore fluid pressure, the macroscopic normal total stress is effectively corrected based on the local pore fluid pressure to obtain the target effective normal stress characterizing the interface stress state. The physical basis for effective stress correction comes from the well-known Terzaghi effective stress principle in geotechnical mechanics: the increase in pore fluid pressure generates an outward spreading effect inside the pores, which is equivalent to offsetting the externally applied compressive stress, thereby increasing the net tensile force actually borne by the material skeleton. In other words, the local pore fluid pressure, as an internal spreading force, is physically opposite in direction to the macroscopic compressive stress.

[0035] The present invention uniformly stipulates that the local pore fluid pressure that generates tensile force is set to a positive value, and the total macroscopic normal stress that generates compressive force is set to a negative value. The two are linearly superimposed to calculate the corresponding target effective normal stress.

[0036] Within this effective stress framework, even if the total macroscopic normal stress is negative (the interface is subjected to overall compression from geostress), as long as the local pore fluid pressure... Large enough, effective normal stress of the target This could potentially exceed zero or even the preset crack initiation threshold, thereby physically triggering microcrack damage within the cemented mineral framework. The target effective normal stress is the core transfer quantity connecting the fluid seepage field and the damage mechanics field, and is the second key data node in the causal coupling link of this invention.

[0037] After obtaining the target effective normal stress, it is compared with the preset crack initiation threshold. A damage accumulation factor characterizing the degree of interface weakening is generated by comparison, and the interface mechanical parameters are obtained based on the damage accumulation factor and the preset tensile benchmark value of the target natural crack interface.

[0038] First, based on the preset tensile benchmark value of the target natural crack interface. and the pre-obtained rock brittleness index The damage evolution coefficient of the target natural crack interface is obtained. The damage evolution coefficient describes the rate of damage evolution of the cemented mineral framework when the effective tensile stress exceeds the crack initiation threshold, and its mathematical relationship satisfies: ; in, Characterizing damage evolution coefficients, Characterizing the brittleness index of rocks, Characterizing the preset tensile reference value, Characterized by a pre-defined calibration constant. Rock brittleness index. It is a dimensionless comprehensive brittleness evaluation parameter, directly calculated from Young's modulus and Poisson's ratio in well logging data, requiring no additional experiments. The formula has extremely strong physical causality: the more brittle the rock (… The larger the value, the more severe the damage evolution after the stress exceeds the threshold; when the preset tensile benchmark value ( The larger the stress span, the more gradual the relative evolution rate. The dimension of is the reciprocal of stress (MPa) -1 ), can be calibrated through regional core laboratory experiments. The value will be determined later.

[0039] Secondly, the effective normal stress of the target With preset crack initiation threshold A comparison is made to determine whether the microcrack damage accumulation process has been triggered: if the target effective normal stress Greater than the preset crack initiation threshold This indicates that the interface has exceeded the microcrack initiation threshold under the drive of local pore pressure and entered a damage accumulation state. At this point, the damage accumulation factor is obtained, and its mathematical relationship satisfies: ; If the target has effective normal stress Less than or equal to the preset crack initiation threshold This indicates that the effective tensile stress at the interface has not yet reached the critical condition required for microcrack initiation, and there is no damage accumulation inside the interface. The damage accumulation factor will be... The value is assigned to zero. Where, The damage accumulation factor is characterized by its range [0,1). =0 means the interface is completely undamaged. A value approaching 1 indicates that the interface is approaching complete failure. This formula adopts the Weibull exponential damage evolution law, which is recognized in the field of continuous damage mechanics. It can smoothly map the stress increment after the effective tensile stress exceeds the crack initiation threshold into a normalized damage metric. It has a clear physical meaning and good mathematical properties.

[0040] Finally, combined with the preset tensile benchmark value The current tensile strength of the interface is reduced and updated to obtain the interface mechanical parameters corresponding to the target natural crack interface. Their mathematical relationships satisfy: ; in, Characterizes the interfacial mechanical parameters (i.e., the dynamic effective tensile strength at the current time step). Characterizing the preset tensile reference value, Characterizing damage accumulation factors.

[0041] This invention represents a dynamic replacement of the static tensile strength constant in the traditional discrimination criterion, and is a direct data product of the core innovation of this invention. Its physical meaning is that when hydraulic fractures approximate cemented natural fractures in a certain way, the actual tensile resistance of the interface is no longer the initial laboratory value. It is not the dynamic value after the damage effect induced by the pre-seepage is reduced. ,and It always holds true.

[0042] After obtaining the interface mechanical parameters, the local tip tensile stress induced from the hydraulic fracture front to the target natural fracture interface is obtained. The local tip tensile stress is compared with the interface mechanical parameters to output the expansion mode results at the current calculation time step.

[0043] The physical essence of localized tip tensile stress is the stress concentration effect at the fracture tip in fracture mechanics: a hydraulic fracture is like an extremely sharp wedge, and the rock skeleton in front of its tip will experience extremely high tensile stress concentration under the drive of a huge spreading force. Unlike the localized pore fluid pressure (fluid seeping into the interface and expanding from the inside out) in the previous steps, localized tip tensile stress is a mechanical tearing force transmitted through the solid rock skeleton itself. Together, they constitute the complete physical picture of the interaction between the hydraulic fracture and the natural fracture interface.

[0044] The local tensile stress at the fracture tip was obtained using the classical analytical model of linear elastic fracture mechanics (LEFM) at the fracture tip. Specifically, the first-order stress intensity factor, which characterizes the singularity amplification of the opening-type stress field at the hydraulic fracture front, was first obtained. The first type of stress intensity factor corresponds to the opening fracture mode, that is, the external force perpendicular to the fracture surface pulls the fracture apart to both sides. This is the typical stress mode in which the fracturing fluid is pressed up in the fracture and drives the fracture wall to open. The calculation is derived from the general analytical formula of linear elastic fracture mechanics, and its mathematical relationship satisfies: ; in, Characterizing the first type of stress intensity factor, The net pressure (i.e., the difference between the fluid pressure inside the fracture and the minimum horizontal principal stress of the target reservoir, is the effective driving force that is actually used to tear the rock) is characterized. Characteristic geometric dimensions of the hydraulic fracture front (e.g., corresponding to fracture half-length in the KGD model and fracture half-height in the PKN model). A dimensionless geometric shape coefficient characterizing the effect of the macroscopic three-dimensional distribution morphology of hydraulic fractures on stress concentration effects, such as standard penny-shaped fractures in deep formations. ).

[0045] In obtaining Then, combined with the shortest normal distance Using Sneddon's classical tip stress field formula, the local tip tensile stress is obtained. Its mathematical relationship satisfies: ; in, Characterizing localized tensile stress at the tip, Characterized by the shortest normal distance, The geometric direction coefficient characterizes the pre-acquired hydraulic fracture front edge, which is used to reflect the projection amplification factor of the stress field in the normal direction when the hydraulic fracture contacts the natural fracture at different approach angles.

[0046] Obtaining local tip tensile stress With interface mechanical parameters Then, the following comparison and judgment are performed: if the local tensile stress at the tip is greater than or equal to the interface mechanical parameter, i.e. The output then represents the first propagation mode result (penetration propagation mode) indicating that the hydraulic fracture front has crossed the target natural fracture interface; if the local tip tensile stress is less than the interface mechanical parameters, i.e. The output then represents the second propagation mode result (deflection or capture propagation mode) that indicates the hydraulic fracture front has not crossed the target natural fracture interface.

[0047] The propagation mode results include the first propagation mode results and the second propagation mode results. The first propagation mode results indicate that the mechanical tearing force of the hydraulic fracture is sufficient to overcome the true tensile resistance of the interface after damage and weakening, and the fracture will directly penetrate the natural fracture and continue to extend. The second propagation mode results indicate that the natural fracture still has sufficient residual tensile strength to resist the current mechanical force, and the hydraulic fracture will be forced to deflect along the weak surface of the natural fracture or be captured and stopped propagating under high closure stress.

[0048] The above steps constitute a complete forward execution chain for the extended mode discrimination within each computation time step. When a microseismic monitoring system is installed at the fracturing site, the following feedback optimization mechanism can be used to perform closed-loop dynamic correction on the key parameters of the discrimination model, namely the preset fracturing initiation threshold, to further improve the model's approximation of the actual underground strata properties.

[0049] First, the actual penetration rate is extracted. Within a preset monitoring period, microseismic event point cloud data is obtained within a preset geographical area corresponding to the target natural fracture interface. Microseismic monitoring is a mainstream technology used in fracturing operations to perceive the dynamics of underground fracture propagation in real time. Its event point cloud data can reflect the spatial distribution of microfractures induced by fracturing fluid in the formation. A spatial capture window of preset width is set on the spatial trace of the target natural fracture interface. The total number of microseismic events occurring on the water-facing side of the natural fracture (i.e., the side of hydraulic fracture propagation) within this window, as well as the number of microseismic events that successfully cross the natural fracture interface and extend to the backwater side (i.e., the other side of the natural fracture), are counted. The ratio of the number of events crossing the interface to the total number of events is used as the actual penetration rate, representing the proportion of fracturing fluid that actually crosses the target natural fracture interface.

[0050] Secondly, the simulated penetration rate is extracted. The proportion of the number of first extension mode results (i.e., the total number of nodes judged as penetration) output by the numerical simulator within the corresponding preset monitoring period is obtained to the total number of all extension mode results (i.e., the total number of nodes that spatially intersect with the target natural crack), and this proportion is used as the simulated penetration rate.

[0051] Next, the feedback optimization parameters are calculated. The difference between the simulated penetration rate and the actual penetration rate is used as the feedback optimization parameters. The feedback optimization parameters are categorized as positive, negative, and zero: a positive value indicates that the simulator predicts too many penetrating nodes, making the prediction too aggressive; a negative value indicates that the simulator predicts too many blocked nodes, making the prediction too conservative; and a zero value indicates that the model prediction matches the actual observations well, requiring no adjustment.

[0052] Finally, when feedback optimizes parameters When the value is not zero, a linear update is performed on the current preset crack initiation threshold, and the mathematical relationship satisfies: ; in, Characterizes the updated preset crack initiation threshold. Characterizing the current preset crack initiation threshold, The preset adjustment step size (in MPa) is used to apply the updated preset crack initiation threshold to the discrimination calculation for the next preset monitoring cycle.

[0053] This update logic constitutes a complete physical negative feedback steady-state convergence closed loop: when the model prediction penetrates too much (i.e. When the preset adjustment step size is reached, If the value is positive, the updated preset crack initiation threshold becomes larger, which raises the threshold for damage triggering in the next iteration, thereby suppressing unnecessary penetration predictions. When the model predicts too little penetration (i.e.) When the updated threshold decreases, damage is more easily triggered, and the number of penetration detection results increases, gradually converging with the actual observation results. Through a continuous monitoring-update-remonitoring cycle, the model's preset fracturing threshold will continuously approach the true physical properties of the target strata, achieving continuous self-correction and improvement in detection accuracy.

[0054] In another implementation, such as Figure 2 As shown, this application also provides a discrimination system for hydraulic fracture propagation modes under the control of cemented natural fractures, including the following modules: The data extraction module is used to acquire hydraulic diffusion parameters characterizing the physical flow constraint boundary of fracturing fluid in the target reservoir matrix pores, obtain the total macroscopic normal stress acting on the target natural fracture interface, and extract the dynamic motion parameters of the hydraulic fracture front at the current calculation time step. This module interacts with the reservoir core experimental database, the well logging interpretation results database, and the real-time coordinate tracking module of the fracturing simulator.

[0055] The data processing module performs diffusion analysis on the hydraulic fracture front based on the hydraulic diffusion parameters and dynamic motion parameters to obtain the local pore fluid pressure exerted by the hydraulic fracture front on the target natural fracture interface. This module achieves efficient numerical calculation of the one-dimensional moving point source diffusion analysis model, and its output directly decouples the erroneous assumption that the traditional interface pore pressure equals the original formation pressure.

[0056] The data correction module is used to effectively correct the total macroscopic normal stress based on the local pore fluid pressure, thereby obtaining the target effective normal stress characterizing the interface stress state. This module follows the Terzaghi effective stress principle to complete the mapping transformation from the fluid field to the stress field.

[0057] The damage assessment module compares the target effective normal stress with a preset crack initiation threshold to generate a damage accumulation factor characterizing the degree of interface weakening. Based on the damage accumulation factor and a preset tensile benchmark value for the target natural crack interface, the module obtains the interface mechanical parameters. This module implements the engineering calculation of the continuous damage mechanics evolution equation, converting stress field information into the actual dynamic tensile strength of the interface.

[0058] The result discrimination module is used to obtain the local tip tensile stress induced from the hydraulic fracture front to the target natural fracture interface, compare the local tip tensile stress with the interface mechanical parameters to output the expansion mode result at the current calculation time step. This module outputs the final discrimination conclusion based on the linear elastic fracture mechanics tip stress field analytical model and transmits the result to the geometry update driver interface of the fracturing simulator.

[0059] The feedback optimization module is used to obtain feedback optimization parameters characterizing the accuracy of the extended mode results and update the preset fracture initiation threshold accordingly. This module receives the actual penetration rate input from the microseismic monitoring data interface to achieve closed-loop dynamic correction of the preset fracture initiation threshold, giving the entire discrimination system the ability to learn and continuously optimize.

[0060] This application achieves the following technical effects: 1) It eliminates the systematic overestimation of the cemented interface resistance, significantly improving the accuracy of penetration mode discrimination. This invention breaks the fundamental assumption of statically constant cemented interface strength in traditional discrimination methods. By introducing the dynamic motion parameters of the hydraulic fracture front into a one-dimensional moving point source diffusion analytical model, it reconstructs and quantifies the local pore fluid pressure acting on the natural fracture interface in real time, thereby accurately describing the time-varying physical process of pre-seepage-interface pressurization. This effectively eliminates the systematic overestimation of cemented interface resistance in traditional models and significantly reduces the misjudgment rate of penetration mode discrimination.

[0061] 2) This invention achieves strict causal coupling across physical fields and establishes a physically complete dynamic discrimination criterion. Based on the continuous damage mechanics framework, this invention constructs a complete quantitative causal link from the fluid seepage field to the effective stress field, then to the damage mechanics field, and finally mapped to the interface mechanical parameters. This allows the actual tensile strength of the natural fracture interface to dynamically evolve with the approach mode of the hydraulic fracture (the combination of velocity and distance), providing a physically complete dynamic resistance threshold for extended mode discrimination. This effectively solves the systematic discrimination error caused by the fragmented treatment of various physical fields in the prior art.

[0062] 3) It possesses adaptive response capabilities to variable displacement fracturing processes, endowing the model with engineering derivative value beyond expectations. Because this invention introduces the propagation velocity parameter of the hydraulic fracture front into the calculation of local pore fluid pressure, the discrimination model can spontaneously respond to changes in fracturing displacement. When a decrease in displacement leads to a slowdown in fracture velocity, the lead time of seepage increases, the weakening of the cemented interface intensifies, and the actual penetration capability is enhanced. This provides underlying algorithmic support for the refined parameter optimization of novel processes such as variable displacement fracturing and pulsed pump injection.

[0063] 4) A closed-loop feedback optimization mechanism based on microseismic monitoring data is introduced to give the model autonomous correction capabilities. Feedback optimization parameters are constructed by the difference between the actual penetration rate and the simulated penetration rate, and the preset fracture initiation threshold is dynamically updated by negative feedback adjustment logic. This achieves closed-loop self-correction of key parameters of the model, enabling the model to continuously approach the true physical properties of the strata as actual engineering observation data is obtained, significantly improving the long-term engineering applicability and reliability of the model.

[0064] In another embodiment, this application also provides a computer storage medium storing computer-executable instructions, which, when executed, implement the method for determining the hydraulic fracture propagation mode under the control of cemented natural fractures.

[0065] The above embodiments are only used to illustrate the technical methods of this application and are not intended to limit it. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical methods of this application without departing from the spirit and scope of the technical methods of this application.

Claims

1. A method for determining the propagation mode of hydraulic fractures under the control of cemented natural fractures, characterized in that, Includes the following steps: Obtain hydraulic diffusion parameters characterizing the physical flow constraint boundary of fracturing fluid in the matrix pores of the target reservoir; Obtain the total macroscopic normal stress acting on the interface of the target natural fracture, and extract the dynamic motion parameters of the hydraulic fracture front at the current calculation time step; Based on the hydraulic diffusion parameters and dynamic motion parameters, diffusion analysis is performed on the hydraulic fracture front to obtain the local pore fluid pressure exerted by the hydraulic fracture front on the target natural fracture interface. The macroscopic normal total stress is effectively corrected based on the local pore fluid pressure to obtain the target effective normal stress characterizing the interface stress state. The target effective normal stress is compared with a preset crack initiation threshold to generate a damage accumulation factor characterizing the degree of interface weakening. The interface mechanical parameters are obtained based on the damage accumulation factor and the preset tensile reference value of the target natural crack interface. The local tip tensile stress induced from the hydraulic fracture front to the target natural fracture interface is obtained, and the local tip tensile stress is compared with the interface mechanical parameters to output the expansion mode result at the current calculation time step; Obtain feedback optimization parameters that characterize the accuracy of the extended mode results, and update the preset crack initiation threshold accordingly.

2. The method for determining the propagation mode of hydraulic fractures under the control of cemented natural fractures according to claim 1, characterized in that, The process of obtaining hydraulic diffusion parameters includes: For the target reservoir matrix, obtain matrix permeability to characterize its fluid conduction capacity, matrix porosity to characterize its microscopic storage space, and comprehensive compressibility coefficient to characterize its rock mass elastic deformation response. For the fracturing fluid, obtain dynamic viscosity to characterize its internal flow resistance. The product of the dynamic viscosity, matrix porosity, and overall compressibility coefficient is used as the denominator, and the ratio of the matrix permeability to the denominator is used as the hydraulic diffusion parameter.

3. The method for determining the propagation mode of hydraulic fractures under the control of cemented natural fractures according to claim 1, characterized in that, The process of extracting dynamic motion parameters includes: The first three-dimensional coordinates of the hydraulic fracture front in the previous calculation time step and the second three-dimensional coordinates in the current calculation time step are obtained. The Euclidean distance between the first three-dimensional coordinates and the second three-dimensional coordinates is divided by the time step length between adjacent calculation time steps to obtain the propagation velocity of the hydraulic fracture front. Based on the second three-dimensional coordinates and the pre-acquired spatial plane equation of the target natural fracture interface, the shortest normal distance from the projection of the hydraulic fracture leading edge to the target natural fracture interface is obtained, and the propagation velocity and the shortest normal distance are output as dynamic motion parameters at the current calculation time step.

4. The method for determining the propagation mode of hydraulic fractures under the control of cemented natural fractures according to claim 3, characterized in that, The process of obtaining local pore fluid pressure includes: Obtain the intra-fracture fluid pressure characterizing the dynamic driving load of the fracturing fluid at the current computational time step. And the original formation pore pressure characterizing the static background energy baseline of the target reservoir matrix. The expansion speed With the shortest normal distance Perform a product operation to obtain seepage characteristic variables; According to the hydraulic diffusion parameters The fluid pressure inside the slit The original formation pore pressure The seepage characteristic variables are used to perform mapping calculations using a natural exponential decay function to obtain the local pore fluid pressure. ; 。 5. The method for determining the propagation mode of hydraulic fractures under the control of cemented natural fractures according to claim 1, characterized in that, The process of obtaining the target effective normal stress includes: The macroscopic normal total stress, which characterizes the closed compression constraint of the background stratigraphic structure on the target natural fracture interface, is obtained. The local pore fluid pressure that produces tensile effect is set to a positive value, and the macroscopic normal total stress that produces compressive effect is set to a negative value. The two are linearly superimposed to calculate and output the corresponding target effective normal stress.

6. The method for determining the propagation mode of hydraulic fractures under the control of cemented natural fractures according to claim 1, characterized in that, The process of obtaining interface mechanical parameters includes: Based on the preset tensile strength benchmark value of the target natural crack interface and the pre-obtained rock brittleness index To obtain the damage evolution coefficient of the target natural crack interface. ,in, These are preset calibration constants; If the target effective normal stress Greater than the preset crack initiation threshold Then, the damage accumulation factor is obtained by combining the damage evolution coefficient. The interfacial mechanical parameters corresponding to the target natural crack interface are obtained by combining the preset tensile benchmark value. If the target effective normal stress Less than or equal to the preset crack initiation threshold Then the damage accumulation factor will be Assign a value of zero and obtain its interface mechanical parameters.

7. The method for determining the propagation mode of hydraulic fractures under the control of cemented natural fractures according to claim 4, characterized in that, The process of outputting the extended mode result includes: Obtain the first type of stress intensity factor characterizing the singularity amplification of the opening stress field at the hydraulic fracture front. And combined with the shortest normal distance Obtain local tip tensile stress ,in, The geometric direction coefficients of the hydraulic fracture front are obtained in advance; If the local tip tensile stress is greater than or equal to the interface mechanical parameter, then the first propagation mode result characterizing the hydraulic fracture front crossing the target natural fracture interface is output. If the local tip tensile stress is less than the interface mechanical parameter, a second propagation mode result is output, which characterizes that the hydraulic fracture front has not crossed the target natural fracture interface. The propagation mode result includes the first propagation mode result and the second propagation mode result.

8. The method for determining the propagation mode of hydraulic fractures under the control of cemented natural fractures according to claim 7, characterized in that, The process of updating the preset crack initiation threshold includes: Within a preset monitoring period, obtain point cloud data of microseismic events corresponding to the target natural fracture interface within a preset geographical range, and extract the actual penetration rate, which represents the proportion of fracturing fluid that actually crosses the target natural fracture interface. The proportion of the number of first extended mode results output within the corresponding preset monitoring period to the total number of all extended mode results is obtained and used as the simulated penetration rate. The value obtained by subtracting the actual penetration rate from the simulated penetration rate is used as the feedback optimization parameter. The feedback optimization parameter is divided into positive, negative and zero values. When the feedback optimization parameters When it is not zero, the current preset crack initiation threshold is... Update to obtain the updated preset crack initiation threshold. And apply it to the next preset monitoring cycle, where, This is a preset adjustment step size.

9. A system for discriminating hydraulic fracture propagation modes under the control of cemented natural fractures, characterized in that, Includes the following modules: The data extraction module is used to obtain hydraulic diffusion parameters characterizing the physical flow constraint boundary of fracturing fluid in the target reservoir matrix pores, obtain the total macroscopic normal stress acting on the target natural fracture interface, and extract the dynamic motion parameters of the hydraulic fracture front at the current calculation time step. The data processing module is used to perform diffusion analysis processing on the hydraulic fracture front based on the hydraulic diffusion parameters and dynamic motion parameters to obtain the local pore fluid pressure of the hydraulic fracture front acting on the target natural fracture interface. The data correction module is used to perform effective stress correction on the total macroscopic normal stress based on the local pore fluid pressure, so as to obtain the target effective normal stress characterizing the stress state of the interface. The damage assessment module is used to compare the target effective normal stress with a preset crack initiation threshold to generate a damage accumulation factor that characterizes the degree of interface weakening, and to obtain the interface mechanical parameters based on the damage accumulation factor and the preset tensile benchmark value of the target natural crack interface. The result discrimination module is used to obtain the local tip tensile stress induced from the hydraulic fracture front to the target natural fracture interface, and compare the local tip tensile stress with the interface mechanical parameters to output the extended mode result at the current calculation time step. The feedback optimization module is used to obtain feedback optimization parameters that characterize the accuracy of the results of the extended mode, and update the preset crack initiation threshold according to them.

10. A computer storage medium storing computer-executable instructions, characterized in that, When the computer-executable instructions are executed, they implement the method for determining the hydraulic fracture propagation mode under the control of cemented natural fractures, as described in any one of claims 1-8.

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