Fracture morphology and net pressure and minimum stress time-dependent evolution inversion method

Abstract

This invention relates to a time-varying evolution inversion method for fracture morphology, net pressure, and minimum geostress, belonging to the field of petroleum technology. The method includes: acquiring on-site bottomhole pressure data, monitoring well information, and LF-DAS data during fracturing operations; discretizing the fracture along its length into several units based on the three-dimensional displacement discontinuity method and constructing a net pressure distribution matrix; using a pre-constructed mechanical response relationship between net pressure and fracture width distribution to solve for the fracture width distribution corresponding to the current net pressure distribution; calculating the axial strain response of the fracture at the monitoring well based on the fracture width distribution to form a forward model for pressure-fracture width-strain solution; iteratively updating the fracture half-length and net pressure using the least squares method, constructing a residual between the inverted strain and the on-site strain to minimize the residual, and obtaining the time-varying inversion results for the fracture half-length and net pressure. The time-varying evolution process of minimum geostress can be characterized by the difference between the on-site bottomhole pressure and the inverted net pressure.

Description

Technical Field

[0001] This invention relates to the field of petroleum technology, and in particular to a time-varying evolution inversion method for fracture morphology, net pressure, and minimum geostress. Background Technology

[0002] Unconventional oil and gas reservoirs are generally characterized by low porosity, low permeability, and strong heterogeneity. They typically require volumetric fracturing to create complex fracture networks within the reservoir, thereby increasing the stimulation volume and hydrocarbon permeability. During fracturing operations, the dynamic changes in fracture propagation morphology, intra-fracture fluid pressure distribution, and minimum geostress directly affect fracture propagation paths, proppant migration and placement, inter-well pressure channeling response, and the final stimulation effect. Therefore, accurately identifying the time-varying evolution of fracture geometry and pressure-stress state during fracturing has always been a key issue in oil and gas field development and dynamic fracturing diagnosis.

[0003] Existing fracturing monitoring methods mainly include construction pressure analysis, microseismic monitoring, tracer monitoring, and distributed fiber optic monitoring. Among them, low-frequency distributed acoustic sensing technology, namely LF-DAS technology, can continuously acquire strain or strain rate responses with high spatiotemporal resolution along the monitoring well. It has advantages such as continuous monitoring, large coverage, and suitability for long-term downhole deployment, and has been gradually applied to fracture propagation identification and inter-well pressure channeling diagnosis in recent years. Compared with microseismic methods, which mainly provide discrete event location information, LF-DAS is more suitable for characterizing the continuous strain response caused by fracture disturbance during fracturing, providing a new data foundation for fracture dynamic inversion.

[0004] However, existing inversion methods based on monitoring data mostly focus on identifying geometric parameters such as fracture width, height, or azimuth, primarily addressing questions like "where is the fracture and how far has it extended?" Their characterization of intra-fracture pressure distribution, fracture length distribution, and dynamic changes in geostress remains limited. On one hand, fracture-induced strain is not only controlled by fracture geometry but also closely related to intra-fracture fluid pressure distribution and fracture opening state; interpretation based solely on geometric parameters often fails to comprehensively describe the fluid-structure interaction mechanical behavior during fracturing. On the other hand, minimum geostress is a crucial parameter influencing fracture initiation, propagation, direction, and inter-fracture competition. However, existing methods typically rely on bottom-hole pressure curves or DFIT test results for interpretation, providing only single-point results at a specific moment or test segment, making it difficult to achieve continuous dynamic characterization corresponding to the fracture propagation process.

[0005] Furthermore, while some existing methods attempt to use numerical models for forward modeling or inversion of net pressure within the fracture, they often treat fracture geometry, intra-fracture pressure, and geostress separately, lacking a unified collaborative inversion framework. This results in insufficient correlation between interpretation results and makes it difficult to fully leverage the temporal advantages of continuous LF-DAS monitoring data. In particular, existing methods do not adequately utilize LF-DAS low-frequency strain data before the fracture impacts the monitoring well. The low-frequency strain response at this stage contains information on fracture tip advancement, fracture opening intensity, and changes in net pressure within the fracture, serving as a crucial data source for characterizing the entire fracture propagation process. Interpreting the data solely based on the response after the fracture impacts the monitoring well is typically insufficient to invert the continuous changes in the fracture half-length and to comprehensively demonstrate the dynamic process of fracture propagation from the fracturing well towards the monitoring well.

[0006] The above content is only used to help understand the technical solution of the present invention and does not represent an admission that the above content is prior art. Summary of the Invention

[0007] The main objective of this invention is to provide a time-varying evolution inversion method for crack morphology, net pressure, and minimum geostress, aiming to solve the aforementioned technical problems in the prior art.

[0008] To achieve the above objectives, the present invention provides a method for time-varying evolution inversion of crack morphology, net pressure, and minimum geostress, comprising: Acquire the bottom hole pressure data, horizontal and vertical well distance between the monitoring well and the fracturing well, fiber gauge length, and LF-DAS strain data before the fracture impacts the monitoring well obtained by distributed fiber measurement in the monitoring well during the fracturing operation. Then, perform synchronous processing on the data and extract the strain response of the monitoring well corresponding to each time step as the inversion target. Based on the three-dimensional displacement discontinuity method, the hydraulic fracture is discretized into several elements along its length and a net pressure distribution matrix inside the fracture is constructed. Then, using the pre-constructed mechanical response relationship between the net pressure inside the fracture and the fracture width distribution, the fracture width distribution corresponding to the current net pressure distribution is solved. Based on the fracture width distribution and the three-dimensional displacement discontinuity method, the axial strain response induced by the fracture at the monitoring well is calculated by forward modeling, forming a forward model for pressure distribution, fracture width distribution, and strain solution. Using the fracture half-fracture length, bottom hole net pressure, and fracture tip pressure as parameters to be inverted, initial values ​​and boundaries are set for inversion. The model strain is calculated using the forward model, and residuals are constructed with the extracted well strain response. The parameters are iteratively updated using the nonlinear least squares method to minimize the residuals, thereby obtaining the optimal fracture half-fracture length and bottom hole net pressure at the current time step, and outputting the pressure distribution and fracture width distribution within the fracture. After entering the next time step, the inversion result of the previous time step is used as the initial value of the current time step. The process is repeated to obtain the optimal fracture half-length and bottom hole net pressure, and the pressure distribution and fracture width distribution within the fracture are output until the set end time is reached, so as to obtain the dynamic inversion results of the evolution of fracture half-length, net pressure distribution and fracture width distribution over time. Based on the net pressure within the fracture obtained from the inversion at each time step, combined with the field bottom hole pressure data, and according to the mechanical relationship between bottom hole pressure, net pressure and minimum in-situ stress, the minimum in-situ stress corresponding to each time step is calculated, and the dynamic evolution result of the minimum in-situ stress changing with time during the fracturing process is obtained.

[0009] Preferably, in the time-varying evolution inversion method of fracture morphology, net pressure, and minimum geostress, the step of discretizing the hydraulic fracture along its length into several units based on the three-dimensional displacement discontinuity method includes: The crack is discretized into N elements, where each element has a height of 2h and a length of l. f Crack half-crack length L f for: L f = N l f Among them, L f The length of half the crack is in meters (m). N represents the number of crack elements; l f Let be the length of the crack element, in meters. The method for constructing the net pressure distribution matrix inside the crack is as follows: ; Where p is the net pressure distribution matrix on each crack element, Pa; p0 is the net pressure at the fracture inlet or bottom of the well, in Pa; S is a dimensionless net pressure shape function used to describe the relative variation of net pressure along the crack length direction.

[0010] Preferably, in the time-varying evolution inversion method of fracture morphology, net pressure, and minimum geostress, before the step of utilizing the pre-constructed mechanical response relationship between intra-fracture net pressure and fracture width distribution, the method further includes: Establish a local coordinate system corresponding to each crack element; The mechanical response relationship between the net pressure within the crack and the crack width distribution is as follows: ; in, Let be the normal stress response induced at the i-th crack element by the unit normal opening of the j-th crack element, i=1,2,...,N; j=1,2,...,N; μ is the shear modulus, in Pa; v is Poisson's ratio; ; ; ; ; ; ; x1, x2, and x3 are the coordinates of element i along the crack length direction, along the crack height direction, and the crack surface normal coordinate in the local coordinate system of element j, respectively. , Represents the angle between X in the global coordinate system and x1 in the local coordinate system for elements i and j; ; , These are the integral variables along the crack length and crack height directions, respectively; The overall stress influence coefficient matrix is ​​as follows: .

[0011] Preferably, in the time-varying evolution inversion method of crack morphology, net pressure, and minimum geostress, the calculation formula in the step of solving the crack width distribution corresponding to the current net pressure distribution is: ; Where w is the crack width distribution matrix, and m; p is the net pressure distribution matrix, Pa.

[0012] Preferably, in the time-varying evolution inversion method of fracture morphology, net pressure, and minimum geostress, the forward modeling calculation of the axial strain response induced by the fracture at the monitoring well based on the fracture width distribution and three-dimensional displacement discontinuity method, forming a forward model of pressure distribution, fracture width distribution, and strain solution, includes: The displacement in the local coordinate system at any position caused by the crack element j with displacement discontinuity is: ; in, This is the kernel function. The commas in the function's subscript represent partial derivatives. c = 1, 2, 3, representing the x1, x2, and x3 directions, respectively. The numbers after the commas indicate the derivative in the corresponding direction. The first digit after the comma is 1, 2, or 3, indicating the derivative in the x1, x2, and x3 directions, respectively. The second digit after the comma is also 1, 2, or 3, indicating the derivative in the x1, x2, and x3 directions, respectively. When there are two numbers after the comma, the derivative is calculated twice. , , Let x1, x2, and x3 be the displacements in m in the local coordinate system at any position caused by the crack element j with displacement discontinuity. ; ; , These are the integral variables along the crack length and crack height directions, respectively; Transform the displacement in the local coordinate system into the displacement in the global coordinate system: ; , , They are respectively , , Displacement in global coordinates; By superimposing the contributions of all crack elements, the displacement of the entire crack at the monitoring point is obtained: ; The axial strain at the b-th monitoring point along the monitoring well is: ; L is the gauge length; for Displacement when X is b+L / 2; for Displacement when X is bL / 2; A strain distribution matrix is ​​formed by calculating the axial strain at each monitoring point.

[0013] Preferably, in the time-varying evolution inversion method of fracture morphology, net pressure, and minimum geostress, the step of calculating the model strain using the forward model, constructing residuals with the extracted well strain response, and iteratively updating parameters using a nonlinear least squares method to minimize the residuals, wherein the formula for the residuals is: ; in, For on-site monitoring of well strain; This is the strain response obtained from the inversion.

[0014] Preferably, in the time-varying evolution inversion method of fracture morphology, net pressure, and minimum geostress, the step of calculating the minimum geostress corresponding to each time step based on the net pressure within the fracture obtained from the inversion at each time step, combined with the field bottom hole pressure data, and according to the mechanical relationship between bottom hole pressure, net pressure, and minimum geostress, uses the following formula: ; in, Minimum ground stress; This refers to the pressure at the bottom of the well. Net pressure; For the friction of the perforation; t is the time step.

[0015] Preferably, in the time-varying evolution inversion method for fracture morphology, net pressure, and minimum geostress, in the step of acquiring on-site bottom hole pressure data, horizontal and vertical well distance between the monitoring well and the fracturing well, fiber gauge length, and LF-DAS strain data obtained from distributed fiber optic measurements in the monitoring well during fracturing operations, and synchronously processing the data to extract the monitoring well strain response corresponding to each time step as the inversion target, when the LF-DAS strain data is a strain rate, the monitoring well strain is calculated based on the strain rate, and the calculation formula is: ; in, For on-site monitoring of well strain; Strain rate, 1 / s; t represents time, s.

[0016] The present invention has at least the following beneficial effects: This invention acquires bottom-hole pressure data, horizontal and vertical well distances between the monitoring well and the fracturing well, fiber gauge length, and LF-DAS strain data obtained from distributed fiber optic measurements in the monitoring well during fracturing operations. The data is processed synchronously, and the strain response of the monitoring well at each time step is extracted as the inversion target. Based on the three-dimensional displacement discontinuity method, the hydraulic fracture is discretized into several elements along its length, and a net pressure distribution matrix inside the fracture is constructed. Using the pre-constructed mechanical response relationship between the net pressure inside the fracture and the fracture width distribution, the fracture width distribution corresponding to the current net pressure distribution is solved. Based on the fracture width distribution and the three-dimensional displacement discontinuity method, the axial strain response induced by the fracture at the monitoring well is calculated using forward modeling, forming a forward model for pressure distribution, fracture width distribution, and strain calculation. Using the fracture half-length, bottom-hole net pressure, and fracture tip pressure as inversion parameters, initial inversion values ​​and boundaries are set. The model strain is calculated using the forward model, and a residual is constructed with the extracted strain response of the monitoring well. A nonlinear maximum value is then used. The least-squares method iteratively updates parameters to minimize the residual, obtaining the optimal fracture half-length and bottom hole net pressure for the current time step, and outputting the pressure distribution and fracture width distribution within the fracture. Upon entering the next time step, the inversion result of the previous time step is used as the initial value for the current time step, and this process is repeated to obtain the optimal fracture half-length and bottom hole net pressure, outputting the pressure distribution and fracture width distribution within the fracture, until a set end time is reached. This yields the dynamic inversion results of the fracture half-length, net pressure distribution, and fracture width distribution evolving over time. Based on the net pressure within the fracture obtained from each time step inversion, combined with field bottom hole pressure data, and according to the mechanical relationship between bottom hole pressure, net pressure, and minimum geostress, the minimum geostress corresponding to each time step is calculated, obtaining the dynamic evolution results of the minimum geostress changing over time during fracturing. This achieves a unified dynamic inversion analysis of fracture half-length, fracture width distribution, internal pressure distribution, and minimum geostress during fracturing, overcoming the shortcomings of existing technologies which are mainly limited to fracture geometric parameter identification and have difficulty simultaneously characterizing the evolution of internal pressure and geostress.

[0017] Furthermore, the time-varying evolution inversion method for fracture morphology, net pressure, and minimum geostress provided by this invention, before the fracture reaches the monitoring well, utilizes continuous low-frequency strain data to jointly invert the fracture half-length, net pressure distribution within the fracture, and fracture width distribution, and further combines this with field bottom-hole pressure data to characterize the time-varying changes in minimum geostress. This method, based on fracture geometric identification, incorporates the net pressure distribution within the fracture, fracture width distribution, and minimum geostress evolution into a unified analysis system, achieving time-varying synergistic characterization of fracture morphology, fracture width distribution, net pressure within the fracture, and minimum geostress. This improves the completeness of fracturing dynamic interpretation and its engineering application value, and provides additional support for field-scale fracture propagation interpretation, fracture modeling, and fracturing construction parameter optimization. Attached Figure Description

[0018] Figure 1 The flowchart of the time-varying evolution inversion method for crack morphology, net pressure, and minimum geostress provided by the present invention in the first embodiment; Figure 2 This is a comparison diagram of the inversion results of fracture half-length and bottom hole net pressure in this invention; Figure 3 Comparison of the inversion results of net pressure distribution within the joint; Figure 4 Comparison of crack width distribution inversion results; Figure 5 To compare the results of simulated strain with actual strain; Figure 6 A comparison and error diagram of the minimum geostress inversion results.

[0019] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0020] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The present invention will be described in detail below with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0021] In this embodiment of the invention, the term "and / or" describes the relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. The character " / " generally indicates that the preceding and following associated objects have an "or" relationship.

[0022] It should be noted that the terms "first," "second," etc., in the specification, claims, and drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0023] In this embodiment of the invention, the term "multiple" refers to two or more, and other quantifiers are similar.

[0024] In this invention, unless otherwise stated, directional terms such as "upper," "lower," "top," and "bottom" are generally used in relation to the direction shown in the accompanying drawings, or in relation to the vertical, perpendicular, or gravitational direction of the component itself; similarly, for ease of understanding and description, "inner" and "outer" refer to the inner and outer contours of each component itself, but the above directional terms are not intended to limit this invention.

[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the various embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, those skilled in the art will understand that many technical details are presented in the embodiments of the present invention to facilitate a better understanding of the invention. However, the technical solutions claimed in the present invention can be implemented even without these technical details and various variations and modifications based on the following embodiments. The division of the following embodiments is for ease of description and should not constitute any limitation on the specific implementation of the present invention. The various embodiments can be combined with and referenced by each other without contradiction.

[0026] This invention provides a method for time-varying evolution inversion of crack morphology, net pressure, and minimum geostress, such as... Figure 1 As shown, the time-varying evolution inversion method of crack morphology, net pressure, and minimum geostress includes steps S1000 to S6000.

[0027] Step S1000 acquires the bottom hole pressure data, horizontal and vertical well distance between the monitoring well and the fracturing well, fiber gauge length, and LF-DAS strain data obtained by distributed fiber measurement in the monitoring well during the fracturing operation. The data is then processed synchronously, and the strain response of the monitoring well corresponding to each time step is extracted as the inversion target.

[0028] It should be noted that when the data obtained on site is in the form of strain rate, it should first be integrated and transformed to obtain strain data for subsequent inversion calculations.

[0029] When LF-DAS strain data is in the form of strain rate, the strain of the monitoring well is calculated based on the strain rate using the following formula: ; in, For on-site monitoring of well strain; Strain rate, 1 / s; t represents time, s.

[0030] Step S2000 uses the three-dimensional displacement discontinuity method to discretize the hydraulic fracture along its length into several elements and construct the net pressure distribution matrix inside the fracture. Then, using the pre-constructed mechanical response relationship between the net pressure inside the fracture and the fracture width distribution, the fracture width distribution corresponding to the current net pressure distribution is solved.

[0031] Specifically, the crack is divided into N elements along its length, where each element has a height of 2h and a length of 2l. fBased on this, considering the non-uniform distribution of net pressure along the crack length, a net pressure distribution matrix inside the crack is constructed. To reduce the number of parameters to be inverted and to improve the physical constraints of the net pressure distribution, the net pressure distribution inside the crack can be further constrained to a specific shape function.

[0032] The method based on three-dimensional displacement discontinuity discretizes the hydraulic fracture along its length into several elements, including: The crack is discretized into N elements, where each element has a height of 2h and a length of l. f Crack half-crack length L f for: L f = N l f Among them, L f The length of half the crack is in meters (m). N represents the number of crack elements; l f Let be the length of the crack element, in meters (m).

[0033] The method for constructing the net pressure distribution matrix inside the crack is as follows: ; Where p is the net pressure distribution matrix on each crack element, Pa; p0 is the net pressure at the fracture inlet or bottom of the well, in Pa; S is a dimensionless net pressure shape function used to describe the relative variation of net pressure along the crack length direction.

[0034] Since different crack elements may have different orientations in space, a local coordinate system is first established for each crack element before calculating the interaction between them. For any crack element, its local coordinate system is defined as follows: x1 is along the crack length direction, x2 is along the crack height direction, and x3 is the normal direction of the crack surface. The transformation relationship between the global coordinate system (X, Y, Z) and the local coordinate system (x1, x2, x3) can be written as: ; Before the step of utilizing the pre-constructed mechanical response relationship between net pressure within the fracture and fracture width distribution, the method further includes: establishing a local coordinate system corresponding to each fracture element; the mechanical response relationship between net pressure within the fracture and fracture width distribution is as follows: ; in, Let be the normal stress response induced at the i-th crack element by the unit normal opening of the j-th crack element, i=1,2,...,N; j=1,2,...,N; μ is the shear modulus, in Pa; v is Poisson's ratio; ; ; ; ; ; ; x1, x2, and x3 are the coordinates of element i along the crack length direction, along the crack height direction, and the crack surface normal coordinate in the local coordinate system of element j, respectively. , Represents the angle between X in the global coordinate system and x1 in the local coordinate system for elements i and j; ; , These are the integral variables along the crack length and crack height directions, respectively; The overall stress influence coefficient matrix is ​​as follows: .

[0035] A 33 This is the overall stress influence coefficient matrix.

[0036] In the step of solving the crack width distribution corresponding to the current net pressure distribution, the calculation formula is as follows: ; Where w is the crack width distribution matrix, and m; p is the net pressure distribution matrix, Pa.

[0037] Step S3000, based on the fracture width distribution and three-dimensional displacement discontinuity method, performs forward modeling to calculate the axial strain response induced by the fracture at the monitoring well, forming a forward modeling model of pressure distribution, fracture width distribution, and strain solution.

[0038] In an infinitely elastic formation, the displacement x = (x1, x2, x3) at any position in the local coordinate system caused by a fracture element j with displacement discontinuity can be expressed as the displacement at any position in the local coordinate system caused by the fracture element j with displacement discontinuity: ; in, This is the kernel function. The commas in the function's subscript represent partial derivatives. c = 1, 2, 3, representing the x1, x2, and x3 directions, respectively. The numbers after the commas indicate the derivative in the corresponding direction. The first digit after the comma is 1, 2, or 3, indicating the derivative in the x1, x2, and x3 directions, respectively. The second digit after the comma is also 1, 2, or 3, indicating the derivative in the x1, x2, and x3 directions, respectively. When there are two numbers after the comma, the derivative is calculated twice. , , Let x1, x2, and x3 be the displacements in m in the local coordinate system at any position caused by the crack element j with displacement discontinuity. ; ; , These are the integral variables along the crack length and crack height directions, respectively; Transform the displacement in the local coordinate system into the displacement in the global coordinate system: ; , , They are respectively , , Displacement in global coordinates; By superimposing the contributions of all crack elements, the displacement of the entire crack at the monitoring point is obtained: ; The axial strain at the b-th monitoring point along the monitoring well is: ; L is the gauge length; for Displacement when X is b+L / 2; for Displacement when X is bL / 2; The fracture width distribution and the strain response of the monitoring well can be expressed in matrix form: By calculating the strain at each monitoring point, a strain distribution matrix is ​​formed.

[0039] Step S4000 uses the fracture half-fracture length, bottom hole net pressure, and fracture tip pressure as parameters to be inverted, sets the initial inversion values ​​and boundaries, calculates the model strain using the forward model, constructs residuals with the extracted well strain response, iteratively updates the parameters using the nonlinear least squares method to minimize the residuals, obtains the optimal fracture half-fracture length and bottom hole net pressure for the current time step, and outputs the pressure distribution and fracture width distribution within the fracture.

[0040] Set the initial values ​​and inversion boundaries for the bottom hole net pressure and fracture half-length inversion. Based on the forward model established in steps S2000 to S3000, calculate the model strain under this condition and construct the residual with the field strain. Use the nonlinear least squares method to iteratively update the fracture half-length and bottom hole net pressure to minimize the residual and obtain the optimal fracture half-length and bottom hole net pressure at the current time step. Output the pressure and width distribution results within the fracture.

[0041] The formula for the residual is: ; in, For on-site monitoring of well strain; This is the strain response obtained from the inversion.

[0042] After step S5000 proceeds to the next time step, the inversion result of the previous time step is used as the initial value for the current time step. This process is repeated to obtain the optimal fracture half-length and bottom hole net pressure, and the pressure distribution and fracture width distribution within the fracture are output until the set end time is reached. This yields the dynamic inversion results of the fracture half-length, net pressure distribution, and fracture width distribution evolving over time. The calculation process from steps S2000 to S4000 is repeated until the set end time is reached, finally yielding the dynamic inversion results of the fracture half-length, net pressure distribution, and fracture width distribution evolving over time.

[0043] Step S6000, based on the net pressure inside the fracture obtained from the inversion at each time step, combined with the field bottom hole pressure data, calculates the minimum ground stress corresponding to each time step according to the mechanical relationship between bottom hole pressure, net pressure and minimum ground stress, and obtains the dynamic evolution result of the minimum ground stress changing with time during the fracturing process.

[0044] The formula for calculating the minimum geostress corresponding to each time step is: ; in, Minimum ground stress; This refers to the pressure at the bottom of the well. Net pressure; For the friction of the perforation; t is the time step.

[0045] Example S1 uses a hydraulic fracturing simulator to generate a single fracture and ensures that it extends to the vicinity of the monitoring well to obtain a synthetic case. The strain data monitored on the monitoring well are then obtained from this synthetic case. The synthetic case is used as the final inversion target of this example, and the data on fracture geometry (length, width, net pressure) and strain variation along the monitoring well over time are saved. The inversion conditions of the synthetic case are shown in Table 1.

[0046] Table 1 Input parameters and subsequent inversion assumptions for the synthetic case S2 discretizes the crack along its length into N elements, each element being 2.4m in size. The net pressure distribution within the crack is assumed to be: P = P0 * (1.0 - (y / L)). f ) 2 ) (1 / 3) Where y is the coordinate of the center point of a certain unit, and L f Let be the half-crack length. A linear set of equations between the pressure inside the crack and the crack width is established using the stress influence coefficient matrix to solve for the crack width distribution corresponding to the current pressure distribution. Then, based on the inverted crack width distribution and the crack-induced strain forward model, the model strain is calculated to construct a forward model of "pressure distribution-crack width distribution-strain solution".

[0047] S3 Residual Function = (Synthetic Case Strain - Inverted Strain) * 10 9 10 9 This method is used solely to scale the residual magnitude to improve the stability of the numerical solution. It assumes an initial bottomhole net pressure of 2 MPa, an initial fracture half-length of 25 m, and inversion boundaries where the net pressure ranges from 0.1 to 20 MPa and the fracture half-length ranges from 0.5 to 150 m. Based on the forward model, the strain distribution is calculated, and a nonlinear least squares method is used to iteratively update the fracture half-length and bottomhole net pressure to minimize the residuals. This yields the optimal fracture half-length and bottomhole net pressure for the current time step, and outputs the net pressure within the fracture, fracture width, and strain distribution along the monitoring well. S4 proceeds to the next time step and repeats the above process until the time step before the fracture impacts the monitoring well is reached, at which point the calculation stops. Finally, the dynamic inversion results of the fracture half-length, net pressure distribution, and fracture width distribution over time are obtained. A comparison chart is then plotted between the inversion results of net pressure within the fracture, fracture width, and strain along the monitoring well at a specific time step and the verification results of the synthetic case. Figures 2 to 5 .

[0048] In the S5 synthetic case, the minimum geostress remains unchanged due to limitations of numerical simulation. However, when combined with the field case, by extracting the bottom hole pressure and perforation friction from the field case, and subtracting the bottom hole pressure at the corresponding time step from the net pressure obtained through inversion, the time-varying evolution process of the minimum geostress can be obtained. Figure 6 As shown.

[0049] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for time-varying evolution and inversion of crack morphology, net pressure, and minimum geostress, characterized in that, include: Acquire the bottom hole pressure data, horizontal and vertical well distance between the monitoring well and the fracturing well, fiber gauge length, and LF-DAS strain data before the fracture impacts the monitoring well obtained by distributed fiber measurement in the monitoring well during the fracturing operation. Then, perform synchronous processing on the data and extract the strain response of the monitoring well corresponding to each time step as the inversion target. Based on the three-dimensional displacement discontinuity method, the hydraulic fracture is discretized into several elements along its length and a net pressure distribution matrix inside the fracture is constructed. Then, using the pre-constructed mechanical response relationship between the net pressure inside the fracture and the fracture width distribution, the fracture width distribution corresponding to the current net pressure distribution is solved. Based on the fracture width distribution and the three-dimensional displacement discontinuity method, the axial strain response induced by the fracture at the monitoring well is calculated by forward modeling, forming a forward model for pressure distribution, fracture width distribution, and strain solution. Using the fracture half-fracture length, bottom hole net pressure, and fracture tip pressure as parameters to be inverted, initial values ​​and boundaries are set for inversion. The model strain is calculated using the forward model, and residuals are constructed with the extracted well strain response. The parameters are iteratively updated using the nonlinear least squares method to minimize the residuals, thereby obtaining the optimal fracture half-fracture length and bottom hole net pressure at the current time step, and outputting the pressure distribution and fracture width distribution within the fracture. After entering the next time step, the inversion result of the previous time step is used as the initial value of the current time step. The process is repeated to obtain the optimal fracture half-length and bottom hole net pressure, and the pressure distribution and fracture width distribution within the fracture are output until the set end time is reached, so as to obtain the dynamic inversion results of the evolution of fracture half-length, net pressure distribution and fracture width distribution over time. Based on the net pressure within the fracture obtained from the inversion at each time step, combined with the field bottom hole pressure data, and according to the mechanical relationship between bottom hole pressure, net pressure and minimum in-situ stress, the minimum in-situ stress corresponding to each time step is calculated, and the dynamic evolution result of the minimum in-situ stress changing with time during the fracturing process is obtained.

2. The time-varying evolution inversion method for crack morphology, net pressure, and minimum geostress as described in claim 1, characterized in that, The method based on three-dimensional displacement discontinuity discretizes the hydraulic fracture along its length into several elements, including: The crack is discretized into N elements, where each element has a height of 2h and a length of l. f Crack half-crack length L f for: L f = N l f Among them, L f The length of half the crack is in meters (m). N represents the number of crack elements; l f Let be the length of the crack element, in meters (m). The method for constructing the net pressure distribution matrix inside the crack is as follows: ； Where p is the net pressure distribution matrix on each crack element, Pa; p0 is the net pressure at the fracture inlet or bottom of the well, in Pa; S is a dimensionless net pressure shape function used to describe the relative variation of net pressure along the crack length direction.

3. The time-varying evolution inversion method for fracture morphology, net pressure, and minimum geostress as described in claim 2, characterized in that, Prior to the step of utilizing the pre-established mechanical response relationship between net intra-crack pressure and crack width distribution, the method further includes: Establish a local coordinate system corresponding to each crack element; The mechanical response relationship between the net pressure within the crack and the crack width distribution is as follows: ； in, Let be the normal stress response induced at the i-th crack element by the unit normal opening of the j-th crack element, i=1,2,...,N; j=1,2,...,N; μ is the shear modulus, in Pa; v is Poisson's ratio; ； ； ； ； ； ； x1, x2, and x3 are the coordinates of element i along the crack length direction, along the crack height direction, and the crack surface normal coordinate in the local coordinate system of element j, respectively. , Represents the angle between X in the global coordinate system and x1 in the local coordinate system for elements i and j; ； , These are the integral variables along the crack length and crack height directions, respectively; The overall stress influence coefficient matrix is ​​as follows: 。 4. The time-varying evolution inversion method for crack morphology, net pressure, and minimum geostress as described in claim 3, characterized in that, In the step of solving the crack width distribution corresponding to the current net pressure distribution, the calculation formula is as follows: ； Where w is the crack width distribution matrix, and m; p is the net pressure distribution matrix, Pa.

5. The time-varying evolution inversion method for fracture morphology, net pressure, and minimum geostress as described in claim 3, characterized in that, The method based on the fracture width distribution and three-dimensional displacement discontinuity is used to perform forward modeling of the axial strain response induced by the fracture at the monitoring well, forming a forward model for pressure distribution, fracture width distribution, and strain solution, including: The displacement in the local coordinate system at any position caused by the crack element j with displacement discontinuity is: ； in, This is the kernel function. The commas in the function's subscript represent partial derivatives. c = 1, 2, 3, representing the x1, x2, and x3 directions, respectively. The numbers after the commas indicate the derivative in the corresponding direction. The first digit after the comma is 1, 2, or 3, indicating the derivative in the x1, x2, and x3 directions, respectively. The second digit after the comma is also 1, 2, or 3, indicating the derivative in the x1, x2, and x3 directions, respectively. When there are two numbers after the comma, the derivative is calculated twice. , , Let x1, x2, and x3 be the displacements in m in the local coordinate system at any position caused by the crack element j with displacement discontinuity. ； ； , These are the integral variables along the crack length and crack height directions, respectively; Transform the displacement in the local coordinate system into the displacement in the global coordinate system: ； , , They are respectively , , Displacement in global coordinates; By superimposing the contributions of all crack elements, the displacement of the entire crack at the monitoring point is obtained: ； The axial strain at the b-th monitoring point along the monitoring well is: ； L is the gauge length; for Displacement when X is b+L / 2; for Displacement when X is bL / 2; A strain distribution matrix is ​​formed by calculating the axial strain at each monitoring point.

6. The time-varying evolution inversion method for crack morphology, net pressure, and minimum geostress as described in claim 3, characterized in that, In the step of calculating the model strain using the forward model, constructing residuals with the extracted well strain response, and iteratively updating the parameters using a nonlinear least squares method to minimize the residuals, the formula for the residuals is: ； in, For on-site monitoring of well strain; This is the strain response obtained from the inversion.

7. The time-varying evolution inversion method for fracture morphology, net pressure, and minimum geostress as described in claim 3, characterized in that, In the step of calculating the minimum in-situ stress corresponding to each time step based on the net pressure within the fracture obtained from the inversion at each time step, combined with the on-site bottom hole pressure data, and according to the mechanical relationship between bottom hole pressure, net pressure, and minimum in-situ stress, the calculation formula is as follows: ； in, Minimum ground stress; This refers to the pressure at the bottom of the well. Net pressure; For the friction of the perforation; t is the time step.

8. The time-varying evolution inversion method for fracture morphology, net pressure, and minimum geostress as described in claim 1, characterized in that, In the step of acquiring on-site bottomhole pressure data, horizontal and vertical well distance between the monitoring well and the fracturing well, fiber gauge length, and LF-DAS strain data obtained from distributed fiber optic measurements in the monitoring well during fracturing operations, and synchronously processing the data to extract the monitoring well strain response corresponding to each time step as the inversion target, when the LF-DAS strain data is the strain rate, the monitoring well strain is calculated based on the strain rate using the following formula: ； in, For on-site monitoring of well strain; Strain rate, 1 / s; t represents time, s.

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