A method for optimizing horizontal well close-cutoff fracturing cluster spacing

Abstract

The application discloses a method for optimizing the interval distance of horizontal well multiple-cutoff multi-fracturing clusters, and belongs to the technical field of oil and gas reservoir reconstruction. The method comprises the following steps: S1, establishing a full-coupling numerical model for horizontal well multiple-cutoff multi-fracture propagation under the consideration of stress interference; S2, discretely solving the fracture propagation numerical model by using the finite element method; S3, establishing a geology-engineering integrated productivity numerical model based on the simulated fracture shape; S4, comprehensively analyzing the fracture propagation and productivity contribution results, and optimizing the interval distance of horizontal well multiple-cutoff multi-fracturing clusters. The method can realize the comprehensive analysis of the influence of different cluster interval distances on the fracture shape, stress interference, productivity contribution and the like from the two aspects of finite element fracture propagation and productivity contribution results based on the parameters of logging, rock and fluid, and can interactively simulate and optimize the interval distance of the clusters, so that a more reliable theoretical basis is provided for the optimization of the cluster interval distance of horizontal well multiple-cutoff multi-fracturing of unconventional oil and gas resources.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas reservoir stimulation technology, specifically relating to an optimization method for the spacing of closely spaced fracturing clusters in horizontal wells. Background Technology

[0002] Literature review revealed that the most relevant technology to this invention is a perforation parameter optimization design method for shale gas horizontal well tight-cutting fracturing invented by Xu Wenjun et al. of Yangtze University (publication number CN113850029A). This invention includes the following steps: Step 1, using the displacement discontinuity method to describe the relationship between stress and fracture width, coupling the fluid flow equations within the wellbore and fractures, and considering the effects of fracturing fluid loss and perforation friction, a fully fluid-structure coupled horizontal well tight-cutting fracturing multi-fracture synchronous propagation model is established; Step 2, using the displacement discontinuity method and the finite volume method to discretize the multi-fracture propagation model, and using the Newton-Raphson iterative method to solve the global nonlinear coupled equation set, and developing a calculation program; Step 3, calculating the uniformity development index of each cluster of hydraulic fractures under different perforation parameter schemes, and selecting the optimal perforation parameter scheme.

[0003] In the existing technology, the current method for analyzing stress interference between fractures to optimize the spacing of fracturing clusters uses the determination of whether stress reversal occurs between fractures as the basis for optimizing the spacing of fracturing clusters. However, this type of method has shortcomings. First, the calculation of induced stress between fractures only considers the deformation of the formation near the fracture, without considering the factor of fluid injection into the fracture that actually leads to fracture propagation. Second, this method assumes that all fractures can effectively extend after fracturing, but in the actual fracturing process, there will inevitably be ineffective fractures. Summary of the Invention

[0004] To address the aforementioned problems, this invention proposes an optimization method for the spacing of fracture clusters in horizontal wells. This method optimizes the spacing of the fracture clusters to obtain the maximum fracture-controlled reserves, and includes the following steps:

[0005] S1. Considering stress interference, establish a fully coupled numerical model for the propagation of multiple fractures in a horizontal well with close cutting;

[0006] S2. The numerical model of crack propagation is solved by discretization using the finite element method.

[0007] S3. Establish an integrated geological and engineering productivity numerical model based on simulated fracture morphology;

[0008] S4. Based on a comprehensive analysis of the fracture propagation and productivity contribution results, optimize the spacing of the horizontal well dense cutting fracturing clusters.

[0009] The beneficial effects of this invention are as follows: This invention can achieve comprehensive analysis based on parameters such as logging, rock, and fluid, from both the finite element fracture propagation and production contribution results, and interactively simulate and optimize the fracturing cluster spacing by considering the effects of different cluster spacings on fracture morphology, stress interference, and production contribution. This provides a more reliable theoretical basis for optimizing the cluster spacing of horizontal well close-cut fracturing in unconventional oil and gas resources.

[0010] 1. Current research on optimizing the cluster spacing in horizontal well fracturing mainly relies on numerical simulation methods. Most of these methods optimize the cluster spacing by studying the stress field variation law of multiple fractures in horizontal wells or the influence of cluster spacing on production capacity. However, these methods consider only one factor. This invention can achieve comprehensive analysis based on parameters such as logging, rock, and fluid, and comprehensively analyze the effects of different cluster spacings on fracture morphology, stress interference, and production capacity contribution from both the finite element fracture propagation and production capacity contribution results. It interactively simulates and optimizes the fracturing cluster spacing.

[0011] 2. The established finite element fracture propagation model considers the full fluid-solid coupling between the reservoir rock mass and the injected fluid, rather than calculating the inter-fracture induced stress by only considering the deformation of the formation near the fracture; it can consider the influence of various friction effects on the fluid inflow of each cluster of fractures, rather than specifying the fluid inflow of a single cluster of fractures; it can obtain the results of the displacement distribution, stress interference, and propagation morphology of each cluster of fractures at different time points during the fracture propagation process, rather than only obtaining the final simulation results.

[0012] 3. The fracture parameters in the integrated geological and engineering production capacity model are derived from the fracture propagation morphology obtained from the finite element fracture propagation model, rather than the rectangular fractures in traditional reservoir numerical simulation software. That is, the final production capacity contribution is the result after considering the interference between fractures. Attached Figure Description

[0013] Figure 1 This is a flowchart illustrating the implementation of the method for optimizing the spacing of horizontal well dense cutting fracturing clusters according to the present invention;

[0014] Figure 2 This is the initial finite element crack propagation model diagram of this invention;

[0015] Figure 3 This is a curve showing the distribution of discharge capacity of each crack in this invention;

[0016] Figure 4 This is a cloud map of reservoir stress distribution around the fracture in this invention;

[0017] Figure 5 This is a single-well section multi-cluster finite element simulation fracture morphology diagram of the present invention;

[0018] Figure 6 This is a cumulative oil production curve under different cluster spacing conditions according to the present invention. Detailed Implementation

[0019] A method for optimizing the spacing of closely spaced fracturing clusters in horizontal wells is proposed, such as... Figure 1-6 As shown, it includes the following steps:

[0020] S1. Establish a fully coupled numerical model for the propagation of multiple fractures in a horizontal well with close cutting, considering stress interference.

[0021] S2. The numerical model of crack propagation is solved by discretization using the finite element method.

[0022] S3. Establish an integrated geological and engineering productivity numerical model based on simulated fracture morphology;

[0023] S4. Based on a comprehensive analysis of the fracture propagation and productivity contribution results, optimize the spacing of the horizontal well dense cutting fracturing clusters.

[0024] Furthermore, step S1 includes: the multi-fracture propagation numerical model is a fully coupled model based on the reservoir rock mass and the injected fluid, which can consider the stress interference between fractures and the influence of various friction effects. It is achieved by simultaneously solving for the fracture width w and the fluid pressure p within the fracture. f and the displacement distribution q in each cluster of cracks i The equation system is used to obtain results such as displacement distribution, reservoir stress distribution around fractures, fracture morphology, and fracture area during the real-time synchronous propagation of multiple fractures in horizontal wells.

[0025] The system of equations includes:

[0026] Equation of relationship between crack width and net fluid pressure

[0027] Fluid flow and filtration equations within the crack

[0028] Fluid pressure equation in wellbore:

[0029] p1+Δp 1,p +Δp 1,t =p2+Δp 1,p +Δp 2,t =...=p N +Δp N,p +Δp N,t (i = 1, 2, ... N);

[0030] Criteria for crack propagation: K I >K IC .

[0031] In each formula, w is the fracture width (mm); r is the distance from the center of the perforation along the fracture propagation direction (m); t is time (s); K represents the integral kernel function dependent on fracture morphology and formation properties; p fσ0 is the fluid pressure within the fracture, MPa; σ0 is the minimum horizontal principal stress, MPa; S0 i (i = 1, 2, ..., N) represents the area of ​​the i-th crack, m 2 ; q and g represent the flow and filtration of fluid within the crack; Here, is the divergence operator defined within the crack surface; μ is the dynamic viscosity, mPa·s; C L The fluid filtration coefficient is given in m / s. 1 / 2 t0 is the arrival time of the fluid leading edge, in seconds; ΔP i,p For perforation friction, MPa; ΔP i,t For bending friction, MPa; p i (i = 1, 2, ..., N) represents the fluid pressure near the wellbore at each fracture, in MPa; K I The stress intensity factor is MPa·m. 1 / 2 ;K IC For rock fracture toughness, MPa·m 1 / 2 .

[0032] Furthermore, step S2 includes: the core of solving the simultaneous equation system is to effectively discretize the equation relating crack width and net fluid pressure using the finite element method. This is achieved by discretizing the analysis domain into a finite number of continuous elements, and then discretizing the equation system using the finite element method.

[0033] The finite element discretization equation for the relationship between crack width and net fluid pressure is: W = KP + C;

[0034] Finite element discretization equations for fracturing fluid flow and filtration:

[0035] K f PΔt+L(WW e )+GΔt+QΔt=0;

[0036] The pressure discretization equation F(q) for each cluster of fractures at the wellbore i ,q i+1 ) = 0 (i = 1, 2, ... N-1).

[0037] In each formula, K is a dense matrix reflecting the global dependence of the fracture width on fluid pressure, C is the fracture closure degree caused by σ0, and P and W are the fluid pressure and fracture width vectors to be determined. f Let L be the overall stiffness matrix of the fluid, G be the crack area matrix, Q be the equivalent nodal filtration vector, and W be the equivalent nodal displacement. e Δt is the crack width vector in the previous balancing step, and Δt is the time step.

[0038] Furthermore, step S3 includes: based on the simulated fracture morphology, and combined with geological parameters such as reservoir permeability, porosity, fluid saturation, reservoir pressure, reservoir temperature, net producing layer thickness, and sub-layer division depth of the target block, engineering parameters such as drilling and completion trajectory and perforation depth, and fluid parameters such as relative permeability curves and PVT, to establish an integrated geological and engineering production capacity numerical model and perform production simulation calculations.

[0039] Furthermore, step S4 includes: establishing a numerical model of multi-fracture propagation with different cluster spacings considering stress interference; obtaining the results of displacement distribution, reservoir stress distribution, and fracture morphology under different cluster spacing conditions through the finite element discretization method; comparing the displacement distribution of each cluster fracture, the magnitude of compressive stress between each cluster fracture, the distribution range of tensile stress at the fracture end, the fracture length, width, height, and area; establishing a production capacity calculation model based on different fracture morphologies obtained from finite element simulation calculations; comparing the production capacity contribution under different cluster spacing conditions; and comprehensively optimizing the spacing of horizontal well dense cutting fracturing clusters by combining the analysis results of the multi-fracture propagation model.

[0040] According to step S1 in the invention description and the accompanying drawings Figure 1 The process described uses a single section of a horizontal well in an oilfield as an example. The fracturing section of this well is 60m long, with a minimum horizontal principal stress of 35.97MPa, an elastic modulus of 28004MPa, a Poisson's ratio of 0.279, and a fracture toughness of 1.24MPa·m. 1 / 2 Tensile strength 8.8 MPa, filtration loss coefficient 0.0005 m / s 1 / 2 12m displacement 3 fracturing fluid density: 1000 kg / m³, number of perforations: 30, orifice diameter: 10 mm. 3 The fracturing fluid viscosity was 6 mPa·s. Finite element fracture propagation models were established with fixed section lengths of 60 m and cluster spacings of 4 m, 6 m, 8 m, 10 m, and 12 m, as shown in the attached figure. Figure 2 As shown, when the segment length is constant, the smaller the cluster spacing, the more perforation clusters per segment.

[0041] Based on the discrete solution method of the equation system described in step S2, results such as the displacement distribution of each fracture, the stress distribution of the reservoir around the fracture, and the fracture morphology can be obtained, as shown in the attached figure. Figures 3-5 As shown.

[0042] A numerical model integrating geology and engineering was established for the well's production capacity according to step S3. The model parameters include: reservoir interval 2187-2194m, reservoir thickness 7m, permeability 0.01-4.37mD, porosity 2.1-15.2%, and oil density 0.89g / cm³. 3The reservoir has a mid-depth temperature of 120℃ and a current formation pressure coefficient of 1.04. Based on drilling and completion data, perforation data, and fracture morphology, relative permeability parameters, and fluid PVT parameters obtained in step S2, production was conducted for one year under a constant bottom hole pressure regime. The production contribution under different cluster spacing conditions was calculated, and the results are shown in the attached figure. Figure 6 As shown.

[0043] The cluster spacing optimization analysis in step S4 shows that a larger cluster spacing leads to more uniform fracture propagation, less inter-fracture stress interference, and a more even distribution of average fracture discharge. However, this results in insufficient stimulation and failure to obtain the optimal fracture-controlled reserves. Conversely, an excessively small cluster spacing leads to uneven fracture propagation, even the presence of ineffective fractures, increased inter-fracture compressive stress, uneven fluid inflow to each cluster, and more severe inter-fracture interference. After comprehensive analysis, a reasonable cluster spacing range for this well is considered to be 6m-8m. Under this condition, the maximum fracture-controlled reserves can be obtained while effectively utilizing inter-fracture stress interference to increase fracture complexity, ultimately achieving the goal of maximizing the stimulation effect.

[0044] The finite element fracture propagation model established in this invention considers the full fluid-solid coupling between the reservoir rock mass and the injected fluid, rather than calculating the inter-fracture induced stress by solely considering the formation deformation near the fracture. It can account for the influence of various frictional effects on the fluid inflow of each fracture cluster, rather than specifying the fluid inflow of a single fracture cluster. It can obtain results such as the displacement distribution, stress interference, and propagation morphology of each fracture cluster at different time points during fracture propagation, rather than only obtaining the final simulation result. Furthermore, the fracture parameters in the integrated geological and engineering productivity model established in this invention are derived from the fracture propagation morphology obtained from the finite element fracture propagation model, rather than the rectangular fractures in traditional reservoir numerical simulation software; that is, the final productivity contribution is the result after considering inter-fracture interference. This invention has been applied in nine wells in the field, and good field feedback results have been obtained.

[0045] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for optimizing the spacing of horizontal well dense-cut fracturing clusters, characterized in that, Includes the following steps: S1. Under stress disturbance conditions, establish a fully coupled numerical model for the propagation of multiple fractures in a horizontal well with close cutting. The following is performed in step S1: By simultaneously solving the crack width Fluid pressure within the crack and the amount of displacement in each cluster of cracks The system of equations for unknowns is used to obtain real-time data on displacement distribution, reservoir stress distribution around fractures, fracture morphology, and fracture area during the synchronous propagation of multiple fractures in horizontal wells. S2. The numerical model of multi-crack propagation is solved by discretization using the finite element method. Step S2 involves the following: effectively discretizing the equation relating crack width to net fluid pressure using the finite element method. This is done by discretizing the analysis domain into a finite number of continuous elements, and then performing finite element discretization on the equation system. Finite element discretization equation for the relationship between crack width and net fluid pressure: ; Finite element discretization equations for fracturing fluid flow and filtration: ； Discrete equations of pressure at the wellbore for each cluster of fractures ; Among the various types, It is a dense matrix that reflects the global dependence of crack width on fluid pressure. yes The resulting crack closure degree and Let the fluid pressure and crack width vectors be the unknowns. The overall stiffness matrix of the fluid. The crack area matrix, Let be the equivalent node filtering vector, and be the equivalent node displacement. The crack width vector from the previous balancing step. For time step; S3. Establish an integrated geological and engineering productivity numerical model based on simulated fracture morphology; In step S3: based on the simulated fracture morphology, geological parameters include reservoir permeability, porosity, fluid saturation, reservoir pressure, reservoir temperature, net production layer thickness, and sub-layer division depth; engineering parameters include drilling and completion trajectory and perforation depth; and fluid parameters include relative permeability curve and PVT. A geological-engineering integrated production capacity numerical model is established by combining the geological parameters, engineering parameters, and fluid parameters of the target block to perform production simulation calculations. S4. Based on a comprehensive analysis of fracture propagation and productivity contribution results, optimize the spacing of horizontal well dense cutting fracturing clusters; The following steps are performed in step S4: A numerical model of multi-fracture propagation with different cluster spacings is established considering stress interference. The displacement distribution, reservoir stress distribution, and fracture morphology under different cluster spacing conditions are obtained by using the finite element discretization method. The displacement distribution of each cluster fracture, the magnitude of compressive stress between each cluster fracture, the distribution range of tensile stress at the fracture end, the fracture length, width, height, and area are compared. Based on the different fracture morphologies obtained by finite element simulation, a production capacity calculation model is established. The production capacity contribution under different cluster spacing conditions is compared. Combining the analysis results of the multi-fracture propagation model, the spacing of horizontal well dense cutting fracturing clusters is comprehensively optimized.

2. The method for optimizing the spacing of horizontal well dense-cut fracturing clusters according to claim 1, characterized in that, The system of equations includes: The equation relating crack width to net fluid pressure is as follows: ; Fluid flow and filtration equations within the crack , ; Fluid pressure equation in wellbore: ； Criteria for crack propagation: ; Among the various types, The crack width is in mm. The distance from the center of the perforation along the direction of crack propagation is expressed in meters (m). Time, in seconds; This represents an integral kernel function that depends on fracture morphology and formation properties; This represents the fluid pressure inside the fracture, expressed in MPa. The minimum horizontal principal stress is given in MPa. Let be the area of ​​the i-th crack, in meters. 2 ; and This indicates the flow and loss of fluid within the crack; Here is a divergence operator defined within the crack surface; Dynamic viscosity, in mPa·s; Fluid filtration coefficient, in m / s 1 / 2 ; The arrival time of the fluid front, in seconds; The frictional resistance of the perforation is expressed in MPa. Bending friction, unit: MPa; The fluid pressure near the wellbore at each fracture is expressed in MPa. Stress intensity factor, in MPa·m 1 / 2 ; Rock fracture toughness, in MPa·m 1 / 2 .

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