A numerical simulation method for directional crossing-layer fracturing of coal seam roof horizontal well with stepwise scale

Abstract

The application discloses a kind of coal seam roof horizontal well directional wear layer fracturing step-by-step scale numerical simulation method, comprising: using finite element simulation software Abaqus to establish single perforation numerical model, perforation area numerical model and whole stratum numerical model;Single perforation numerical model crack result after single perforation simulation end processing is introduced into perforation area numerical model as the initial crack of perforation area simulation calculation;Then the crack result of perforation area numerical model after perforation area simulation end processing is introduced into whole stratum numerical model, as the initial crack of whole stratum simulation calculation, finally obtain wear layer extended crack, indirectly realize the simulation of cross-scale directional wear layer fracturing crack propagation process.The method solves the problem of huge number of grid division and crack propagation simulation across interface caused by the large gap between perforation size and fracturing crack size, and has broad application prospect in the numerical simulation of interface fracturing in broken soft coal seam roof horizontal well.

Description

Technical Field

[0001] This invention relates to the field of surface coalbed methane development technology, and in particular to a step-by-step numerical simulation method for directional cross-layer fracturing in horizontal wells in the roof of a coal seam. Background Technology

[0002] Horizontal wells in the roof are an effective method for coalbed methane development in soft, low-permeability coal seams. How to accurately simulate and evaluate the propagation of pre-fracturing fractures is a concern for field engineers. Physical simulation results show that directional perforation has the effect of inducing hydraulic fractures in the roof to propagate into the coal seam. Directional perforation parameters and interface properties have a significant impact on the propagation of hydraulic fractures. However, existing directional cross-layer fracturing simulation methods for horizontal wells in the roof of coal seams cannot accurately simulate complex formations and process conditions, nor can they capture the propagation path and morphology of hydraulic fractures well. This is mainly because: (1) the numerical influence of perforation parameters is limited. The simulation is mostly limited to two-dimensional simulation and cannot perform three-dimensional simulation. The difficulty lies in the large difference between the size of the perforation and the fracturing fracture, which leads to a huge number of meshes in the formation fracturing calculation model after considering the perforation, making it difficult to perform efficient calculation; (2) Existing hydraulic fracturing simulations are mostly for single homogeneous formations, and can only simulate two situations: hydraulic fractures directly pass through the interface or cannot pass through at all. However, the actual formation is generally not a single homogeneous layer, but is composed of multiple rock layers with different thicknesses, properties and stress states. This leads to the fact that when the fracture extends in the height direction, the morphology of the cross-layer fracturing fracture is often different.

[0003] In summary, existing directional cross-layer fracturing techniques for horizontal wells in coal seams cannot be simulated in three dimensions, resulting in a lack of quantitative and reliable basis for designing construction parameters. This makes it difficult to achieve the goal of efficiently transforming soft and fractured coal seams and enhancing coalbed methane production in horizontal wells in the roof. Summary of the Invention

[0004] To address the aforementioned problems in the prior art, the present invention aims to provide a step-by-step numerical simulation method for directional cross-layer fracturing in horizontal wells of coal seams, providing a quantitative and reliable basis for the design of directional cross-layer fracturing construction parameters, reducing the difficulty of fracturing construction in horizontal wells of the roof, improving the transformation effect of horizontal wells of the roof, and realizing the production enhancement design of surface coalbed methane horizontal wells with enhanced extraction effect.

[0005] To address the aforementioned problems, this invention discloses a step-by-step numerical simulation method for directional cross-layer fracturing in horizontal wells of coal seam roofs, the method comprising the following steps:

[0006] Step 1: Obtain the geometric parameters, physical and mechanical parameters, formation condition parameters, and construction parameters used to construct the single perforation numerical model, the perforation zone numerical model, and the overall formation numerical model. Input the obtained parameters into the finite element simulation software Abaqus to establish the single perforation numerical model, the perforation zone numerical model, and the overall formation numerical model.

[0007] Step 2: Using the constructed single-perforation numerical model, the XFEM method is used to simulate the propagation process of the crack from the initiation of a single perforation to the point before it merges with the cracks of other perforations, thus obtaining the single-perforation crack.

[0008] Step 3: After smoothing the single perforation crack obtained in Step 2, input it into the numerical model of the perforation area established in Step 1 as the initial crack for the simulation of the perforation area. Use the XFEM method to simulate the merging process of multiple initial cracks in the perforation area to obtain the intermediate crack formed by the merging of multiple initial cracks in the perforation area.

[0009] Step 4: After smoothing the intermediate fracture obtained in Step 3, input it as the initial fracture for the overall formation simulation into the overall formation numerical model established in Step 1. Simulate the cross-layer propagation of the intermediate fracture in the overall formation to obtain the cross-layer propagation fracture in the overall formation.

[0010] In the overall stratigraphic numerical model, zero-thickness cohesive unit layers are embedded between adjacent strata as stratigraphic interfaces.

[0011] The present invention also has the following technical features:

[0012] Specifically, the tensile strength of the stratum interface in the horizontal direction is equal to the tensile strength of the rock stratum to be traversed, and the tensile strength of the stratum interface in the vertical direction is equal to the bond strength of the adjacent rock strata.

[0013] Furthermore, the length of the single perforation numerical model along the wellbore direction is less than or equal to the spacing between adjacent perforations.

[0014] Furthermore, the length of the numerical model of the perforation zone along the height direction is less than or equal to the rock layer thickness of the horizontal well deployment layer.

[0015] Furthermore, the geometric parameters of the single-perforation numerical model include the length, width, height, perforation direction, perforation depth, and aperture diameter of the single-perforation numerical model.

[0016] Furthermore, the geometric parameters of the perforation zone numerical model include the length, width, and height of the perforation zone numerical model, the perforation zone range, and the perforation density.

[0017] Furthermore, the geometric parameters of the overall stratigraphic numerical model are the length, width, height, and stratigraphic thickness, and the stratigraphic layers include at least, from top to bottom, sandstone, sandy mudstone, mudstone, coal seam, and mudstone.

[0018] Furthermore, the physical and mechanical parameters include the elastic modulus, tensile strength, permeability, and Poisson's ratio of the formation and its interfaces; the formation condition parameters include vertical stress, maximum horizontal stress, and minimum horizontal stress; and the construction parameters include the flow rate, fracturing fluid viscosity, perforation azimuth, horizontal well location, and injection volume.

[0019] Furthermore, the smoothing process described in steps 3 and 4 is implemented using GeoMagic or Solidworks software.

[0020] The present invention has the following effects:

[0021] (1) The method of the present invention solves the problem of the huge number of grids in the formation fracturing calculation model after perforation caused by the large difference between the perforation size and the fracturing fracture size in the prior art by constructing three fracturing numerical models and adopting a step-by-step simulation method.

[0022] (2) When constructing the overall formation numerical model, the method of the present invention sets the interface between adjacent formations as a cohesive unit layer with zero thickness, which solves the problem of simulating the expansion of rock fractures across the interface of multiple rock layers with different thicknesses, properties and stress states. This makes the simulation results more consistent with the actual formation in the project and provides stronger guidance for cross-layer fracturing construction.

[0023] (3) The method of the present invention provides a step-by-step three-dimensional numerical simulation method for directional cross-layer fracturing in horizontal wells, which can realize the evaluation of the fracturing fracture propagation effect and the optimization of construction parameters. The optimization results can improve the fracturing fracture cross-interface propagation effect, achieve the purpose of enhancing the production of coalbed methane in horizontal wells in the roof. Moreover, the method of the present invention is stable and reliable, easy to operate, and has broad application prospects in cross-interface fracturing of horizontal wells in soft coal seams.

[0024] The details of this invention will become apparent from the following description and the accompanying drawings. Attached Figure Description

[0025] The accompanying drawings are provided to further illustrate the present disclosure and form part of the specification. They are used together with the following detailed description to explain the present disclosure, but do not constitute a limitation thereof. In the drawings:

[0026] Figure 1 This is a flowchart of the method of the present invention;

[0027] Figure 2 This is a schematic diagram of a single-perforation numerical model;

[0028] Figure 3 A three-dimensional cross-sectional view of the numerical model of a single perforation.

[0029] Figure 4 To import the numerical model of the perforation zone after the crack;

[0030] Figure 5 This is a three-dimensional cross-sectional view of the numerical model of the perforation zone;

[0031] Figure 6 To import the overall formation numerical model after the fracture;

[0032] Figure 7 This is a three-dimensional cross-section of the overall stratigraphic numerical model;

[0033] Figure 8 This is a cross-sectional view of the fracturing fractures in the perforation zone;

[0034] Figure 9 This is a cross-sectional view of the overall formation fracturing fractures;

[0035] Figure 10 The numerical simulation results are for directional cross-layer fracturing of a horizontal well in the top plate. Detailed Implementation

[0036] Following the above technical solutions, specific embodiments of the present invention are given below. It should be noted that the present invention is not limited to the following specific embodiments, and all equivalent modifications made based on the technical solutions of this application fall within the protection scope of the present invention.

[0037] In the description of this invention, the terms "comprising," "including," and "having" are intended to indicate inclusion. Furthermore, it should be understood that the terms "inner," "outer," "upper," and "lower," etc., indicate the positional relationship based on the positional relationship shown in the accompanying drawings, and are not intended to limit the invention.

[0038] The technical terms used in this invention are explained as follows:

[0039] The rock stratum to be crossed: refers to the rock stratum that the crack is about to cross. For example, when the crack is about to cross the coal seam from the roof, the rock stratum to be crossed is the coal seam; when the crack is about to enter the mudstone from the coal seam, the rock stratum to be crossed is the mudstone stratum.

[0040] The technical concept of this invention is as follows: a step-by-step simulation method for perforation fracturing is adopted. The fracture results of the single-perforation numerical model after the single-perforation simulation is processed are imported into the perforation zone numerical model as the initial fractures for the perforation zone simulation calculation. Then, the fracture results of the perforation zone numerical model after the perforation zone simulation is processed are imported into the overall formation numerical model as the initial fractures for the overall formation simulation calculation. Finally, cross-layer propagation fractures are obtained, indirectly realizing the simulation of the cross-scale directional cross-layer fracturing fracture propagation process.

[0041] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0042] Example 1

[0043] To better illustrate this, in this embodiment, a horizontal well on the roof of the No. 3 coal seam in a certain mine is selected. The distance between the horizontal well and the coal seam is less than or equal to 1.0m, and the length of the horizontal section is 748.6m. Cross-layer fracturing construction is carried out in 8 sections. The strata include coal seam, coal-rock strata, mudstone strata, sandy mudstone strata, and sandstone strata.

[0044] like Figure 1 As shown, this invention discloses a step-by-step numerical simulation method for directional cross-layer fracturing in horizontal wells of coal seam roof, comprising the following steps:

[0045] Step 1: Obtain the geometric parameters, physical and mechanical parameters, formation condition parameters, and construction parameters used to construct the single perforation numerical model, the perforation zone numerical model, and the overall formation numerical model. Input the obtained parameters into the finite element simulation software Abaqus to establish the single perforation numerical model, the perforation zone numerical model, and the overall formation numerical model.

[0046] In the overall formation numerical model, zero-thickness cohesive unit layers are embedded between adjacent formations as formation interfaces. Cohesive unit layers that comply with the traction-separation criterion are used to simulate the interface layers. The damage-failure behavior of cohesive units after being subjected to stress is used to simulate the failure behavior of the interface layers. The initial damage criterion and the damage evolution criterion are introduced into the material properties of the formation interface. The damage criterion of cohesive units is set using a keyword editor, and user-defined subroutines are then allowed to control the process, thus realizing the simulation of different modes of fracture penetration.

[0047] Specifically, the geometric parameters of the single perforation numerical model include the length, width, and height of the single perforation numerical model, the perforation direction, the perforation depth, and the borehole diameter; the geometric parameters of the perforation zone numerical model include the length, width, and height of the perforation zone numerical model, the perforation zone extent, and the borehole density; the geometric parameters of the overall stratigraphic numerical model are the length, width, and height of the overall stratigraphic numerical model and the stratigraphic thickness, and the stratigraphic layers include at least, from top to bottom, sandstone, sandy mudstone, mudstone, coal seam, and mudstone.

[0048] Physical and mechanical parameters include the elastic modulus, tensile strength, permeability, and Poisson's ratio of the formation and its interfaces; formation condition parameters include formation thickness, interface properties, vertical stress, maximum horizontal stress, and minimum horizontal stress; construction parameters include flow rate, fracturing fluid viscosity, perforation azimuth, horizontal well location, and injection volume.

[0049] The physical and mechanical parameters, formation condition parameters, and construction parameters of the strata and formation interfaces obtained in this embodiment are as follows:

[0050] As shown in Tables 1 and 2.

[0051]

[0052] Table 1. Physical and mechanical parameters of strata and stratigraphic interfaces

[0053] parameter numerical values parameter numerical values Vertical ground stress / MPa 16 Fracturing fluid viscosity (cp) 1 Maximum horizontal ground stress / MPa 12 Horizontal well location 1.0m inside the roof of the coal seam Minimum horizontal ground stress / MPa 10 <![CDATA[Liquid injection volume / m 3 / min]]> 800 <![CDATA[Displacement / m 3 / min]]> 10 Perforation azimuth Vertically downward

[0054] Table 2. Formation condition parameters and construction parameters

[0055] The geometric parameters of the single perforation numerical model, the perforation zone numerical model, and the overall formation numerical model in this embodiment are shown in Tables 3 to 5.

[0056] Model dimensions (length × width × height, in meters) 0.1×1.5×0.5 Perforation direction Vertically downward Perforation depth (m) 0.8 Orifice diameter (m) 0.01

[0057] Table 3. Geometric parameters of the single-perforation numerical model

[0058] Model dimensions (length × width × height, in meters) 3×4×3 Perforation zone range (m) 2 Pore ​​density (m) 10

[0059] Table 4. Geometric parameters of the numerical model for the perforation zone

[0060]

[0061] Table 5. Geometric parameters of the overall stratigraphic numerical model

[0062] The single-perforation numerical model constructed in this embodiment includes a single perforation, followed by mesh generation, as shown below. Figure 2 As shown, the single-perforation numerical model is ultimately divided into 12,500 elements after meshing.

[0063] Step 2: Using the constructed single-perforation numerical model, the XFEM method is used to simulate the propagation process of the crack from its initiation in a single perforation to its merging with cracks in other perforations, thus obtaining the single-perforation crack. After simulation, the size, direction, and spatial location of the single-perforation crack can also be obtained.

[0064] In finite element simulation, the `part` module is used to create the components of the simulated formation, and each component is partitioned. The `mesh` module is used to mesh the generated partitions. The `interaction` module sets the fracture control zone and initial fractures. The simulation considers the presence of single perforations in the formation, taking into account the perforation depth and borehole diameter. Figure 3 As shown, the square section below the wellbore simulates the top stratum, and the rectangle in the center of the square represents the perforation. As the fluid pressure inside the perforation increases, the fracture eventually initiates from the perforation, forming an initial fracture, i.e., an elliptical region. The fracture propagation direction is perpendicular to the direction of minimum geostress, ultimately resulting in a single-perforation fracture, as shown in the diagram. Figure 3The figure shows a three-dimensional cross-section of the single-perforation numerical model.

[0065] Finally, the single-perforation fracture calculated using the XFEM method can be exported using VRML. The results are then smoothed using GeoMagic and Solidworks software to facilitate importing into the next numerical model for simulation. The final simulation results of the single-perforation numerical model are obtained. The model is cut along the direction of maximum geostress, with the central part being the perforation. When the perforation is subjected to increasing pressure applied by fracturing fluid, it will eventually initiate and propagate to form a fracture.

[0066] Step 3: After smoothing the single-perforation crack obtained in Step 2, input it as the initial crack into the numerical model of the perforation zone established in Step 1 to obtain the following result. Figure 4 The numerical model of the perforation zone after introducing the crack is shown. Figure 5 The figure shows a three-dimensional cross-sectional view of the numerical model of the perforation zone; the XFEM method is used to simulate the confluence process of multiple initial cracks in the perforation zone, and the intermediate crack formed by the confluence of multiple initial cracks in the perforation zone is obtained; the size, direction and spatial location of the intermediate crack can also be obtained after simulation.

[0067] The numerical model of the perforation zone was also simulated using the XFEM method. In the simulation, the single-perforation cracks output from the single-perforation numerical model obtained in step 2 were assembled to simulate the process of multiple perforations initiating and merging simultaneously.

[0068] The final result is as follows Figure 8 The diagram shown is a cross-sectional view of the fracturing fracture in the perforated area, which is a Mises stress cloud diagram after multiple fractures merge into a single intermediate fracture. When fracturing fluid continues to be injected into the formation, the fluid volume inside the fracture increases, causing the fracture to expand and merge with other fractures to form a fracture surface, with the direction perpendicular to the direction of minimum geostress.

[0069] Step 4: After smoothing the intermediate fracture obtained in Step 3, input it as the initial fracture for the overall formation simulation into the overall formation numerical model established in Step 1. Simulate the cross-layer propagation of the intermediate fracture within the overall formation to obtain the cross-layer propagation fractures within the overall formation, as shown below. Figure 6 The overall formation numerical model after introducing fractures is shown. Figure 7 The three-dimensional cross-section of the overall stratigraphic numerical model shown can also be obtained after simulation, including the size, direction and spatial location of the cross-strata propagation fractures.

[0070] Material properties are set according to rock type and assigned to the corresponding partitions of the formation components. The interface layer of the overall formation numerical model is set in the keyword editor, enabling it to call user subroutines for calculations during simulation. In the step module, the geostatic stress equilibrium analysis step and the soil flow and seepage analysis step are set, the total simulation time is input, and output variables are set. In the load module, the injection point, stress / pore pressure boundary conditions, and geostress field are set. Finally, the job module performs calculations and outputs the results to obtain the final fracturing results.

[0071] The final result is as follows Figure 9 The overall formation fracturing fracture profile diagram is shown, in which the thin line in the center of the profile represents the specific shape of the fracture surface that is ultimately formed by fracturing.

[0072] The resulting file can be further processed to extract data and plot data such as... Figure 10 The image shown.

[0073] The method of this invention realizes the step-by-step numerical simulation from small fractures in a single perforation to larger fractures in the perforated area, and then to large cross-layer fractures in the entire formation. By adopting the step-by-step simulation method, it solves the problem of huge number of grids in the formation fracturing calculation model after perforation caused by the large difference between the perforation size and the fracturing fracture size in the prior art.

[0074] It is obvious that the above description and account are merely illustrative and not intended to limit the disclosure, application, or use of this invention. Although embodiments have been described and illustrated in the accompanying drawings, the invention is not limited to the specific examples exemplified by the drawings and described in the embodiments as currently considered the best mode for carrying out the teachings of the invention. The scope of the invention will include any embodiments falling within the foregoing description and the appended claims.

[0075] The preferred embodiments of the present disclosure have been described in detail above with reference to the accompanying drawings. However, the present disclosure is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present disclosure, various simple modifications can be made to the technical solutions of the present disclosure, and these simple modifications all fall within the protection scope of the present disclosure.

[0076] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, this disclosure will not describe the various possible combinations separately.

[0077] Furthermore, various different embodiments of this disclosure can be combined in any way, as long as they do not violate the spirit of this disclosure, they should also be regarded as the content disclosed in this disclosure.

Claims

1. A step-by-step numerical simulation method for directional cross-layer fracturing in horizontal wells of coal seam roof, characterized in that, The method includes the following steps: Step 1: Obtain the geometric parameters, physical and mechanical parameters, formation condition parameters, and construction parameters used to construct the single perforation numerical model, the perforation zone numerical model, and the overall formation numerical model. Input the obtained parameters into the finite element simulation software Abaqus to establish the single perforation numerical model, the perforation zone numerical model, and the overall formation numerical model. Step 2: Using the constructed single-perforation numerical model, the XFEM method is used to simulate the propagation process of the crack from the initiation of a single perforation to the point before it merges with the cracks of other perforations, thus obtaining the single-perforation crack. Step 3: After smoothing the single perforation crack obtained in Step 2, input it into the numerical model of the perforation area established in Step 1 as the initial crack for the simulation of the perforation area. Use the XFEM method to simulate the merging process of multiple initial cracks in the perforation area to obtain the intermediate crack formed by the merging of multiple initial cracks in the perforation area. Step 4: After smoothing the intermediate fracture obtained in Step 3, input it as the initial fracture for the overall formation simulation into the overall formation numerical model established in Step 1. Simulate the cross-layer propagation of the intermediate fracture in the overall formation to obtain the cross-layer propagation fracture in the overall formation. In the overall formation numerical model, zero-thickness cohesive unit layers are embedded between adjacent formations as formation interfaces. The tensile strength of the stratum interface in the horizontal direction is equal to the tensile strength of the rock stratum to be traversed, and the tensile strength of the stratum interface in the vertical direction is equal to the bond strength of the adjacent rock strata. The length of the single perforation numerical model along the wellbore direction is less than or equal to the distance between adjacent perforations.

2. The step-by-step numerical simulation method for directional cross-layer fracturing in horizontal wells of coal seam roof as described in claim 1, characterized in that, The length of the numerical model of the perforation zone along the height direction is less than or equal to the rock layer thickness of the horizontal well deployment layer.

3. The step-by-step numerical simulation method for directional cross-layer fracturing in horizontal wells of coal seam roof as described in claim 1, characterized in that, The geometric parameters of the single-perforation numerical model include the length, width, height, perforation direction, perforation depth, and aperture diameter.

4. The step-by-step numerical simulation method for directional cross-layer fracturing in horizontal wells of coal seam roof as described in claim 1, characterized in that, The geometric parameters of the perforation zone numerical model include the length, width, and height of the perforation zone numerical model, the perforation zone range, and the perforation density.

5. The step-by-step numerical simulation method for directional cross-layer fracturing in horizontal wells of coal seam roof as described in claim 1, characterized in that, The geometric parameters of the overall stratigraphic numerical model are the length, width, height, and stratigraphic thickness. The stratigraphic layers include, from top to bottom, sandstone, sandy mudstone, mudstone, coal seam, and mudstone.

6. The step-by-step numerical simulation method for directional cross-layer fracturing in horizontal wells of coal seam roof as described in claim 1, characterized in that, Physical and mechanical parameters include the elastic modulus, tensile strength, permeability, and Poisson's ratio of the formation and its interfaces; formation condition parameters include vertical stress, maximum horizontal stress, and minimum horizontal stress; construction parameters include flow rate, fracturing fluid viscosity, perforation azimuth, horizontal well location, and injection volume.

7. The step-by-step numerical simulation method for directional cross-layer fracturing in horizontal wells of coal seam roof as described in claim 1, characterized in that, The smoothing process described in steps 3 and 4 is implemented using GeoMagic or Solidworks software.

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