Method and system for simulating proppant migration in unconventional natural gas fracturing production

Abstract

The present invention proposes a proppant migration simulation method and system in unconventional natural gas fracturing exploitation, comprising: obtaining material parameters, including material parameters of proppant particles, material parameters of fracturing fluid, and material parameters of shale gas reservoirs; calibrating the material parameters of proppant particles and shale gas reservoirs respectively; after the parameters are calibrated, based on the morphology of rock mass and fracture network after shale gas reservoir fracturing, establishing a discrete element particle calculation model of the fracture network structure of shale gas reservoirs after fracturing, and establishing a computational fluid dynamics calculation grid along the fracture network of the calculation model; obtaining shale gas reservoir ground stress data and fracturing fluid injection pressure data, and setting the boundary conditions of the model based on the obtained data; and simulating proppant migration and the effect of proppant fracture network.

Description

Technical Field

[0001] The present invention belongs to the technical field of numerical simulation of energy exploitation, and in particular relates to a proppant migration simulation method and system in unconventional natural gas fracturing exploitation. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] As an ideal replacement for conventional oil and natural gas, shale gas mining has become one of the levers for changing global energy supply and prices. Vigorously developing shale gas can not only increase the proportion of new energy and reduce the high dependence on high-carbon emission coal and oil, but also help improve the energy consumption structure and enhance energy security.

[0004] The rapid development of shale gas extraction in the past 20 years mainly relies on breakthroughs in key technologies such as hydraulic fracturing and horizontal drilling. Fracturing technology is a key technology in shale gas extraction. It is the key to whether shale gas extraction can achieve large-scale commercial development and ensure shale gas production. Its fracturing costs generally account for about 30% of the total extraction costs. Fracturing technology is divided into hydraulic fracturing, carbon dioxide / nitrogen fracturing, foam fracturing and other fracturing methods. The fracturing process mainly includes the fracturing fluid splitting the rock under high pressure. Under continuous high pressure, the cracks in the rock will continue to expand, and under the action of multiple joint fracturing, a reservoir fracture network will eventually be formed.

[0005] Due to the increase in reservoir closure pressure under deep burial conditions, the initial fracture width becomes narrower and the conductivity decreases rapidly, making it more difficult to maintain the long-term conductivity of the fracture network. Proppant, as a supporting carrier for fracturing, ensures the long-term conductivity of the fracture. It operates with the fracturing fluid in larger fractures and accumulates in fractures smaller than the proppant particle size to prevent the fractures from closing under high ground stress.

[0006] According to the inventors' understanding, domestic and foreign scholars have studied this process through theoretical calculations, model tests and numerical simulation methods. However, there is little consideration of the impact mechanism of the dynamic expansion of the fracture network on the migration and transportation of the proppant, and a single calculation method is difficult to simulate the two processes of proppant particle flow and supporting the fracture network at the same time, making it difficult to simulate the migration and action process of the proppant in a complex fracture network. Summary of the invention

[0007] In order to overcome the deficiencies of the above-mentioned prior art, the present invention provides a proppant migration simulation method in unconventional natural gas fracturing production, which is convenient for analyzing the migration and supporting effect of proppant particles, can consider the interaction between fracturing fluid, rock mass and proppant particles, and realize the simulation of the migration and action process of proppant in complex fracture networks.

[0008] To achieve the above objectives, one or more embodiments of the present invention provide the following technical solutions:

[0009] In a first aspect, a proppant migration simulation method in unconventional natural gas fracturing production is disclosed, comprising:

[0010] Obtaining material parameters, including material parameters of proppant particles and material parameters of shale gas reservoirs;

[0011] Calibrate the material parameters of proppant particles and shale gas reservoir material parameters respectively;

[0012] After the parameters are calibrated, a discrete element particle calculation model of the fracture network structure of the shale gas reservoir after fracturing is established based on the morphology of the rock mass and fracture network of the shale gas reservoir after fracturing, and a computational fluid dynamics calculation grid is established along the fracture network of the calculation model;

[0013] Obtain shale gas reservoir in-situ stress data and fracturing fluid injection pressure data, and set the model boundary conditions based on the acquired data;

[0014] Simulate proppant migration and proppant network effects.

[0015] As a further technical solution, when obtaining the material parameters of the shale gas reservoir, rock samples of the shale gas reservoir are obtained, and the strength and elastic modulus parameters of the reservoir rock mass are measured through indoor tests as the simulated solid domain material parameters.

[0016] As a further technical solution, when calibrating the material parameters of the proppant particles, the static friction coefficient and rolling friction coefficient between the particles are calibrated according to the static and dynamic repose angles of the samples measured by the drop test and the roller test.

[0017] As a further technical solution, when calibrating the parameters of shale gas reservoir materials, the elastic modulus, normal stiffness, tangential stiffness, normal bond strength and tangential bond strength of the parallel bond are calibrated according to the elastic modulus, Poisson's ratio, friction coefficient, compressive strength and tensile strength of the rock and soil samples measured by uniaxial compression, triaxial compression and Brazilian splitting tests, so that the strength indicators of the model under the bonding action of parallel bond are consistent with those of the rock and soil samples.

[0018] As a further technical solution, the boundary conditions of the model are set based on the acquired data, specifically:

[0019] The geostress in the model area is calculated according to the burial depth of the shale gas reservoir and the lateral pressure coefficient of the rock formation, and the geostress is evenly applied to each boundary of the model;

[0020] Based on the selected production pressure, the corresponding fluid pressure is set at the inlet of the model fracturing fluid.

[0021] As a further technical solution, the simulation of proppant migration and support fracture network effect includes:

[0022] Step 1: Calculate the fluid domain: Use the NS equation to calculate the flow rate and pressure information of the fluid at a certain moment;

[0023] Step 2: Calculate the simulation data of the solid part of the model: the ground stress on the boundary, the pressure of the fracturing fluid on the boundary of the fracture network, and the contact force of the supporting particles;

[0024] Step 3: By retrieving the fracture of parallel bonding bonds between solid DEM particles in the rock formation, the area where the fracture exceeds the lower limit of fracture generation is regarded as the newly generated fracture area, so as to simulate the expansion of the fracture network during the fracturing process;

[0025] Step 4: Based on the continuously expanding fracture network during the fracturing process, update the fluid calculation grid to cover all fracture areas;

[0026] Step 5: Repeat step 1 to step 4 until the model reaches overall equilibrium.

[0027] As a further technical solution, after the model reaches overall equilibrium, the migration and deposition of proppant particles are statistically analyzed:

[0028] By obtaining the velocity of the proppant particles at each calculation time, the proppant particles with a velocity less than a set value are regarded as deposited proppant, and the proppant particles with a velocity greater than the set value are regarded as proppant particles still migrating with the fracturing fluid.

[0029] In the second aspect, a proppant migration simulation system in unconventional natural gas fracturing production is disclosed, including:

[0030] The parameter acquisition module is configured to: acquire material parameters, including material parameters of proppant particles, material parameters of fracturing fluid, and material parameters of shale gas reservoirs;

[0031] The parameter calibration module is configured to: calibrate the material parameters of the proppant particles and the material parameters of the shale gas reservoir respectively;

[0032] The model building module is configured to: after parameter calibration, establish a discrete element particle calculation model of the fracture network structure of the shale gas reservoir after fracturing based on the morphology of the rock mass and fracture network after fracturing of the shale gas reservoir, and establish a computational fluid dynamics calculation grid along the fracture network of the calculation model;

[0033] Obtain shale gas reservoir in-situ stress data and fracturing fluid injection pressure data, and set the model boundary conditions based on the acquired data;

[0034] The simulation module is configured to simulate proppant migration and propping network effects.

[0035] One or more of the above technical solutions have the following beneficial effects:

[0036] The proppant migration simulation scheme in unconventional natural gas fracturing production provided by the present invention facilitates the analysis of the migration and supporting effect of proppant particles, can take into account the interaction between fracturing fluid, rock mass and proppant particles, and realizes the simulation of the migration and action process of proppant in complex fracture networks.

[0037] Advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0039] Figure 1 The calculation flow chart of proppant migration and support seam network realization in the embodiment of the present invention;

[0040] Figure 2 It is a schematic diagram of the calculation model of unconventional natural gas fracturing production according to an embodiment of the present invention. DETAILED DESCRIPTION

[0041] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0042] It should be noted that the terms used herein are for describing specific embodiments only and are not intended to be limiting of exemplary embodiments according to the present invention.

[0043] In the absence of conflict, the embodiments of the present invention and the features of the embodiments may be combined with each other.

[0044] The overall idea proposed by the present invention is:

[0045] 1. Obtain proppant particle density, particle size, friction angle, and elastic modulus parameters;

[0046] 2. Obtain fracturing fluid density and viscosity parameters;

[0047] 3. Obtain shale gas reservoir strength parameters;

[0048] 4. Calibrate macro and micro parameters of shale gas reservoirs and proppant particles;

[0049] 5. Establish a DEM (discrete element) particle calculation model of the fracture network structure of the shale gas reservoir after fracturing, establish a CFD (computational fluid dynamics) calculation grid along the model fracture network, and set the inlet of the fracturing fluid and proppant particles;

[0050] 6. Obtain reservoir in-situ stress and fracturing fluid injection pressure, and set the boundary conditions of the model;

[0051] 7. Based on the interaction between proppant particles, rock particles and fracturing fluid, simulate the proppant migration and proppant fracture network.

[0052] Embodiment 1

[0053] This embodiment discloses a method for simulating proppant migration in unconventional natural gas fracturing production. Figure 1 As shown, the following steps are included:

[0054] Step 1: Obtain the material parameters of the proppant particles.

[0055] The required proppant type is selected, and through indoor experiments, the density, particle size, elastic modulus and other parameters of the proppant particles are measured as the simulated particle material parameters.

[0056] Step 2: Obtain material parameters of the fracturing fluid.

[0057] The type of fracturing fluid required for the fracturing production process is selected, and the density, viscosity and other parameters of the fracturing fluid are measured through indoor tests as the simulated fluid material parameters.

[0058] Step 3: Obtain shale gas reservoir material parameters.

[0059] Obtain rock samples from shale gas reservoirs, and perform indoor tests such as uniaxial compression, triaxial compression, and Brazilian splitting to measure parameters such as the strength and elastic modulus of the reservoir rock mass as the simulated solid domain material parameters.

[0060] Step 4: Macro and micro parameter calibration

[0061] Since macroscopic parameters such as elastic modulus and compressive strength cannot be directly applied to the material in discrete element calculations, it is necessary to adjust the microscopic parameters between particles to match the macroscopic mechanical properties. Therefore, indoor experiments are carried out to calibrate the parameters of the calculation model.

[0062] Step 4.1, proppant particle parameter calibration.

[0063] There is no mutual adhesion between proppant particles, that is, they can be regarded as non-sticky particles. Macro- and micro-parameter calibration is carried out. According to the macro-indicators of static and dynamic repose angles α and β of the samples measured by the drop test and the roller test, the static friction coefficient μ between the particles is calibrated. s and rolling friction coefficient μ r .

[0064] Step 4.2, shale gas reservoir parameter calibration.

[0065] The parallel bonding model in DEM is used to simulate the strong bonding between rock particles. The elastic modulus E, Poisson's ratio ν, friction coefficient c, compressive strength σ of the rock and soil samples measured by uniaxial compression, triaxial compression and Brazilian splitting tests are used to calculate the elastic modulus E, Poisson's ratio ν, friction coefficient c, compressive strength σ t , tensile strength σ l , calibrate the elastic modulus E of its parallel bond c , normal stiffness k n , tangential stiffness k s , normal bond strength σ c , tangential bond strength τ c , so that the strength index of the model under the bonding action of parallel bonding bonds is consistent with that of the rock and soil sample.

[0066] During calibration, by adjusting the parameters of the parallel bond, simulations of uniaxial compression, Brazilian splitting, and triaxial compression tests are carried out in the discrete element to test the macroscopic mechanical strength (compressive strength, tensile strength, elastic modulus, etc.) of the discrete element model. When the values ​​are consistent with those measured in indoor tests, the calibration is successful.

[0067] Step 5: Establish the calculation model, see Appendix Figure 2 shown.

[0068] Based on the morphology of the rock mass and fracture network after fracturing of the shale gas reservoir, a DEM calculation model of the shale gas reservoir is established. In the DEM model, there is no interaction between adjacent particles at the position corresponding to the fracture network. A CFD fluid calculation model is established based on the fracture network morphology, and the inlet of the fracturing fluid is set. The fracturing fluid inlet is used as the proppant particle inlet, and proppant particles are circulated and generated at this inlet.

[0069] The DEM calculation model of the shale gas reservoir is established using modeling software such as Rhino, which is the core model of numerical simulation; the CFD fluid calculation model is established using the commonly used CFD modeling software in ANSYS.

[0070] Step 6: Set boundary conditions.

[0071] The geostress in the model area is calculated according to the burial depth of the shale gas reservoir and the lateral pressure coefficient of the rock formation, and the geostress is evenly applied to each boundary of the model; based on the selected production pressure, the corresponding fluid pressure is set at the inlet of the model fracturing fluid.

[0072] The above boundary conditions are the prerequisite for the convergence of numerical simulation calculations.

[0073] Step 7: Simulation of proppant migration and proppant network effect.

[0074] Step 7.1, calculate the fluid domain.

[0075] The NS equation is used to calculate the flow rate, pressure and other information of the fluid, which is the basic information to describe the flow state of the fluid, as shown in Formula 1.

[0076]

[0077] Where u is the fluid velocity, t is the time, p is the fluid pressure, and ρ f is the fluid density, R pf is the fluid-solid coupling force, and τ is the fluid viscosity.

[0078] During the simulation, each moment is calculated according to the time when the fluid velocity and pressure are collected in equation (1).

[0079] Step 7.2, calculate the solid fraction.

[0080] The DEM calculation model includes two parts of solids, one is the bonded rock solids, and the other is the unbonded supporting particles. The rock solids are mainly affected by the ground stress on the boundary, the pressure of the fracturing fluid on the fracture network boundary, and the contact force of the supporting particles. The pressure of the fracturing fluid can be obtained from the fluid pressure calculated in step 7.1, and the contact force of the proppant particles can be calculated by the built-in contact model of DEM. The proppant particles migrating in the fracture network are mainly affected by the drag force of the fracturing fluid and the collision between particles. The migration, stress, and velocity of the proppant can be calculated by Newton's second theorem, as shown in Formula 2.

[0081]

[0082] In the formula, m i is the proppant particle mass, is the acceleration of the proppant particle, F i，n 、F i，f They are the interaction force between particles and the interaction force between particles and fluid, respectively.

[0083] This step is used to calculate the stress of shale gas reservoirs under the action of fracture seepage. The advantage is that it can accurately calculate the expansion of fractures and the interaction between proppant particles and rock formations.

[0084] Step 7.3, search for parallel bond breaks.

[0085] By retrieving the fracture of parallel bonding bonds between solid DEM particles in the rock formation, the area where the fracture exceeds the lower limit of fracture generation ξ is regarded as the newly generated fracture area, thereby simulating the expansion of the fracture network during the fracturing process.

[0086] Step 7.4, fluid grid update.

[0087] Based on the continuously expanding fracture network morphology during the fracturing process, the fluid calculation grid is updated to cover all fracture areas.

[0088] Step 7.5: Repeat steps 6.1-6.4 until the model reaches equilibrium. Specifically, when the velocity and pressure of the CFD flow field no longer change and the unbalanced force between DEM particles is less than 10 -5 When N, the model is in equilibrium.

[0089] Step 8: Count the migration and deposition of proppant particles.

[0090] By obtaining the velocity of the proppant particles at each calculation moment, the velocity less than v min The proppant particles with a diameter greater than v are considered as deposited proppant, which mainly play a supporting role in the fracture network. min The proppant particles that are still migrating with the fracturing fluid are regarded as proppant particles.

[0091] The above DEM calculation part can directly obtain the particle velocity.

[0092] Embodiment 2

[0093] The purpose of this embodiment is to provide a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the steps of the above method when executing the program.

[0094] Embodiment 3

[0095] The purpose of this embodiment is to provide a computer-readable storage medium.

[0096] A computer-readable storage medium stores a computer program, which executes the steps of the above method when executed by a processor.

[0097] Embodiment 4

[0098] The purpose of this embodiment is to provide a proppant migration simulation system in unconventional natural gas fracturing production, including:

[0099] The parameter acquisition module is configured to: acquire material parameters, including material parameters of proppant particles, material parameters of fracturing fluid, and material parameters of shale gas reservoirs;

[0100] The parameter calibration module is configured to: calibrate the material parameters of the proppant particles and the material parameters of the shale gas reservoir respectively;

[0101] The model building module is configured to: after parameter calibration, establish a discrete element particle calculation model of the fracture network structure of the shale gas reservoir after fracturing based on the morphology of the rock mass and fracture network after fracturing of the shale gas reservoir, and establish a computational fluid dynamics calculation grid along the fracture network of the calculation model;

[0102] Obtain shale gas reservoir in-situ stress data and fracturing fluid injection pressure data, and set the model boundary conditions based on the acquired data;

[0103] The simulation module is configured to simulate proppant migration and propping network effects.

[0104] The steps involved in the apparatuses of the above embodiments 2, 3 and 4 correspond to the method embodiment 1, and the specific implementation methods can refer to the relevant description part of embodiment 1. The term "computer-readable storage medium" should be understood as a single medium or multiple media including one or more instruction sets; it should also be understood to include any medium that can store, encode or carry an instruction set for execution by a processor and enable the processor to execute any method in the present invention.

[0105] Those skilled in the art should understand that the modules or steps of the present invention described above can be implemented by a general-purpose computer device, or alternatively, they can be implemented by a program code executable by a computing device, so that they can be stored in a storage device and executed by the computing device, or they can be made into individual integrated circuit modules, or multiple modules or steps therein can be made into a single integrated circuit module for implementation. The present invention is not limited to any specific combination of hardware and software.

[0106] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. Simulation method of proppant migration in unconventional natural gas fracturing production, Its characteristics are: include: Obtaining material parameters, including material parameters of proppant particles and material parameters of shale gas reservoirs; Calibrate the material parameters of proppant particles and shale gas reservoir material parameters respectively; After the parameters are calibrated, a discrete element particle calculation model of the fracture network structure of the shale gas reservoir after fracturing is established based on the morphology of the rock mass and fracture network of the shale gas reservoir after fracturing, and a computational fluid dynamics calculation grid is established along the fracture network of the calculation model; Obtain shale gas reservoir in-situ stress data and fracturing fluid injection pressure data, and set the model boundary conditions based on the acquired data; Simulate proppant migration and proppant network effects; Among them, the simulation of proppant migration and support fracture network effect includes: Step 1: Calculate the fluid domain: Use the NS equation to calculate the flow rate and pressure information of the fluid at a certain moment; Step 2: Calculate the simulation data of the solid part of the model: the ground stress on the boundary, the pressure of the fracturing fluid on the boundary of the fracture network, and the contact force of the supporting particles; Step 3: By retrieving the fracture of parallel bonding bonds between solid DEM particles in the rock formation, the area where the fracture exceeds the lower limit of fracture generation is regarded as the newly generated fracture area, so as to simulate the expansion of the fracture network during the fracturing process; Step 4: Based on the continuously expanding fracture network during the fracturing process, update the fluid calculation grid to cover all fracture areas; Step 5: Repeat step 1 to step 4 until the model reaches overall equilibrium.

2. The method for simulating proppant migration in unconventional natural gas fracturing production according to claim 1, Its characteristics are: When obtaining the material parameters of the shale gas reservoir, a rock sample of the shale gas reservoir is obtained, and the strength and elastic modulus parameters of the reservoir rock mass are measured through indoor tests as the simulated solid domain material parameters.

3. The method for simulating proppant migration in unconventional natural gas fracturing production according to claim 1, Its characteristics are: When calibrating the material parameters of the proppant particles, the static friction coefficient and rolling friction coefficient between the particles are calibrated according to the static and dynamic repose angles of the samples measured by the drop test and the roller test.

4. The method for simulating proppant migration in unconventional natural gas fracturing production according to claim 1, Its characteristics are: When calibrating the parameters of shale gas reservoir materials, the elastic modulus, normal stiffness, tangential stiffness, normal bond strength and tangential bond strength of the parallel bond are calibrated according to the elastic modulus, Poisson's ratio, friction coefficient, compressive strength and tensile strength of the rock and soil samples measured by uniaxial compression, triaxial compression and Brazilian splitting tests, so that the strength index of the model under the bonding action of parallel bond is consistent with that of the rock and soil samples.

5. The method for simulating proppant migration in unconventional natural gas fracturing production according to claim 1, Its characteristics are: The boundary conditions of the model are set based on the acquired data, specifically: The geostress in the model area is calculated according to the burial depth of the shale gas reservoir and the lateral pressure coefficient of the rock formation, and the geostress is evenly applied to each boundary of the model; Based on the selected production pressure, the corresponding fluid pressure is set at the inlet of the model fracturing fluid.

6. The method for simulating proppant migration in unconventional natural gas fracturing production according to claim 1, Its characteristic is that the model After the overall balance is reached, the migration and deposition of proppant particles are counted: By obtaining the velocity of the proppant particles at each calculation time, the proppant particles with a velocity less than a set value are regarded as deposited proppant, and the proppant particles with a velocity greater than the set value are regarded as proppant particles still migrating with the fracturing fluid.

7. Proppant migration simulation system in unconventional natural gas fracturing production, Its characteristics are: include: The parameter acquisition module is configured to: acquire material parameters, including material parameters of proppant particles, material parameters of fracturing fluid, and material parameters of shale gas reservoirs; The parameter calibration module is configured to: calibrate the material parameters of the proppant particles and the material parameters of the shale gas reservoir respectively; The model building module is configured to: after parameter calibration, establish a discrete element particle calculation model of the fracture network structure of the shale gas reservoir after fracturing based on the morphology of the rock mass and fracture network after fracturing of the shale gas reservoir, and establish a computational fluid dynamics calculation grid along the fracture network of the calculation model; Obtain shale gas reservoir in-situ stress data and fracturing fluid injection pressure data, and set the model boundary conditions based on the acquired data; The simulation module is configured to: simulate proppant migration and propping seam network effects; Among them, the simulation of proppant migration and support fracture network effect includes: Step 1: Calculate the fluid domain: Use the NS equation to calculate the flow rate and pressure information of the fluid at a certain moment; Step 2: Calculate the simulation data of the solid part of the model: the ground stress on the boundary, the pressure of the fracturing fluid on the boundary of the fracture network, and the contact force of the supporting particles; Step 3: By retrieving the fracture of parallel bonding bonds between solid DEM particles in the rock formation, the area where the fracture exceeds the lower limit of fracture generation is regarded as the newly generated fracture area, so as to simulate the expansion of the fracture network during the fracturing process; Step 4: Based on the continuously expanding fracture network during the fracturing process, update the fluid calculation grid to cover all fracture areas; Step 5: Repeat step 1 to step 4 until the model reaches overall equilibrium.

8. A computer device comprising a memory, a processor and a computer program stored in the memory and executable on the processor, Its characteristics are: When the processor executes the program, the steps of the method described in any one of claims 1 to 6 are implemented.

9. A computer-readable storage medium having a computer program stored thereon, Its characteristics are: When the program is executed by a processor, the steps of the method described in any one of claims 1 to 6 are performed.

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