A method for optimizing the injection process of hydrate reservoir pore expansion and permeability enhancement working fluid

Abstract

The present invention discloses a method for optimizing the injection process of a hydrate reservoir pore expansion and permeability enhancement working fluid, comprising the following steps: S1: constructing a physical model for the injection of a hydrate reservoir pore expansion and permeability enhancement working fluid, and performing matrix grid division and physical parameter assignment on the physical model; S2: constructing a numerical calculation model for the injection of a hydrate reservoir pore expansion and permeability enhancement working fluid based on the finite volume method; S3: determining a process scheme for the injection of the pore expansion and permeability enhancement working fluid, loading boundary conditions, and performing numerical simulation according to the numerical calculation model for the injection of the hydrate reservoir pore expansion and permeability enhancement working fluid; S4: obtaining numerical simulation results of the current process scheme, determining whether the numerical simulation results meet the pore expansion and permeability enhancement requirements, and if not, changing the process scheme and repeating S3-S4 until the current process scheme meets the pore expansion and permeability enhancement requirements. The present invention can optimize the process and construction parameters of hydrate reservoir pore expansion and permeability enhancement measures, obtain an optimized process, and provide technical support for hydrate mining.

Description

Technical Field

[0001] The present invention relates to the technical field of hydrate reservoir mining, and in particular to a method for optimizing the injection process of a hydrate reservoir pore expansion and permeability enhancement working fluid. Background Art

[0002] Natural gas hydrates are gaining global attention as an alternative to oil and coal, and their mining value is increasingly recognized. Furthermore, hydrate mining can increase energy supply, reduce dependence on traditional fossil fuels, and promote energy diversification. However, in current hydrate mining, due to the low initial permeability and strong heterogeneity of porosity and permeability, secondary hydrate formation occurs in the near-wellbore area as mining progresses, blocking the channels and causing a decrease in permeability around the wellbore. Therefore, to address this issue, pore-expansion and permeability-enhancing working fluids are injected into the pore-permeability spaces of natural gas hydrates to improve the reservoir's porosity and permeability, thereby increasing the efficiency of gas-liquid development in the reservoir and achieving pore-expansion and permeability enhancement in the hydrate reservoir.

[0003] Pore ​​expansion and permeability enhancement is a complex multi-physics coupling problem. The reaction between the working fluid and the matrix rock releases heat, which promotes the decomposition of natural gas hydrates. This decomposition transforms the flow into a gas-liquid two-phase flow. Simultaneously, the matrix's porosity and permeability properties change as the working fluid is injected. Therefore, determining the effectiveness of pore expansion and permeability enhancement and ensuring the applicability and efficiency of the process are pressing technical challenges. Summary of the Invention

[0004] In view of the above problems, the present invention aims to provide a method for optimizing the injection process of working fluid for pore expansion and permeability enhancement in hydrate reservoirs.

[0005] The technical solutions of the present invention are as follows:

[0006] A method for optimizing the injection process of a hydrate reservoir pore expansion and permeability enhancement working fluid comprises the following steps:

[0007] S1: Constructing a physical model for hydrate reservoir pore expansion and permeability enhancement working fluid injection, and performing matrix grid division and physical parameter assignment on the physical model;

[0008] S2: constructing a numerical calculation model for hydrate reservoir pore expansion and permeability enhancement working fluid injection based on the finite volume method, wherein the numerical calculation model for hydrate reservoir pore expansion and permeability enhancement working fluid injection includes a hydrate phase change decomposition model, a gas-liquid two-phase seepage model, a temperature field calculation model, a chemical reaction model, and a pore permeability evolution model;

[0009] S3: Determine a process plan for injecting the pore-expanding and permeability-enhancing working fluid, load boundary conditions, and perform numerical simulation based on the numerical calculation model for injecting the pore-expanding and permeability-enhancing working fluid into the hydrate reservoir;

[0010] S4: Obtain the numerical simulation results of the current process solution, and determine whether the numerical simulation results meet the requirements of pore expansion and permeability increase. If not, change the process solution and repeat S3-S4 until the current process solution meets the requirements of pore expansion and permeability increase.

[0011] Preferably, in step S1, the physical parameters include initial flow porosity, absolute permeability, reservoir temperature and reservoir pressure of the formation.

[0012] Preferably, the hydrate phase change decomposition model is:

[0013] (1)

[0014] (2)

[0015] Where: 、 、 are the gas phase and water phase formation rates and hydrate dissociation rates, respectively. ; 、 、 are the molar masses of gas, water and hydrate respectively, ; is the number of water molecules in a hydrate molecule; is the hydration reaction constant; is the hydration activation energy, J ; is the universal gas constant, 8.314 J ; is the reservoir temperature, ; is the hydrate decomposition specific surface area, ; 、 are the pressure of the hydrate reservoir under equilibrium conditions and the actual pressure at any time, respectively. .

[0016] Preferably, the gas-liquid two-phase seepage model is:

[0017] (3)

[0018] (4)

[0019] (5)

[0020] Where: is the divergence operator; is the absolute permeability, ; 、 are the water phase relative permeability and the gas phase relative permeability, ; 、 、 are the densities of water phase, gas phase and hydrate respectively, ; 、 are the water phase viscosity and the gas phase viscosity, respectively. ; is the gradient operator; For pressure, ; 、 、 are the gas phase and water phase consumption rates and the hydrate formation rate, respectively. ; is the flow porosity; 、 、 are the saturations of water phase, gas phase and hydrate respectively; For time, .

[0021] Preferably, the temperature field calculation model is:

[0022] (6)

[0023] Where: 、 、 、 They are the enthalpy changes of water, gas, hydrate formation or decomposition, and chemical reactions between working fluid and minerals; 、 are the water phase pressure and the gas phase pressure, respectively. ; is the average apparent thermal conductivity of the hydrate reservoir, J ; is the temperature, ; is the working fluid consumption rate during mineral dissolution, ; 、 、 、 are the internal energies of water phase, gas phase, hydrate and rock, respectively. .

[0024] Preferably, the chemical reaction model includes a mass conservation equation for the working fluid and a mass conservation equation for salt generated by mineral dissolution;

[0025] The working fluid mass conservation equation is:

[0026] (7)

[0027] (8)

[0028] Where: is the concentration of the working fluid chemical, ; is the diffusion coefficient of the working fluid chemical in the water phase, ; is the chemical consumption rate, ; 100% chemical solubility. ; is the average molar mass of dissolved minerals, ; is the stoichiometric coefficient of the dissolved mineral;

[0029] The mass conservation equation for the salt generated by mineral dissolution is:

[0030] (9)

[0031] (10)

[0032] Where: is the average concentration of salts generated by mineral dissolution, ; is the average diffusion coefficient of the generated salt in the aqueous phase, ; is the rock density, .

[0033] Preferably, the porosity and permeability evolution model is:

[0034] (11)

[0035] (12)

[0036] (13)

[0037] (14)

[0038] Where: is the updated flow porosity; is the initial flow porosity; For a time step, ; is the initial absolute permeability, ; is an empirical constant; 、 are the pore radius and the initial pore radius, ; is the initial specific surface area, .

[0039] Preferably, in step S3, the pore-expanding and permeation-increasing working fluid injection process includes any one or more of injection displacement, injection fluid volume, working fluid formula, and pore-expanding and permeation-increasing chemical agent concentration.

[0040] The beneficial effects of the present invention are:

[0041] The present invention combines the finite volume method to construct a numerical calculation model for the injection of hydrate reservoir pore expansion and permeability enhancement working fluid, which takes into account the effects of hydrate phase change decomposition, gas-liquid two-phase seepage, temperature field influence, chemical reaction and porosity and permeability evolution. This can accurately simulate and evaluate the porosity and permeability changes of the hydrate reservoir pore expansion and permeability enhancement working fluid after injection, and can optimize the hydrate reservoir pore expansion and permeability enhancement working fluid injection process, which has broad market prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0043] Figure 1 Schematic diagram of the process flow of the method for optimizing the injection process of the hydrate reservoir pore expansion and permeability enhancement working fluid according to the present invention;

[0044] Figure 2 A schematic diagram of a physical model for injecting working fluid to expand the pores and increase the permeability of a hydrate reservoir in a specific implementation;

[0045] Figure 3 Schematic diagram of the pore expansion and permeability enhancement effect when the mass fraction of the working fluid is 0.15 in a specific implementation;

[0046] Figure 4 Schematic diagram of the pore expansion and permeability enhancement effect when the mass fraction of the working fluid is 0.2 in a specific implementation;

[0047] Figure 5 Schematic diagram of the pore expansion and permeability enhancement effect when the mass fraction of the working fluid is 0.25 in a specific implementation;

[0048] Figure 6 For a specific implementation, the injection strength is 0.5m 3 Schematic diagram of the pore expansion and permeability enhancement effect of / m;

[0049] Figure 7 For a specific implementation, the injection strength is 1.5m 3Schematic diagram of the pore expansion and permeability enhancement effect of / m;

[0050] Figure 8 For a specific implementation, the injection strength is 3.0m 3 Schematic diagram of the pore expansion and permeability enhancement effect of / m. DETAILED DESCRIPTION

[0051] The present invention is further described below with reference to the accompanying drawings and examples. It should be noted that, in the absence of conflict, the embodiments in this application and the technical features in the embodiments can be combined with each other. It should be noted that, unless otherwise specified, all technical and scientific terms used in this application have the same meanings as those commonly understood by those of ordinary skill in the art to which this application belongs. The use of similar words such as "include" or "comprising" in the present invention means that the elements or objects appearing before the word cover the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0052] like Figure 1 As shown, the present invention provides a method for optimizing the injection process of a hydrate reservoir pore expansion and permeability increasing working fluid, comprising the following steps:

[0053] S1: Construct a physical model for the injection of working fluid for pore expansion and permeability enhancement in hydrate reservoirs, and perform matrix grid division and physical parameter assignment on the physical model.

[0054] In a specific embodiment, the physical model includes a hydrate phase change decomposition physical model, a gas-liquid two-phase seepage physical model, a temperature field calculation physical model, a chemical reaction physical model, and a porosity and permeability evolution physical model.

[0055] It should be noted that the physical models of the above embodiments have the same structure and are used for numerical simulation calculations in subsequent steps. Five identical physical models can be established, or only one physical model can be constructed and simulated five times.

[0056] In a specific embodiment, the physical parameters include initial flow porosity, absolute permeability, reservoir temperature, and reservoir pressure.

[0057] S2: A numerical calculation model for the injection of pore-expansion and permeability-enhancing working fluid for hydrate reservoirs is constructed based on the finite volume method. The numerical calculation model for the injection of pore-expansion and permeability-enhancing working fluid for hydrate reservoirs includes a hydrate phase change decomposition model, a gas-liquid two-phase seepage model, a temperature field calculation model, a chemical reaction model, and a pore-permeability evolution model.

[0058] In a specific embodiment, the hydrate phase change decomposition model is:

[0059] (1)

[0060] (2)

[0061] Where: 、 、 are the gas phase and water phase formation rates and hydrate dissociation rates, respectively. ; 、 、 are the molar masses of gas, water and hydrate respectively, ; is the number of water molecules in a hydrate molecule; is the hydration reaction constant; is the hydration activation energy, J ; is the universal gas constant, 8.314 J ; is the reservoir temperature, ; is the hydrate decomposition specific surface area, ; 、 are the pressure of the hydrate reservoir under equilibrium conditions and the actual pressure at any time, respectively. .

[0062] The gas-liquid two-phase seepage model is:

[0063] (3)

[0064] (4)

[0065] (5)

[0066] Where: is the divergence operator; is the absolute permeability, ; 、 are the water phase relative permeability and the gas phase relative permeability, ; 、 、 are the densities of water phase, gas phase and hydrate respectively, ; 、 are the water phase viscosity and the gas phase viscosity, respectively. ; is the gradient operator; For pressure, ; 、 、 are the gas phase and water phase consumption rates and the hydrate formation rate, respectively. ; is the flow porosity; 、 、 are the saturations of water phase, gas phase and hydrate respectively; For time, .

[0067] The temperature field calculation model is:

[0068] (6)

[0069] Where: 、 、 、 They are the enthalpy changes of water, gas, hydrate formation or decomposition, and chemical reactions between working fluid and minerals; 、 are the water phase pressure and the gas phase pressure, respectively. ; is the average apparent thermal conductivity of the hydrate reservoir, J ; is the temperature, ; is the working fluid consumption rate during mineral dissolution, ; 、 、 、 are the internal energies of water phase, gas phase, hydrate and rock, respectively. .

[0070] The chemical reaction model includes a mass conservation equation for the working fluid and a mass conservation equation for salt generated by mineral dissolution;

[0071] The working fluid mass conservation equation is:

[0072] (7)

[0073] (8)

[0074] Where: is the concentration of the working fluid chemical (defined as the mass of the working fluid chemical per unit mass of water), ; is the diffusion coefficient of the working fluid chemical in the water phase, ; is the chemical consumption rate, ; is the solvency of the chemical at 100% (defined as the ratio of the mass of dissolved mineral to the mass of the chemical at 100% reaction), ; is the average molar mass of dissolved minerals, ; is the stoichiometric coefficient of the dissolved mineral;

[0075] The mass conservation equation for the salt generated by mineral dissolution is:

[0076] (9)

[0077] (10)

[0078] Where: is the average concentration of salts generated by mineral dissolution (defined as the average mass of salts generated by mineral dissolution), ; is the average diffusion coefficient of the generated salt in the aqueous phase, ; is the rock density, .

[0079] The porosity evolution model is:

[0080] (11)

[0081] (12)

[0082] (13)

[0083] (14)

[0084] Where: is the updated flow porosity; is the initial flow porosity; For a time step, ; is the initial absolute permeability, ; is an empirical constant; 、 are the pore radius and the initial pore radius, ; is the initial specific surface area, .

[0085] In the above porosity and permeability evolution model, the matrix porosity and absolute permeability are updated by equations (11) and (12), and equations (13) and (14) are the expressions of the pore radius and specific surface area and their corresponding initial values.

[0086] S3: Determine a process plan for injecting the pore expansion and permeability enhancement working fluid, load boundary conditions, and perform numerical simulation based on the numerical calculation model for injecting the pore expansion and permeability enhancement working fluid into the hydrate reservoir.

[0087] In a specific embodiment, the pore-expanding and permeation-enhancing working fluid injection process includes any one or more of injection displacement, injection fluid volume, working fluid formula, and pore-expanding and permeation-enhancing chemical agent concentration.

[0088] When performing numerical simulation, parameter data such as relative permeability and chemical reaction rate are calculated and updated according to the grid unit; the multi-field coupled linear equation group in the numerical calculation model of the hydrate reservoir expansion and permeability enhancement working fluid injection is used to solve the pressure, temperature, saturation, etc.; the iteration error is calculated to determine whether the iteration has converged in the current time step. Optionally, the difference in the pressure, temperature, and saturation distribution between the previous and next iterations is calculated to determine whether the iteration has converged. If not, the iterative calculation is repeated until the current time step converges; the matrix porosity and absolute permeability are calculated and updated; and it is determined whether the numerical simulation of the current process solution is completed. If not, the numerical simulation is repeated until all time step simulations are completed.

[0089] S4: Obtain the numerical simulation results of the current process solution, and determine whether the numerical simulation results meet the requirements of pore expansion and permeability increase. If not, change the process solution and repeat S3-S4 until the current process solution meets the requirements of pore expansion and permeability increase.

[0090] It should be noted that the requirements for pore expansion and permeability increase are generally determined based on the actual reservoir, primarily by determining whether the change in pore permeability reaches a threshold. Furthermore, the impact of the increase in pore permeability is also considered. This is conventional technology, and the specific requirements are not elaborated here.

[0091] In a specific embodiment, taking a hydrate reservoir as an example, the present invention is used to optimize the pore expansion and permeability enhancement working fluid injection process, which specifically includes the following sub-steps:

[0092] (1) Construct a physical model for the hydrate reservoir expansion and permeability enhancement working fluid injection, and perform matrix grid division and physical parameter assignment on the physical model. In this embodiment, the physical model constructed is as follows Figure 2 As shown;

[0093] (2) Determine a set of hydrate reservoir pore expansion and permeability enhancement working fluid injection process scheme. In this embodiment, the main focus is on optimizing the working fluid mass fraction and injection intensity in the pore expansion and permeability enhancement working fluid injection process scheme. Among them, the working fluid mass fractions studied are 0.150, 0.200, and 0.250, and the injection intensity studied is 0.5m 3 / m、1.5m 3 / m、3.0m 3 / m;

[0094] (3) initializing or updating the physical parameters of the physical model for the pore expansion and permeability enhancement working fluid injection into the hydrate reservoir;

[0095] (4) Using the hydrate phase transition decomposition model shown in equations (1) and (2), simulate and calculate the hydrate dissociation rate and the gas phase and water phase generation rates;

[0096] (5) Using the gas-liquid two-phase seepage model shown in equations (3)-(5), the gas-liquid two-phase flow velocity distribution is simulated and calculated;

[0097] (6) Using the chemical reaction model shown in equations (7)-(10), simulate and calculate the working fluid concentration, consumption rate, and dissolved mineral concentration;

[0098] (7) Using the temperature field calculation model shown in formula (6), simulate and calculate the temperature distribution;

[0099] (8) Using the porosity and permeability evolution model shown in equations (11)-(14), calculate and update the changes in porosity and absolute permeability;

[0100] (9) Repeat steps (4) to (8) in one time step until the calculation converges. After convergence, determine whether the simulation is complete;

[0101] (10) Repeat steps (4)-(9) to simulate the next time step until all time step simulations are completed, calculate the changes in the final porosity and permeability of the current process solution, and determine whether the current process solution meets the requirements of pore expansion and permeability increase. If it does not meet the requirements, repeat steps (2)-(10) until the process solution that meets the requirements of pore expansion and permeability increase is optimized.

[0102] In this embodiment, the pore expansion and permeability enhancement effects under different working fluid mass fractions are as follows: Figure 3-Figure 5 As shown. Figure 3-Figure 5 It can be seen that the larger the mass fraction of the working fluid, the larger the pore expansion and permeation radius. However, when the mass fraction increases to a certain value, the pore expansion and permeation radius will not continue to increase. Therefore, the working fluid mass fraction can be optimized to be between 0.2-0.25.

[0103] The pore expansion and permeability enhancement effects under different injection intensities are as follows Figure 6-Figure 8 As shown. Figure 6-Figure 8 It can be seen that increasing the injection intensity is beneficial to increasing the depth of pore expansion and permeability, so the injection intensity can be optimized to be 3m 3 / m.

[0104] In summary, the present invention can optimize the injection process of working fluid for pore expansion and permeability enhancement in hydrate reservoirs, providing technical support for chemical extraction of working fluid from hydrate reservoirs. Compared with existing technologies, the present invention represents a significant improvement.

[0105] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with the present profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A method for optimizing the injection process of a hydrate reservoir pore expansion and permeability enhancement working fluid, characterized in that: The following steps are involved: S1: Constructing a physical model for hydrate reservoir pore expansion and permeability enhancement working fluid injection, and performing matrix grid division and physical parameter assignment on the physical model; S2: constructing a numerical calculation model for hydrate reservoir pore expansion and permeability enhancement working fluid injection based on the finite volume method, wherein the numerical calculation model for hydrate reservoir pore expansion and permeability enhancement working fluid injection includes a hydrate phase change decomposition model, a gas-liquid two-phase seepage model, a temperature field calculation model, a chemical reaction model, and a pore permeability evolution model; The hydrate phase change decomposition model is: （1） （2） Where: 、 、 are the gas phase and water phase formation rates and hydrate dissociation rates, respectively. ; 、 、 are the molar masses of gas, water and hydrate respectively, ; is the number of water molecules in a hydrate molecule; is the hydration reaction constant; is the hydration activation energy, J ; is the universal gas constant, 8.314 J ; is the reservoir temperature, ; is the hydrate decomposition specific surface area, ; 、 are the pressure of the hydrate reservoir under equilibrium conditions and the actual pressure at any time, respectively. ; The gas-liquid two-phase seepage model is: （3） （4） （5） Where: is the divergence operator; is the absolute permeability, ; 、 are the water phase relative permeability and the gas phase relative permeability, ; 、 、 are the densities of water phase, gas phase and hydrate respectively, ; 、 are the water phase viscosity and the gas phase viscosity, respectively. ; is the gradient operator; For pressure, ; 、 、 are the gas phase and water phase consumption rates and the hydrate formation rate, respectively. ; is the flow porosity; 、 、 are the saturations of water phase, gas phase and hydrate respectively; For time, ; The temperature field calculation model is: （6） Where: 、 、 、 are the enthalpy changes of water, gas, hydrate formation or decomposition, and chemical reaction between working fluid and minerals; P w 、P g are the water phase pressure and the gas phase pressure, respectively. ; is the average apparent thermal conductivity of the hydrate reservoir, J ; is the temperature, ; is the working fluid consumption rate during mineral dissolution, ; 、 、 、 are the internal energies of water phase, gas phase, hydrate and rock, respectively. ; The chemical reaction model includes a mass conservation equation for the working fluid and a mass conservation equation for salt generated by mineral dissolution; The working fluid mass conservation equation is: （7） （8） Where: is the concentration of the working fluid chemical, ; is the diffusion coefficient of the working fluid chemical in the water phase, ; is the chemical consumption rate, ; 100% chemical solubility. ; is the average molar mass of dissolved minerals, ; is the stoichiometric coefficient of the dissolved mineral; The mass conservation equation for the salt generated by mineral dissolution is: （9） （10） Where: is the average concentration of salts generated by mineral dissolution, ; is the average diffusion coefficient of the generated salt in the aqueous phase, ; is the rock density, ; The porosity evolution model is: （11） （12） （13） （14） Where: is the updated flow porosity; is the initial flow porosity; For a time step, ; is the initial absolute permeability, ; is an empirical constant; 、 are the pore radius and the initial pore radius, ; is the initial specific surface area, ; S3: Determine a process plan for injecting the pore-expanding and permeability-enhancing working fluid, load boundary conditions, and perform numerical simulation based on the numerical calculation model for injecting the pore-expanding and permeability-enhancing working fluid into the hydrate reservoir; S4: Obtain the numerical simulation results of the current process solution, and determine whether the numerical simulation results meet the requirements of pore expansion and permeability increase. If not, change the process solution and repeat S3-S4 until the current process solution meets the requirements of pore expansion and permeability increase.

2. The method for optimizing the injection process of hydrate reservoir pore expansion and permeability enhancement working fluid according to claim 1, characterized in that: In step S1, the physical parameters include initial flow porosity, absolute permeability, reservoir temperature and reservoir pressure of the formation.

3. The method for optimizing the injection process of hydrate reservoir pore expansion and permeability enhancement working fluid according to claim 1 or 2, characterized in that: In step S3, the pore-expanding and permeation-increasing working fluid injection process includes any one or more of injection displacement, injection fluid volume, working fluid formula, and pore-expanding and permeation-increasing chemical agent concentration.

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