A method for predicting the decomposition and transport properties of a natural gas hydrate core

Abstract

The application discloses a kind of natural gas hydrate core decomposition and transport property prediction method, comprising: constructing core geometric model and carrying out grid division;Chemical kinetics model and transport multiphase percolation model of natural gas hydrate decomposition are constructed;Decomposition chemical kinetics model and transport multiphase percolation model are compiled respectively using C language, and embedded in Fluent software in the form of mass and energy source term;The initial distribution of natural gas hydrate, water, gas three phases in core is inverted, and the initial distribution law of natural gas hydrate is obtained;Based on the initial distribution law of natural gas hydrate, the decomposition and transport property of natural gas hydrate core under different exploitation conditions is predicted using Fluent software.The prediction method fills the blank of existing core natural gas hydrate core decomposition and transport prediction method.

Description

Technical Field

[0001] This invention relates to the field of natural gas hydrate extraction technology, and in particular to a method for predicting the decomposition and transport properties of natural gas hydrate cores. Background Technology

[0002] Natural gas hydrate, also known as combustible ice, is a crystalline substance formed by the adsorption of methane-based gases into the crystal lattice of ice under high pressure and low temperature conditions. It has the advantages of large resource reserves, high energy density, and low pollution, and is currently a research hotspot in the global energy field.

[0003] Natural gas hydrates decompose into water and gas at ambient temperature and pressure, posing significant challenges to natural core drilling, preservation, and laboratory experiments. Numerical simulation can provide an economical and efficient research method for predicting future hydrate reservoir properties and optimizing development strategies. However, there is a limited availability of specialized software for hydrate formation-decomposition simulation, and most of it is commercial software. The extraction process of natural gas hydrates is a complex heat and mass transfer problem in porous media involving phase transitions, dynamic boundaries, chemical kinetics, and multiphase fluid flow coupling. Existing simulation methods sometimes assume that the gas produced after hydrate decomposition is completely dissolved in water; other methods employ finite difference calculations, which often result in lower accuracy in computational fluid dynamics simulations compared to finite element or finite volume methods.

[0004] Therefore, existing methods for predicting the decomposition and transport properties of natural gas hydrate cores need further improvement and development. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of this invention is to provide a method for predicting the decomposition and transport properties of natural gas hydrate cores, aiming to fill the gap in the method for predicting the decomposition and transport properties of natural gas hydrate cores.

[0006] A method for predicting the decomposition and transport properties of natural gas hydrate cores, comprising:

[0007] Construct a core geometric model and mesh the core geometric model;

[0008] A chemical kinetic model for the decomposition of natural gas hydrate and a multiphase flow model for its transport were constructed. The chemical kinetic model for the decomposition and the multiphase flow model for its transport were compiled using C language and embedded into the finite volume method software in the form of mass and energy source terms.

[0009] Based on the core geometric model and the finite volume method software, the initial distribution of natural gas hydrate, water and gas phases in the core is inverted to obtain the initial distribution law of natural gas hydrate.

[0010] Based on the initial distribution pattern of natural gas hydrates, the natural gas hydrates in the core under different mining conditions to be simulated are injected into the core geometric model. The finite volume method software is then used to predict the decomposition and transport properties of the natural gas hydrate core under different mining conditions.

[0011] Optionally, in the method for predicting the decomposition and transport properties of natural gas hydrate cores, the decomposition process of natural gas hydrate cores follows the following formula (1):

[0012] CH4·N h H2O→CH4+N h H2O (1)

[0013] The generated gas follows the Peng-Robinson equation, and the core decomposition rate of the natural gas hydrate in the core follows the nonlinear Arrhenius chemical kinetic model as shown in equation (2):

[0014]

[0015] Among them, It is the intrinsic constant, ΔE is the activation energy, R is the universal gas constant, and f is the eigenvalue constant. eh It is the equilibrium pressure, f g It is the pressure of methane, A d It is the reaction surface area of ​​the hydrate.

[0016] Optionally, in the method for predicting the decomposition and transport properties of natural gas hydrate cores, the natural gas hydrate cores follow the NS basic equations after decomposition, which include: the mass conservation equation, the momentum conservation equation, and the energy conservation control equation.

[0017] Optionally, in the method for predicting the decomposition and transport properties of natural gas hydrate cores, the mass conservation equation is shown in equation (3), the momentum conservation equation is shown in equation (4), and the energy conservation control equation is shown in equation (5).

[0018]

[0019]

[0020]

[0021] Among them, P k The momentum source term due to the hydrate decomposition reaction is calculated based on equations (1) and (2); Q h The heat absorbed by the decomposition of hydrates.

[0022] Optionally, in the method for predicting the decomposition and transport properties of natural gas hydrate cores, the step of constructing a core geometric model and performing mesh generation specifically includes:

[0023] Based on the size and experimental conditions of the natural gas hydrate core used in the indoor experiment, a core geometric model was constructed using ICEM modeling software, and the core geometric model was meshed. The corresponding boundary conditions of the core geometric model were set with reference to the temperature and pressure conditions of the inlet, outlet, and side of the core in the indoor experiment.

[0024] Optionally, the method for predicting the decomposition and transport properties of natural gas hydrate cores, wherein the step of inverting the initial distribution of the three phases of natural gas hydrate, water, and gas within the core based on the core geometric model and the finite volume method software to obtain the initial distribution law of natural gas hydrate specifically includes:

[0025] Using the basic porosity, density, and thermodynamic parameters of the rock cores used in the indoor experiments, and based on the corresponding procedures of different indoor rock core natural gas hydrate synthesis methods, the same amount of water and methane gas as in the indoor experiments were injected into the rock core geometric model in sequence to simulate the cooling process of the rock core natural gas hydrate synthesis experiment.

[0026] The natural gas hydrate formation process was simulated using an adaptive time step until the total time reached the synthesis time of natural gas hydrate from the indoor experimental core. The distribution patterns of natural gas hydrate, residual water, and gas obtained from the indoor synthesis experiment were then obtained through inversion.

[0027] Optionally, in the method for predicting the decomposition and transport properties of natural gas hydrate cores, Q... h Calculated from equation (6):

[0028]

[0029] Wherein, Hd is the latent heat of the core decomposition reaction of natural gas hydrate in the core; Equations (2) and (6) are compiled using C language and embedded into the finite volume method software in the form of mass and energy source terms, respectively.

[0030] Optionally, the method for predicting the decomposition and transport properties of natural gas hydrate cores, wherein the step of predicting the decomposition and transport properties of natural gas hydrate cores under different mining conditions based on the initial distribution law of natural gas hydrates and using the finite volume method software specifically includes:

[0031] Based on the initial distribution pattern of natural gas hydrates, and according to different natural gas hydrate mining simulation experiments, the core boundary pressure and temperature conditions of the core geometric model are adjusted to simulate the depressurization or heat injection mining process of natural gas hydrates.

[0032] Different temperature and pressure monitoring surfaces or points are set up to obtain in real time the distribution law of natural gas hydrate, gas and water three-phase saturation, temperature, pressure and fluid velocity field distribution inside the rock core;

[0033] Predict the chemical reactions, phase changes, and heat and mass transfer mechanisms of multiphase flow in porous media during the decomposition and transport of natural gas hydrates at the core scale.

[0034] Beneficial effects: This invention constructs a core geometric model and, based on C language and finite volume method software, develops a method that can predict the decomposition and transport properties of natural gas hydrate cores under different conditions. This prediction method fills the gap in existing methods for predicting the decomposition and transport of natural gas hydrate cores, and it has high prediction accuracy. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the process for predicting the decomposition and transport properties of natural gas hydrate rock cores in this invention;

[0036] Figure 2 This is a schematic diagram of the rock sample grid model, boundary conditions, and temperature and pressure monitoring point locations provided in an embodiment of the present invention.

[0037] Figure 3 A schematic diagram of the process for indoor synthesis of natural gas hydrate cores provided in an embodiment of the present invention;

[0038] Figure 4 The heterogeneous distribution of natural gas hydrate, water, and gas in the core obtained through simulation in an embodiment of the present invention;

[0039] Figure 5 A comparison curve of simulation results and experimental results of cumulative gas production during the decomposition process of natural gas hydrate cores provided in this embodiment of the invention;

[0040] Figure 6 A comparison curve of simulation results and experimental results of the outlet pressure of the hydrate core decomposition process provided in the embodiments of the present invention;

[0041] Figure 7 The simulation results and experimental results of the temperature at the monitoring point of the hydrate core decomposition process provided in the embodiments of the present invention are compared. Detailed Implementation

[0042] This invention provides a method for predicting the decomposition and transport properties of natural gas hydrate cores. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0043] Please see Figure 1 , Figure 1 This is a schematic flowchart of a method for predicting the decomposition and transport properties of natural gas hydrate cores, as shown in the figure. The method for predicting the decomposition and transport properties of natural gas hydrate cores includes:

[0044] S10. Construct a core geometric model and perform mesh generation on the core geometric model.

[0045] Specifically, a core geometry model for simulation is first constructed. This model can be built with reference to the shape and size of the cores used in laboratory experiments. That is, based on the dimensions of the natural gas cores used in laboratory experiments and the corresponding experimental conditions, the core geometry model is established and meshed using ICEMCFD (The Integrated Computer Engineering and Manufacturing code for Computational Fluid Dynamics) modeling software. Figure 2 As shown, after the mesh is generated, the corresponding boundary conditions are set with reference to the temperature and pressure conditions at the inlet and outlet and sides of the indoor experimental rock core.

[0046] Following step S10 is step S20, which involves constructing a decomposition chemical kinetic model and a transport multiphase flow model for natural gas hydrates. The decomposition chemical kinetic model is compiled using C language and embedded into the finite volume method software in the form of mass and energy source terms.

[0047] Specifically, this includes following the natural gas hydrate decomposition process.

[0048] CH4·N h H2O→CH4+N h H2O (1)

[0049] The generated gas follows the Peng-Robinson equation, and the decomposition rate of the hydrate follows a nonlinear Arrhenius chemical kinetic model.

[0050]

[0051] Among them, It is the intrinsic constant, ΔE is the activation energy, R is the universal gas constant, and f is the eigenvalue constant. eh It is the equilibrium pressure, f g It is the pressure of methane, A d It is the reaction surface area of ​​the hydrate.

[0052] The mathematical model described above was implemented and compiled using the C language, and then embedded into the finite volume method software (Fluent) as a mass source term.

[0053] In this embodiment, the decomposition of natural gas hydrate follows the basic Navier-Stokes equations, mainly including the governing equations for mass conservation, momentum conservation, and energy conservation, which are as follows:

[0054]

[0055]

[0056]

[0057] Among them, P k The momentum source term due to the hydrate decomposition reaction is calculated according to formulas (1) and (2); Q h The heat absorbed by the decomposition of hydrates can be calculated using the following formula.

[0058]

[0059] Among them, H d The latent heat of the hydrate decomposition reaction is denoted as . Equation (6) is implemented and compiled using C language and embedded into Fluent software as an energy source term. It should be noted that the technologies involved in implementing and compiling the above equation using C language and embedding the C-compiled equation into Fluent software as an energy source term are existing technologies and are not limited here.

[0060] In this embodiment, by compiling the above-mentioned equations involved in the decomposition process of natural gas hydrate cores using C language, and embedding the C-compiled equations into Fluent software in the form of mass and energy source terms, the accuracy of the prediction can be improved, making the prediction method more applicable to the calculation of chemical kinetics, rock physics parameters, and thermodynamic parameters of natural gas hydrate core decomposition.

[0061] Following step S20, step S30 is also included: based on the core geometric model and the finite volume method software, the initial distribution of natural gas hydrate, water, and gas phases in the core is inverted to obtain the initial distribution law of natural gas hydrate.

[0062] Specifically, the basic porosity, permeability, density, and thermodynamic parameters of the rock core in this embodiment are shown in Table 1.

[0063] Table 1 Basic physical properties of rock samples

[0064] parameter value Initial permeability of rock core 97.98mD Core porosity (φ) 0.182 <![CDATA[Average hydrate saturation (S h )]]> 0.501 <![CDATA[Average water saturation (S w )]]> 0.351 <![CDATA[Average gas hydrate saturation (S g )]]> 0.148 <![CDATA[Air bath temperature (T air )]]> 274.15K <![CDATA[Skeleton density (ρ R )]]> <![CDATA[2650kg / m 3 ]]> <![CDATA[Thermal conductivity of the framework (λ R )]]> 3.0 W / m·K <![CDATA[Skeleton specific heat (C R )]]> 800J / kg·K

[0065] in accordance with Figure 3The experimental procedure shown involves sequentially injecting the same amount of water and methane gas as in the experiment into the core, followed by simulating the cooling process of the hydrate synthesis experiment. An adaptive time step is used to simulate the hydrate formation process until the total time reaches the experimental hydrate synthesis time. At this point, the distribution patterns of hydrates, residual water, and gas in the core obtained from the indoor synthesis experiment are retrieved, as shown below. Figure 4 As shown.

[0066] Following step S30, step S40 is also included: based on the initial distribution law of natural gas hydrate, the natural gas hydrate core samples under different mining conditions to be simulated are injected into the core geometric model, and the finite volume method software is used to predict the decomposition and transport properties of the natural gas hydrate core samples under different mining conditions.

[0067] Specifically, based on Figure 4 The initial distribution of hydrates is shown. Based on different hydrate mining simulation experiments, the core boundary pressure and temperature conditions are adjusted to simulate the depressurization or thermal injection mining process of hydrates. By setting up different temperature and pressure monitoring surfaces or points, the distribution law of hydrate, gas, and water three-phase saturation, temperature, pressure, and fluid velocity field distribution inside the core are obtained in real time. This embodiment selects the results of the hydrate core decomposition experiment conducted by Masuda in 1999 as a comparative verification, in which... Figure 5 , Figure 6 , Figure 7 The figure shows a comparison curve of cumulative gas production and monitoring points at different temperatures and pressures. It can be seen that the simulation results agree well with the experimental data. Based on the above process, effective prediction of the heat and mass transfer mechanisms of porous media, including chemical reactions, phase changes, and multiphase flow during hydrate decomposition and transport at the core scale, can be achieved.

[0068] It should be understood that the application of the present invention is not limited to the examples above. Those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A method for predicting the decomposition and transport properties of natural gas hydrate cores, characterized in that, include: Construct a core geometric model and mesh the core geometric model; Construct a chemical kinetic model for the decomposition of natural gas hydrates and a multiphase flow model for their transport; The decomposition chemical kinetic model and the transport multiphase flow model were compiled using C language and embedded into the finite volume method software in the form of mass and energy source terms. Based on the core geometric model and the finite volume method software, the initial distribution of natural gas hydrate, water and gas phases in the core is inverted to obtain the initial distribution law of natural gas hydrate. Based on the initial distribution pattern of natural gas hydrates, the natural gas hydrates in the core under different mining conditions to be simulated are injected into the core geometric model. The finite volume method software is used to predict the decomposition and transport properties of natural gas hydrate cores under different mining conditions. The steps of constructing the core geometric model and performing mesh generation specifically include: Based on the size and experimental conditions of the natural gas hydrate core used in the indoor experiment, a core geometric model was constructed using ICEM modeling software, and the core geometric model was meshed. Based on the temperature and pressure conditions at the inlet and outlet and sides of the indoor experimental rock core, the corresponding boundary conditions of the rock core geometric model are set. The step of inverting the initial distribution of natural gas hydrate, water, and gas phases within the core based on the core geometric model and the finite volume method software to obtain the initial distribution law of natural gas hydrate specifically includes: Using the basic porosity, density, and thermodynamic parameters of the rock cores used in the indoor experiments, and based on the corresponding procedures of different indoor rock core natural gas hydrate synthesis methods, the same amount of water and methane gas as in the indoor experiments were injected into the rock core geometric model in sequence to simulate the cooling process of the rock core natural gas hydrate synthesis experiment. The formation process of natural gas hydrate was simulated using an adaptive time step until the total time reached the synthesis time of natural gas hydrate from the indoor experimental core. The distribution patterns of natural gas hydrate, residual water, and gas obtained from the indoor synthesis experiment were then obtained through inversion. The steps of injecting natural gas hydrate core samples under different mining conditions into the core geometric model based on the initial distribution law of the natural gas hydrate, and using the finite volume method software to predict the decomposition and transport properties of the natural gas hydrate core under different mining conditions, specifically include: Based on the initial distribution pattern of natural gas hydrates, and according to different natural gas hydrate mining simulation experiments, the core boundary pressure and temperature conditions of the core geometric model are adjusted to simulate the depressurization or heat injection mining process of natural gas hydrates. Different temperature and pressure monitoring surfaces or points are set up to obtain in real time the distribution law of natural gas hydrate, gas and water three-phase saturation, temperature, pressure and fluid velocity field distribution inside the rock core; Predict the chemical reactions, phase changes, and heat and mass transfer mechanisms of multiphase flow in porous media during the decomposition and transport of natural gas hydrates at the core scale.

2. The method for predicting the decomposition and transport properties of natural gas hydrate cores according to claim 1, characterized in that, The decomposition process of natural gas hydrate cores follows the following formula (1): (1) The generated gas follows the Peng-Robinson equation, and the core decomposition rate of the natural gas hydrate in the core follows the nonlinear Arrhenius chemical kinetic model as shown in equation (2): (2) in, These are eigenvalues. ΔE It is activation energy. R It is the universal gas constant. T It is absolute temperature. f eh It is to balance the pressure. f g It is the pressure of methane. A d It is the reaction surface area of ​​the hydrate.

3. The method for predicting the decomposition and transport properties of natural gas hydrate cores according to claim 2, characterized in that, After the natural gas hydrate core is decomposed, its flow and heat transfer processes follow a set of governing equations consisting of the mass conservation equation, the Navier-Stokes momentum conservation equation, and the energy conservation equation.

4. The method for predicting the decomposition and transport properties of natural gas hydrate cores according to claim 3, characterized in that, The mass conservation equation is shown in equation (3), the momentum conservation equation is shown in equation (4), and the energy conservation control equation is shown in equation (5). in, P k The momentum source term resulting from the hydrate decomposition reaction is calculated based on equations (1) and (2). The heat absorbed by the decomposition of hydrates.

5. The method for predicting the decomposition and transport properties of natural gas hydrate cores according to claim 4, characterized in that, The Calculated from equation (6): in, H d The latent heat of the decomposition reaction of natural gas hydrate in the rock core is used; the formulas (2) and (6) are compiled using C language and embedded into the finite volume method software in the form of mass and energy source terms, respectively.

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