Sea area natural gas hydrate and underlying gas multi-gas commingling production numerical simulation method

Abstract

The invention discloses a sea area natural gas hydrate and underlying gas multi-gas commingling production numerical simulation method, which comprises the following steps of S1, selecting a target block, and enabling natural gas hydrate and underlying gas in the target block to be symbiotic; s2, establishing a multi-gas commingling production numerical simulation physical model of the natural gas hydrate and underlying gas in the target block, and determining simulation initial parameters; s3, mesh generation is carried out, and boundary conditions are set; s4, setting a shaft, perforation information and a production system; and S5, solving the established model and carrying out sensitivity analysis on a result. According to the numerical simulation method provided by the invention, the resource conditions of multi-gas coupling symbiosis of the sea area natural gas hydrate and underlying gas are comprehensively considered, the process is clear and simple, the operability is high, and the method also has universality in other natural gas hydrate development areas.

Description

Technical Field

[0001] The present invention belongs to the technical field of offshore natural gas hydrate exploitation, and in particular relates to a numerical simulation method for combined exploitation of offshore natural gas hydrate and underlying gas. Background Art

[0002] Natural gas hydrate (NGH) is a cage-shaped crystalline compound formed by hydrocarbon gases such as methane and water under high pressure and low temperature conditions. It has the characteristics of high energy density, wide distribution, large scale, high combustion value, clean and pollution-free. It is an energy source that has not yet been effectively developed on a large scale on the earth. It is also known as the new alternative energy with the most development potential in the 21st century. Developing efficient hydrate resources from marine sediments is of great significance to solving the world's energy shortage problem.

[0003] The exploitation of natural gas hydrates is complex, involving a variety of physical and chemical reactions such as the formation, decomposition and migration of hydrates. At present, the main methods for hydrate exploitation include depressurization, thermal stimulation, injection of inhibitors, N2 / CO2 displacement, and solid fluidization exploitation methods proposed for the development of non-diagenetic natural gas hydrates in the sea. Among the above methods, the depressurization method and the heating method are technically easy to implement, but compared with the heating method, the depressurization method has the advantages of low cost and high gas production efficiency. It is the main method used in recent trial production and the main method for exploiting natural gas hydrates in the future. my country completed short-term trial production in the Shenhu area of ​​the South China Sea in 2017 and 2020, respectively, and achieved a series of breakthrough results. Some technologies have reached international leading levels, becoming the world's first country to use horizontal well drilling technology to test the production of natural gas hydrates in the sea.

[0004] Domestic and foreign studies have confirmed that the reservoir formation mechanisms of marine hydrates, shallow gas and deep oil and gas are related, and most of them have regional symbiosis and spatial coexistence, and have the resource conditions for joint development. However, the existence of the initial underlying free gas in such hydrate reservoirs makes its exploitation process more complicated. The difference in the seepage of gas and water itself will affect the pressure propagation and heat transfer process in the sedimentary layer. The possibility of the initial free gas and water combining in situ to form hydrates will also interfere with the decomposition process of hydrates. However, if this part of free gas can be fully utilized, it can also become a new source of gas supply. In order to improve the gas production efficiency and total output of such natural gas hydrate reservoirs, the joint exploitation of natural gas hydrates and underlying free gas has become a new marine hydrate reservoir development plan. Therefore, the comprehensive exploration and three-dimensional development of multiple types of natural gas resources such as marine natural gas hydrates, shallow gas, and deep gas is an important direction for the industrial development of marine natural gas hydrates, and it is also one of the core areas of world cutting-edge technological innovation.

[0005] In the prior art, a method for joint exploitation of marine natural gas hydrates by heat injection-fracturing-depressurization (202211357181.4) established a third-type hydrate reservoir model, and used the method of joint exploitation of marine natural gas hydrates by heat injection-fracturing-depressurization to conduct numerical simulation research and influencing factor analysis, providing reference value for the exploitation of marine hydrate reservoirs. A numerical simulation method for low-frequency electric field heating to exploit natural gas hydrate reservoirs (202010992536.1) discloses a numerical simulation method for low-frequency electric field heating to exploit third-type natural gas hydrate reservoirs. However, the numerical models constructed by the above-mentioned technology are mostly uniform hydrate layers without underlying free gas, and the mutual influence mechanism under the conditions of joint exploitation of hydrates and underlying free gas cannot be analyzed. Therefore, it is urgent to form a numerical simulation method for the joint exploitation of marine natural gas hydrates and underlying gas, which lays a foundation for the research on the joint exploitation of natural gas hydrates and underlying free gas. Summary of the invention

[0006] The present invention provides a numerical simulation method for combined production of marine natural gas hydrate and underlying gas, comprising the following steps:

[0007] Step S1: selecting a target block, where natural gas hydrates coexist with underlying gas;

[0008] Step S2: establishing a numerical simulation physical model for combined production of natural gas hydrate and underlying gas in the target block, and determining initial simulation parameters;

[0009] Step S3: Meshing and setting boundary conditions;

[0010] Step S4: Setting wellbore and perforation information and production system;

[0011] Step S5: Solve the constructed model and perform sensitivity analysis on the results.

[0012] Preferably, in step S1, the development of underlying gas is quantitatively evaluated by geostatistical methods.

[0013] Preferably, in step S2, the initial simulation parameters are determined using field data, wherein the field data include seismic interpretation, inversion results, logging and completion comprehensive processing interpretation result diagram, logging interpretation result table, and formation test result table data;

[0014] Preferably, in the present invention, the multi-gas combined production numerical simulation physical model in step S2 includes: an overburden layer, a hydrate layer, an interlayer, an underlying gas layer, an underburden layer, a wellbore position, and a perforation position.

[0015] Preferably, the initial simulation parameters include: seawater depth, seabed temperature, depth and temperature of the top of the hydrate layer, temperature gradient, pressure gradient or pressure coefficient, thickness of each layer, size of model geometric boundaries, porosity of each layer, permeability of each layer, water saturation, gas saturation, hydrate saturation of each layer, pore water salinity of each layer, rock mechanical properties and thermal conductivity characteristics of each layer.

[0016] Preferably, in the present invention, the grid division in step S3 also includes the steps of: performing grid encryption near the wellbore, gradually increasing the grid size radially outward, performing grid encryption at the connection between each layer, and setting the grids at the upper and lower boundaries of the upper and lower covering layers as inactive grids with a grid size of 0.01m.

[0017] Preferably, in the present invention, step S4 further includes: setting the position, porosity, longitudinal and axial permeability of the wellbore, and setting the position, porosity, longitudinal and axial permeability of the perforation area.

[0018] Preferably, the production system of the present invention includes: the location of the production well, the bottom hole pressure, and the total production time.

[0019] Preferably, in the present invention, grey theory is used to perform sensitivity analysis in step S5.

[0020] Preferably, the present invention also includes analyzing the following parameters: wellhead gas production rate, wellhead cumulative gas production, wellhead water production rate, wellhead cumulative water production, hydrate decomposition rate, cumulative hydrate decomposition amount, remaining methane amount, remaining hydrate amount, reservoir pressure distribution law, reservoir temperature distribution law, reservoir hydrate distribution law, reservoir gas distribution law, and reservoir sedimentation law.

[0021] On the other hand, the present invention provides a computer device, comprising a memory, a processor, and a computer program stored in the memory and running on the processor, characterized in that when the processor executes the computer program, the steps of the numerical simulation method for multi-gas combined production of marine natural gas hydrates and underlying gas as described in any one of the above claims are implemented.

[0022] On the other hand, the present invention provides a computer-readable storage medium having a computer program stored thereon, characterized in that when the computer program is executed by a processor, the steps of the numerical simulation method for multi-gas combined production of marine natural gas hydrates and underlying gas as described in any one of the above claims are implemented.

[0023] The present invention has the following advantages: (1) The present invention comprehensively considers the resource conditions of the coupled symbiosis of marine natural gas hydrates and underlying gas. The process is clear and simple, and the operability is strong. The method can be used to predict the production capacity under the conditions of hydrate and underlying gas co-production, the spatiotemporal evolution of reservoir temperature, pressure saturation, and the decomposition and secondary generation of hydrates. (2) The numerical simulation method of the present invention can improve the simulation accuracy and calculation speed, so as to study the influence of the interlayer on the gas production law of each layer by changing the physical properties of the interlayer. (3) The parameter sensitivity analysis method of the present invention can quickly analyze the influence of each parameter on the final mining effect, which is conducive to the rapid optimization of the production plan. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The numerical simulation flow chart of the multi-gas production of marine natural gas hydrate and underlying gas provided by the present invention;

[0025] Figure 2 It is a schematic diagram of a physical model for combined production of marine natural gas hydrate and underlying gas in an embodiment of the present invention;

[0026] Figure 3 It is a longitudinal cross-sectional schematic diagram of a physical model for multi-gas co-production of natural gas hydrates and underlying gas in the sea area according to an embodiment of the present invention;

[0027] Figure 4 It is a schematic diagram of a physical model for combined production of natural gas hydrate and underlying gas in an embodiment of the present invention;

[0028] Figure 5 It is a schematic diagram of grid division of a physical model for combined production of natural gas hydrate and underlying gas in an embodiment of the present invention;

[0029] Figure 6 This is a graph showing changes in the cumulative gas production at the wellhead over production time under different production pressure differences according to an embodiment of the present invention;

[0030] Figure 7 This is a graph showing the change of the cumulative decomposition gas volume of hydrate with production time under different production pressure differences in an embodiment of the present invention;

[0031] Figure 8 A diagram showing the evolution of reservoir pressure distribution over time during the mining process of an embodiment of the present invention;

[0032] Fig. 9 This is a diagram showing the evolution of reservoir temperature distribution over time during the mining process of an embodiment of the present invention;

[0033] Fig.10 This is a diagram showing the evolution of reservoir hydrate saturation distribution over time during the mining process of an embodiment of the present invention;

[0034] Fig.11This is a diagram showing the evolution of reservoir gas saturation distribution over time during the mining process of an embodiment of the present invention. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with an example of combined production of natural gas hydrate and underlying gas in a certain sea area. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0036] According to a numerical simulation method for combined production of marine natural gas hydrate and underlying gas, Figure 1 The specific implementation process is as follows:

[0037] Step S1: selecting a target block, where natural gas hydrates coexist with underlying gas;

[0038] Not all sea areas have developed natural gas hydrate reservoirs, and specific analysis is required for specific working conditions. In the present invention, in order to simulate the co-production of natural gas hydrates and underlying gas, a target block with symbiotic natural gas hydrates and underlying gas is selected as the study area.

[0039] Step S2: establishing a numerical simulation physical model for combined production of natural gas hydrate and underlying gas in the target block, and determining initial simulation parameters;

[0040] Specifically, a numerical simulation physical model for the combined production of natural gas hydrates and underlying gas in the sea area of ​​the study area was established based on the field data. The field data included seismic interpretation, inversion results, logging and completion comprehensive processing interpretation results maps, logging interpretation results tables, formation testing results tables and other data.

[0041] The physical model of multi-gas production of natural gas hydrate and underlying gas in the sea area established based on well logging data includes not only the hydrate layer, barrier layer and underlying gas, but also the overburden and underburden. The production well is located in the middle of the cylindrical model, and the perforation section is set from the hydrate layer to the underlying gas layer. Figure 2 shown.

[0042] After the physical model of multi-gas production is established, it is necessary to determine the simulation parameters of the overburden thickness a, hydrate layer thickness b, barrier layer thickness c, underlying gas thickness d, underlying layer thickness e, model radius r, and the depth h of the top of the hydrate layer from the sea level. The longitudinal section of the physical model of multi-gas production of natural gas hydrate and underlying gas in the sea area is as follows: Figure 3 shown.

[0043] The initial simulation parameters include: seawater depth, seabed temperature, depth and temperature of the top of the hydrate layer, temperature gradient, pressure gradient or pressure coefficient, thickness of each layer, size of model geometric boundaries, porosity of each layer, permeability of each layer, water saturation, gas saturation, hydrate saturation of each layer, and salinity of pore water in each layer.

[0044] Among them, the hydrate decomposition kinetic equation is:

[0045] CH4+N H H2O=CH4·N H H2O

[0046] Where N H is the hydration number; the integrated finite difference method is used to divide the simulation domain, and the mass and energy conservation equations in the grid block are expressed as:

[0047]

[0048] Where V n is the volume of grid block n, m 3 ;M k is the mass accumulation term of component k, kg·m -3 ; k is hydrate component (h), methane component (m), water component (w), chemical component (i) or energy (θ); F k is the Darcy vector of component k, kg·m -2 ·s -1 Γ n is the surface area m of grid block n 2 , n is the unit normal vector of the surface element; q k is the source and sink term of the component, kg·m -3 ·s -1 ; t is time, s;

[0049] Under kinetic conditions, the mass accumulation term M of component k k The calculation formula is as follows:

[0050]

[0051] In the formula, is the porosity, dimensionless; β is the solid hydrate (H) or water phase (A) or gas phase (G) or ice phase (I); s β is the saturation of the β phase, dimensionless; ρ β is the β phase density, kg·m -3 ;X β k is the mass fraction of component, k = w,m,i, in phase β, kg / kg;

[0052] The mass fluxes of water, CH4 and inhibitor include the contributions of water phase and gas phase, i.e.

[0053]

[0054] Since they are immobile, the contribution of the two solid phases (β = I, H) to the fluid flux is zero. The heat flux density takes into account conduction, advection and radiation heat transfer and is given by

[0055]

[0056] In the formula is the composite thermal conductivity of the medium / fluid system, W·m -1 ·K -1 ;h β is the specific enthalpy of phase β=A,G, J·kg -1 ;f σ is the radiosity factor, dimensionless; σ0 is the Boltzmann constant, 5.6687×10 -8 J.m -2 ·K -4 ;

[0057] Under equilibrium conditions, the rate of heat removal or addition includes the contribution of heat associated with fluid removal or addition, as well as direct heat input or extraction, and is given by

[0058]

[0059] Where q β is the yield of β phase, kg·m -3 ; Under different temperature and pressure conditions, the hydrate system is in different phases. When the equilibrium state is broken, the state of the system will change. The phase equilibrium relationship between hydrate decomposition and generation is as follows:

[0060]

[0061] The effect of salinity on the decomposition equilibrium pressure-temperature relationship is described as

[0062]

[0063] Where, T is temperature, K; p e is the equilibrium pressure at temperature T, Pa; T e is the equivalent equilibrium temperature when inhibitor K exists; ΔT D is the temperature drop caused by the corrosion inhibitor, K; ΔT D，r is the reference mole fraction X mol,A c Temperature gradient at, K; X mol,A c The molar fraction of the inhibitor in the aqueous phase; Xmol,A,r c is the reference molar fraction of the inhibitor in the aqueous phase; the inhibitor in the present invention is NaCl.

[0064] In a preferred embodiment, considering the influence of pressure and temperature on the porosity coefficient, the corresponding variation equation is established:

[0065]

[0066]

[0067] in, is the effective porosity; is the initial porosity; k e is the effective permeability; k0 is the initial permeability; α p is the rock compression coefficient; α T is the thermal expansion coefficient of rock; ΔP is the pressure change; ΔT is the temperature change; S H is the hydrate saturation; γ is the first permeability variation index, which is related to temperature and pressure; n is the second permeability variation index, which is related to solid phase saturation; is the critical porosity.

[0068] Step S3: Meshing and setting boundary conditions;

[0069] The grid division needs to be encrypted near the wellbore, and then the grid can be gradually increased as it moves radially outward, and the grid must also be encrypted at the connection between each layer to improve the calculation efficiency while meeting the simulation accuracy. The grid size at the upper and lower boundaries of the upper and lower covering layers of the model is 0.01m, and it is set as an inactive grid, so as to limit the temperature and pressure conditions at the boundary and not change with the decomposition of hydrates.

[0070] Step S4: Setting wellbore and perforation information and production system;

[0071] The setting of the wellbore and perforation information includes: the grid position of the wellbore in step S3, the porosity of the wellbore, the longitudinal and axial permeability of the wellbore, the grid position of the perforation area in step S3, the porosity of the perforation area, and the longitudinal and axial permeability of the perforation area.

[0072] The production system includes: the location of the production well, the bottom hole pressure, and the total duration of production.

[0073] Step S5: Solve the constructed model and perform sensitivity analysis on the results.

[0074] Solving the constructed model through numerical simulation software requires assigning the simulation initial parameters in step S2 and the wellbore and perforation information and production system in step S4 to the grid constructed in S3 and calculating in the simulator.

[0075] To solve the constructed model through numerical simulation software, it is necessary to set the time step in the simulator, the time to output the required cloud map and other parameters.

[0076] Furthermore, in order to simulate the production conditions under different working conditions, the boundary conditions can be limited by fixing the bottom hole pressure or the production rate.

[0077] Under the condition of constant bottom hole pressure, the production of gas and water phases in the grid is calculated as follows:

[0078]

[0079] Where q is the production of gas and water phases; subscript β is the fluid type, A is the water phase, G is the gas phase; PI is the production index; k r is the relative permeability, μ is the viscosity, ρ is the density, p is the formation pressure, and p wf is the bottom hole pressure.

[0080] Under the condition of fixed production, the production of gas phase and water phase in each grid is distributed according to the percentage of the flow capacity of gas and water phase to the total flow capacity, and is calculated in the following way:

[0081]

[0082]

[0083] Among them, q A,p,i is the yield of water phase in grid i, q G,p,i is the gas phase production in grid i; Q p is the given total production, PI is the production index; the subscript β is the fluid type, A is the water phase, and G is the gas phase.

[0084] A sensitivity analysis was conducted on the numerical simulation results of combined production of offshore natural gas hydrates and underlying gas.

[0085] According to the target reservoir and development requirements, the numerical simulation results of offshore natural gas hydrate and underlying gas co-production are analyzed, including wellhead gas production rate, wellhead cumulative gas production, wellhead water production rate, wellhead cumulative water production, hydrate decomposition rate, cumulative hydrate decomposition amount, remaining methane amount, remaining hydrate amount, reservoir pressure distribution law, reservoir temperature distribution law, reservoir hydrate distribution law, reservoir gas distribution law, and reservoir settlement law.

[0086] In a preferred embodiment, in order to analyze the influence of various parameters on the recovery effect, various sensitive parameters and recovery effect parameters are first preprocessed, and then analyzed using the grey theory, wherein the correlation coefficient calculation method can be:

[0087]

[0088] Among them, γ i is the correlation coefficient, m is the number of sensitivity parameters, xd0(k) is the preprocessed harvesting effect parameter sequence, xd i (k) is the sensitivity parameter sequence after preprocessing, k is the sequence number;

[0089] The pre-processing methods available are:

[0090]

[0091] Among them, x(k) is the parameter sequence before preprocessing, and xd(k) is the parameter sequence after preprocessing.

[0092] After preprocessing, the difference between the sensitivity parameter and the harvesting effect parameter can also be calculated.

[0093] Δ i (k) = (xd0(k) - xd i (k))

[0094] Then determine the extreme values ​​and the relationship coefficients:

[0095] x max =max(xd(k)*Δ i (k))

[0096] x min =min(xd(k)*Δ i (k))

[0097]

[0098] Among them, x max and x min is an extreme value, ε i is the relationship coefficient.

[0099] Determine the relationship between each sensitivity parameter and harvesting effect parameter:

[0100]

[0101] Among them, θ i is the degree of relationship, m is the number of sensitivity parameters, ω i is the weight of the sensitivity parameter, ε i is the relationship coefficient.

[0102] By obtaining the relationship degree, the influence of each sensitivity parameter on the harvesting effect can be quickly determined, which is conducive to the rapid formulation of simulation optimization plans and further guides the on-site production.

[0103] In the above steps, all or part of the steps can be implemented by software, hardware, firmware or any combination thereof. When the steps are implemented in whole or in part in the form of a computer program product, the computer program product includes one or more computer instructions. When the computer program instructions are loaded or executed on a computer, the process or function described in the embodiment of the present invention is generated in whole or in part. The computer may be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions may be stored in a computer-readable storage medium or transmitted from one computer-readable storage medium to another computer-readable storage medium. For example, the computer instructions may be transmitted from one website, computer, server or data center to another website, computer, server or data center by wired (e.g., coaxial cable, optical fiber, digital subscriber line (DSL) or wireless (e.g., infrared, wireless, microwave, etc.)). The computer-readable storage medium may be any available medium that a computer can access or a data storage device such as a server or data center that includes one or more available media integrated therein. The available medium may be a magnetic medium (e.g., a floppy disk, a hard disk, a tape), an optical medium (e.g., a DVD), or a semiconductor medium (e.g., a solid-state drive Solid State Disk (SSD)), etc.

[0104] The technical solution of the present invention is further described below in conjunction with embodiments.

[0105] like Figure 4 As shown, it is a schematic diagram of the physical model of natural gas hydrate and underlying gas multi-gas co-production in an embodiment of the present invention. In this embodiment, the seawater depth is 1549.95m, the seabed temperature is 3°C, the initial pressure at the bottom of the hydrate layer is 17.93MPa, the initial temperature is 16.22°C, the temperature gradient is 0.04°C / m, the pressure coefficient is 1.04, the model radius is 120m, the depth of the top of the hydrate layer from the sea level is 1766.9m, the thickness of the overburden layer is 5m, the thickness of the hydrate layer is 3.6m, the thickness of the interlayer is 0.5m, the thickness of the underlying gas layer is 13m, and the thickness of the lower overburden layer is 5m. The porosity of the overburden layer, hydrate layer, interlayer, underlying gas layer and lower overburden are 28.8%, 44.8%, 40%, 41.8% and 28.8% respectively, and the permeability is 100mD, 500mD, 100mD, 680mD and 100mD respectively.

[0106] The cylindrical simulation area near the development well in this embodiment has radial and vertical depths of 120m and 26.6m respectively. The wellbore is divided into 30 grids radially, the first production well grid is 0.1m, and it is divided into 31 grids vertically. The cylindrical system is discretized into 30×31=930 grid blocks radially and vertically. The gas-water flow and temperature and pressure changes in the area around the production well are more drastic. The dense and refined reservoir grids around the well are helpful to analyze the changes in characteristic parameters of the reservoir around the well. The vertical well unit, the uppermost unit of the upper cover layer and the lowermost unit of the lower cover layer are all inactive units. Their reservoir characteristic parameters such as temperature and pressure remain constant. During the simulation, hydrate formation is considered to be decomposed into a phase equilibrium reaction. The schematic diagram of the grid division of the physical model of natural gas hydrate and underlying gas multi-gas production established in the embodiment of the present invention is shown in the figure. Figure 5 shown.

[0107] This example adopts a single vertical well constant pressure reduction mining method, a vertical well penetrating the hydrate layer and the underlying gas layer for joint mining, and perforating at the location of the hydrate layer and the underlying gas layer. The production pressure difference is set to 0.7MPa, 0.8MPa, 0.9MPa, 1.0MPa, 2.0MPa, 3.0MPa, and 4.0MPa, respectively, and the short-term test production lasts for 5 days. The vertical well is located at the center of the cylindrical hydrate model with a radius of r w It is assumed that the fluid flow in the production well is a gas-liquid permeation process, following Darcy's law, the porosity is 1.0, the longitudinal permeability of the wellbore is 5000D, the radial permeability of the perforation area is 10D, and the capillary pressure is 0.

[0108] The simulation initial parameters in step S2 and the wellbore and perforation information and production system in step S4 are assigned to the grid built in S3, and finally the solution is performed. The capacity analysis is performed based on the simulation data, and the cumulative gas production at the wellhead changes with the production time under different production pressure differences and the cumulative decomposition gas volume of hydrates changes with the production time under different production pressure differences are plotted as follows: Figure 6 , 7 According to the simulation results, the reservoir physical property parameters are evolved to obtain the distribution evolution law of reservoir pressure, temperature, hydrate saturation and gas saturation over time, as shown in Figure 8-11 shown.

[0109] It can be seen that in this example, the average gas production rate at the wellhead increases significantly when the production pressure difference increases from 0.6MPa to 1.5MPa, and the average gas production rate at the wellhead increases slightly when the production pressure difference increases from 1.5MPa to 4MPa. The thickness of the hydrate layer in the reservoir of this example is small, while the underlying free gas layer is thick and has high permeability, which makes it easy for the free gas to flow into the wellbore, hindering the contact between the decomposition front of the hydrate layer and the nearby thermal fluid, and weakening the heat conduction effect. In the early stage of the test, a higher production pressure difference will inhibit the decomposition of hydrates. During the production process, the floating of free gas will form a highly saturated gas belt near the interface between the hydrate layer and the underlying free gas layer. In the early stage of the test, when the production pressure difference reaches a certain level, the large amount of floating of free gas will generate hydrates in the hydrate layer for the second time. In this example, the production pressure difference needs to be controlled within 1.5MPa under the formation conditions to ensure that the hydrate decomposition rate remains positive during the test. There is a significant difference in the distance of pressure propagation between the hydrate layer and the underlying free gas layer. The presence of free gas hinders the propagation of pressure in the hydrate layer, resulting in a reduced pressure propagation distance, while the pressure can be easily propagated in the underlying free gas layer. In addition, in this example, the heat absorption of hydrate decomposition and the "scorched soup effect" caused by the influx of free gas into the wellbore lead to the formation of a low-temperature zone below the interface between the hydrate layer near the wellbore and the underlying free gas layer.

[0110] The present invention comprehensively considers the resource conditions of the coupled symbiosis of marine natural gas hydrates and the underlying gas multi-gas, establishes a numerical simulation physical model of marine natural gas hydrates and the underlying gas multi-gas co-production in the target area, including hydrate layers, interlayers and underlying free gas layers, establishes the study area according to seismic interpretation, inversion results and logging data, determines the initial simulation parameters, and then performs grid division, sets the wellbore and perforation information, and production system, solves the established model through numerical simulation software, and finally analyzes the numerical simulation results of marine natural gas hydrates and the underlying gas multi-gas co-production. The method has a clear and simple process, strong operability, and is also universal in other natural gas hydrate development areas.

[0111] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A numerical simulation method for combined production of marine natural gas hydrates and underlying gas, characterized in that: The steps include: Step S1: selecting a target block, where natural gas hydrates coexist with underlying gas; Step S2: establishing a numerical simulation physical model for combined production of natural gas hydrate and underlying gas in the target block, and determining initial simulation parameters; Step S3: Meshing and setting boundary conditions; Step S4: Setting wellbore and perforation information and production system; Step S5: Solve the constructed model and perform sensitivity analysis on the results.

2. The numerical simulation method for combined production of marine natural gas hydrate and underlying gas according to claim 1 is characterized by: In step S1, the development of underlying gas is quantitatively evaluated by geostatistical methods.

3. The numerical simulation method for combined production of marine natural gas hydrate and underlying gas according to claim 1 is characterized by: In step S2, the initial simulation parameters are determined using field data, which include seismic interpretation, inversion results, logging and completion comprehensive processing interpretation results map, logging interpretation results table, and formation test result table data.

4. The numerical simulation method for combined production of marine natural gas hydrate and underlying gas according to claim 1 is characterized by: The multi-gas combined production numerical simulation physical model in step S2 includes: an overburden layer, a hydrate layer, an interlayer, an underlying gas layer, an underburden layer, a wellbore position, and a perforation position.

5. The numerical simulation method for combined production of marine natural gas hydrate and underlying gas according to claim 1 is characterized in that: The grid division in step S3 also includes the steps of: performing grid encryption near the wellbore, gradually increasing the grid size radially outward, performing grid encryption at the connection between each layer, and setting the grids at the upper and lower boundaries of the upper and lower covering layers as inactive grids.

6. The method for numerical simulation of multi-gas production of marine natural gas hydrate and underlying gas according to claim 1, characterized in that: Step S4 also includes: setting the position, porosity, longitudinal and axial permeability of the wellbore, and setting the position, porosity, longitudinal and axial permeability of the perforation area.

7. The numerical simulation method for combined production of marine natural gas hydrate and underlying gas according to claim 1 is characterized by: In step S5, the grey theory is used to perform sensitivity analysis.

8. The method for numerical simulation of multi-gas production of marine natural gas hydrate and underlying gas according to claim 7, characterized in that: The following parameters are analyzed: wellhead gas production rate, cumulative wellhead gas production, wellhead water production rate, cumulative wellhead water production, hydrate decomposition rate, cumulative hydrate decomposition, remaining methane amount, remaining hydrate amount, reservoir pressure distribution law, reservoir temperature distribution law, reservoir hydrate distribution law, reservoir gas distribution law, and reservoir sedimentation law.

9. A computer device comprising a memory, a processor, and a computer program stored in the memory and running on the processor, characterized in that: When the processor executes the computer program, the steps of the numerical simulation method for combined production of marine natural gas hydrates and underlying gas are implemented as described in any one of claims 1 to 8.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the numerical simulation method for combined production of marine natural gas hydrates and underlying gas are implemented as described in any one of claims 1 to 8.

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