A method for calculating gas production from hydrate decomposition

Abstract

The application discloses a hydrate decomposition gas production calculation method, S1: obtaining natural gas hydrate corresponding parameters, and defining natural gas area, hydrate area and moving decomposition front; S2: establishing hydrate decomposition kinetics equation; S3: establishing hydrate decomposition material balance relationship, calculating decomposition gas volume and decomposition gas production rate; S4: obtaining expression of moving decomposition front position changing with time; S5: establishing radial seepage and pressure distribution model in natural gas area and determining natural gas area pressure distribution expression; S6: calculating natural gas hydrate decomposition gas production. Through coupling of hydrate decomposition kinetics, moving decomposition front propulsion and natural gas area radial seepage process, the gas production corresponding to hydrate decomposition is directly calculated; meanwhile, the relationship among decomposition front position, pressure distribution and gas production is described by means of analytic derivation, and the hydrate decomposition gas production can be rapidly predicted under different production systems and reservoir conditions.

Description

Technical Field

[0001] This invention relates to the field of natural gas hydrate development technology, and in particular to a method for calculating the gas production from hydrate decomposition. Background Technology

[0002] During the depressurization extraction of natural gas hydrates, the reduction in reservoir pressure disrupts the stability conditions of the hydrates, causing them to decompose and release methane gas. The gas production from hydrate decomposition is a crucial indicator for evaluating the development potential of hydrate reservoirs, analyzing the effectiveness of depressurization extraction, and optimizing production regimes. Existing methods for evaluating hydrate gas production mainly include numerical simulation, experimental fitting, and wellhead gas production statistics. Among these, numerical simulation relies on complex grids, phase parameters, and iterative solution processes; experimental fitting typically has limited applicability; and wellhead gas production statistics are easily affected by factors such as free gas, wellbore confluence, and boundary recharge, making it difficult to independently characterize the gas production generated by hydrate decomposition.

[0003] Furthermore, while calculating the decomposition gas yield solely based on changes in hydrate saturation can reflect the hydrate phase reduction process, it fails to capture the coupling effect between the decomposition front advancement, decomposition kinetics, and radial gas flow driven by depressurization. Therefore, an analytical calculation method is needed to simultaneously describe hydrate decomposition kinetics and natural gas radial flow for calculating the decomposition gas production during the depressurization extraction process of natural gas hydrates. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for calculating the gas production from hydrate decomposition.

[0005] The objective of this invention is achieved through the following technical solution: a method for calculating the gas production from hydrate decomposition, comprising the following steps:

[0006] S1: Obtain the corresponding parameters of natural gas hydrates, and define the natural gas zone, hydrate zone and moving decomposition front according to the physical process of hydrate depressurization extraction;

[0007] S2: Establish the kinetic equation for hydrate decomposition;

[0008] S3: Establish the equilibrium relationship of hydrate decomposition substances and calculate the volume of decomposition gas and the decomposition gas production rate;

[0009] S4: By combining the hydrate decomposition kinetic equation with the hydrate decomposition material equilibrium relationship, we can obtain an expression for the change of the position of the moving decomposition front over time.

[0010] S5: Establish a radial seepage and pressure distribution model in the natural gas zone and determine the pressure distribution expression in the natural gas zone;

[0011] S6: Calculate the gas production from the decomposition of natural gas hydrates under standard conditions.

[0012] Preferably, in step S1, the parameters include reservoir parameters, fluid parameters, and development parameters. The reservoir parameters include reservoir radius, wellbore radius, reservoir thickness, porosity, permeability, hydrate saturation, and reservoir temperature. The fluid parameters include gas viscosity, gas compressibility factor, hydrate decomposition kinetic coefficient, and hydrate phase equilibrium pressure. The development parameters include initial reservoir pressure, bottom hole pressure, standard state pressure, standard state temperature, and production time.

[0013] Preferably, in step S1, the moving decomposition front is the radial interface between the natural gas zone and the hydrate zone, and the position of the moving decomposition front moves from near the wellbore towards the outer boundary of the reservoir as the hydrate decomposition process proceeds.

[0014] Preferably, in step S2, the hydrate decomposition rate is determined by the pressure difference between the hydrate phase equilibrium pressure and the reservoir pressure at the moving decomposition front, and the hydrate decomposition kinetic equation is:

[0015] ;

[0016] in, This represents the initial amount of gaseous material that can be released from the hydrate before extraction. This represents the amount of gaseous substances still remaining in the undecomposed hydrate at any given time. The coefficient for hydrate decomposition kinetics. The area of ​​the hydrate decomposition interface. For the equilibrium pressure of the hydrate phase, To move the reservoir pressure at the decomposition front, This represents the order of the decomposition reaction.

[0017] Preferably, in step S3, the equilibrium relationship of hydrate decomposition substances includes the initial gas volume, the remaining gas volume, and the decomposition gas volume;

[0018] The formula for calculating the initial amount of releasable gas in the hydrate before extraction is as follows:

[0019] ;

[0020] in, This represents the initial amount of gaseous material that can be released from the hydrate before extraction. The radius of the outer boundary of the reservoir is . For reservoir thickness, Porosity For hydrate saturation, For the formation factor of natural gas hydrates, For standard state pressure, The standard compressibility coefficient of a gas. The gas constant is... Standard temperature;

[0021] The formula for calculating the amount of residual gas remaining in the undecomposed hydrate at any given time is:

[0022] ;

[0023] in, This represents the amount of gaseous substances still remaining in the undecomposed hydrate at any given time. This represents the equivalent radius of the decomposed region or the advancing distance of the moving decomposed leading edge.

[0024] The formula for calculating the amount of gas produced by the decomposition of hydrates is:

[0025] .

[0026] Preferably, in step S3, the formula for calculating the volume of the decomposed gas is:

[0027] ;

[0028] in, This represents the volume of gas produced during the decomposition of hydrates.

[0029] The formula for calculating the gas production rate of hydrate decomposition is:

[0030] ;

[0031] in, This represents the rate of gas production from hydrate decomposition under standard conditions. This refers to the mining time.

[0032] Preferably, step S4 further includes the following step:

[0033] S41: By combining the hydrate decomposition kinetic equations with the mass balance relationship, a control relationship for the propulsion of the moving decomposition front is established:

[0034] ;

[0035] S42: Construct the differential equation of the moving decomposition front as a function of time:

[0036] ;

[0037] in, To determine the moving decomposition leading edge propulsion velocity, The correlation coefficient is the decomposition kinetic coefficient.

[0038] S43: Analytical expression for the position of the moving decomposition leading edge obtained by combining the initial conditions:

[0039] ;

[0040] in, This refers to the bottom hole pressure.

[0041] Preferably, step S5 further includes the following step:

[0042] S51: Establish radial gas flow equations within the already decomposed natural gas zone.

[0043] ;

[0044] in, This refers to the pressure in the natural gas zone or reservoir I. Radial distance, For mining time, The hydrodynamic diffusion constant of reservoir I region;

[0045] S52: The pressure distribution expression for the natural gas zone is obtained by combining the wellbore boundary conditions and the moving decomposition front boundary conditions. The wellbore boundary conditions constrain the pressure at the wellbore radius, and the moving decomposition front boundary conditions constrain the pressure at the decomposition front.

[0046] ;

[0047] in, For the pressure distribution in the natural gas zone, The dimensionless variable corresponding to the radial position after the variable transformation. The dimensionless variable corresponding to the decomposition front after the variable transformation. The dimensionless variable corresponding to the wellbore boundary after variable transformation. It is an exponential integral function.

[0048] Preferably, in step S6, the gas production from the decomposition of natural gas hydrate under standard conditions is calculated based on Darcy's law, the square form of pressure, and standard state parameters.

[0049] ;

[0050] in, This represents the rate of gas production from hydrate decomposition under standard conditions. For reservoir permeability, For reservoir thickness, For gas viscosity, The gas compressibility factor, For reservoir temperature, Where is the wellbore radius. is the hydrodynamic diffusion constant.

[0051] This invention has the following advantages: By coupling hydrate decomposition kinetics, the advancement of the moving decomposition front, and the radial seepage process in the natural gas zone, this invention can directly calculate the gas production corresponding to hydrate decomposition without introducing the calculation of free gas contribution rate or shallow gas contribution rate; at the same time, this invention uses an analytical derivation method to describe the relationship between the decomposition front position, pressure distribution, and gas production, which can be used for rapid prediction of hydrate decomposition gas production under different production regimes and reservoir conditions. Attached Figure Description

[0052] Figure 1 A schematic diagram of the process for calculating the gas production from hydrate decomposition;

[0053] Figure 2 A schematic diagram of the natural gas region, hydrate region, and moving decomposition front;

[0054] Figure 3 This is a schematic diagram showing the calculated results of the gas production from hydrate decomposition over time. Detailed Implementation

[0055] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0056] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0057] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other.

[0058] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0059] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this invention is in use, or the orientation or positional relationship commonly understood by those skilled in the art. They are only used for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. In addition, the terms "first," "second," etc., are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0060] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0061] In this embodiment, as Figure 1 As shown, a method for calculating the gas production from hydrate decomposition includes the following steps:

[0062] S1: Obtain the corresponding parameters of natural gas hydrates, and define the natural gas zone, hydrate zone and moving decomposition front according to the physical process of hydrate depressurization extraction;

[0063] S2: Establish the kinetic equation for hydrate decomposition;

[0064] S3: Establish the equilibrium relationship of hydrate decomposition substances and calculate the volume of decomposition gas and the decomposition gas production rate;

[0065] S4: Combine the hydrate decomposition kinetic equation with the hydrate decomposition substance equilibrium relationship to obtain an expression for the change of the position of the moving decomposition front over time;

[0066] S5: Establish a radial seepage and pressure distribution model in the natural gas zone and determine the pressure distribution expression in the natural gas zone;

[0067] S6: Calculate the gas production from natural gas hydrate decomposition under standard conditions. By coupling hydrate decomposition kinetics, the advancement of the moving decomposition front, and the radial seepage process in the natural gas zone, the gas production corresponding to hydrate decomposition can be directly calculated without introducing the contribution rate of free gas or shallow gas. At the same time, this invention uses an analytical derivation method to describe the relationship between the decomposition front location, pressure distribution, and gas production, which can be used for rapid prediction of hydrate decomposition gas production under different production regimes and reservoir conditions.

[0068] Furthermore, in step S1, the parameters include reservoir parameters, fluid parameters, and development parameters. The reservoir parameters include reservoir radius, wellbore radius, reservoir thickness, porosity, permeability, hydrate saturation, and reservoir temperature. The fluid parameters include gas viscosity, gas compressibility factor, hydrate decomposition kinetic coefficient, and hydrate phase equilibrium pressure. The development parameters include initial reservoir pressure, bottom hole pressure, standard state pressure, standard state temperature, and production time. Specifically, such as... Figure 2 As shown, during depressurization extraction, the area near the wellbore where hydrates have decomposed and formed gas flow is defined as the natural gas zone, and the area where hydrates have not yet completely decomposed is defined as the hydrate zone. The radial interface between the two is defined as the moving decomposition front. As extraction progresses, the moving decomposition front advances from near the wellbore toward the outer boundary of the reservoir. In other words, the moving decomposition front is the radial interface between the natural gas zone and the hydrate zone, and the position of the moving decomposition front moves from near the wellbore toward the outer boundary of the reservoir as the hydrates decompose.

[0069] Furthermore, in step S2, the hydrate decomposition rate is determined by the pressure difference between the hydrate phase equilibrium pressure and the reservoir pressure at the moving decomposition front. A hydrate decomposition kinetic equation is established based on this pressure difference to characterize the relationship between the hydrate decomposition rate and the pressure difference, the decomposition interface area, and the kinetic index. The hydrate decomposition kinetic equation is as follows:

[0070] ;

[0071] in, This represents the initial amount of gaseous material that can be released from the hydrate before extraction. This represents the amount of gaseous substances still remaining in the undecomposed hydrate at any given time. The coefficient for hydrate decomposition kinetics. The area of ​​the hydrate decomposition interface. For the equilibrium pressure of the hydrate phase, To move the reservoir pressure at the decomposition front, This formula represents the decomposition reaction order and is used to indicate the rate at which the hydrate decomposes and releases gas per unit time.

[0072] In this embodiment, in step S3, the equilibrium relationship of hydrate decomposition substances includes the initial gas volume, the remaining gas volume, and the decomposition gas volume;

[0073] The formula for calculating the initial amount of releasable gas in the hydrate before extraction is as follows:

[0074] ;

[0075] in, This represents the initial amount of gaseous material that can be released from the hydrate before extraction. The radius of the outer boundary of the reservoir is . For reservoir thickness, Porosity For hydrate saturation, For the formation factor of natural gas hydrates, For standard state pressure, The standard compressibility coefficient of a gas. The gas constant is... Standard temperature;

[0076] The formula for calculating the amount of residual gas remaining in the undecomposed hydrate at any given time is:

[0077] ;

[0078] in, This represents the amount of gaseous substances still remaining in the undecomposed hydrate at any given time. This represents the equivalent radius of the decomposed region or the advancing distance of the moving decomposed leading edge.

[0079] The formula for calculating the amount of gas produced by the decomposition of hydrates is:

[0080] .

[0081] Furthermore, in step S3, the formula for calculating the volume of the decomposed gas is:

[0082] ;

[0083] in, This represents the volume of gas produced during the decomposition of hydrates.

[0084] The formula for calculating the gas production rate of hydrate decomposition is:

[0085] ;

[0086] in, This represents the rate of gas production from hydrate decomposition under standard conditions. This refers to the mining time.

[0087] Furthermore, step S4 also includes the following steps:

[0088] S41: By combining the hydrate decomposition kinetic equations with the mass balance relationship, a control relationship for the propulsion of the moving decomposition front is established:

[0089] ;

[0090] S42: Construct the differential equation of the moving decomposition front as a function of time:

[0091] ;

[0092] in, To determine the moving decomposition leading edge propulsion velocity, The correlation coefficient is the decomposition kinetic coefficient.

[0093] S43: Analytical expression for the position of the moving decomposition leading edge obtained by combining the initial conditions:

[0094] ;

[0095] in, This refers to the bottom hole pressure. Specifically, this step establishes a correlation between the hydrate decomposition range and the extraction time, providing leading-edge location parameters for subsequent calculations of natural gas zone pressure distribution and production.

[0096] In this embodiment, step S5 further includes the following step:

[0097] S51: Establish a radial gas seepage equation within the already decomposed natural gas zone to describe the change in pressure within the natural gas zone with radial distance and time.

[0098] ;

[0099] in, This refers to the pressure in the natural gas zone or reservoir I. Radial distance, For mining time, The hydrodynamic diffusion constant of reservoir I region;

[0100] S52: The pressure distribution expression for the natural gas zone is obtained by combining the wellbore boundary conditions and the moving decomposition front boundary conditions. The wellbore boundary conditions constrain the pressure at the wellbore radius, and the moving decomposition front boundary conditions constrain the pressure at the decomposition front.

[0101] ;

[0102] in, For the pressure distribution in the natural gas zone, The dimensionless variable corresponding to the radial position after the variable transformation. The dimensionless variable corresponding to the decomposition front after the variable transformation. The dimensionless variable corresponding to the wellbore boundary after variable transformation. It is an exponential integral function.

[0103] Furthermore, in step S6, the gas production from the decomposition of natural gas hydrate under standard conditions is calculated based on Darcy's law, the square form of pressure, and standard state parameters. This production is determined by the square gradient of pressure at the wellbore, reservoir permeability, reservoir thickness, gas viscosity, gas compressibility factor, reservoir temperature, and standard state parameters.

[0104] ;

[0105] in, This represents the rate of gas production from hydrate decomposition under standard conditions. For reservoir permeability, For reservoir thickness, For gas viscosity, The gas compressibility factor, For reservoir temperature, Where is the wellbore radius. This is the hydrodynamic diffusion constant. Specifically, such as... Figure 3 As shown, with the increase of extraction time, the moving decomposition front gradually advances outward, the pressure gradient of the natural gas zone gradually decreases, and the calculated gas production from hydrate decomposition shows a decreasing trend, which can reflect the change law of decomposition gas production during the depressurization extraction process of natural gas hydrate.

[0106] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for calculating the gas production from hydrate decomposition, characterized in that: Includes the following steps: S1: Obtain the corresponding parameters of natural gas hydrates, and define the natural gas zone, hydrate zone and moving decomposition front according to the physical process of hydrate depressurization extraction; S2: Establish the kinetic equation for hydrate decomposition; S3: Establish the equilibrium relationship of hydrate decomposition substances and calculate the volume of decomposition gas and the decomposition gas production rate; S4: Combine the hydrate decomposition kinetic equation with the hydrate decomposition substance equilibrium relationship to obtain an expression for the change of the position of the moving decomposition front over time; S5: Establish a radial seepage and pressure distribution model in the natural gas zone and determine the pressure distribution expression in the natural gas zone; S6 : Calculate the gas production from the decomposition of natural gas hydrates under standard conditions; In step S1, the parameters include reservoir parameters, fluid parameters, and development parameters. The reservoir parameters include reservoir radius, wellbore radius, reservoir thickness, porosity, permeability, hydrate saturation, and reservoir temperature. The fluid parameters include gas viscosity, gas compressibility factor, hydrate decomposition kinetic coefficient, and hydrate phase equilibrium pressure. The development parameters include initial reservoir pressure, bottom hole pressure, standard state pressure, standard state temperature, and production time. In step S1, the moving decomposition front is the radial interface between the natural gas zone and the hydrate zone, and the position of the moving decomposition front moves from near the wellbore towards the outer boundary of the reservoir as the hydrate decomposition process proceeds. In step S2, the hydrate decomposition rate is determined by the pressure difference between the hydrate phase equilibrium pressure and the reservoir pressure at the moving decomposition front. The hydrate decomposition kinetic equation is: ； in, This represents the initial amount of gaseous material that can be released from the hydrate before extraction. This represents the amount of gaseous substances still remaining in the undecomposed hydrate at any given time. The coefficient for hydrate decomposition kinetics. The area of ​​the hydrate decomposition interface. For the equilibrium pressure of the hydrate phase, To move the reservoir pressure at the decomposition front, This represents the order of the decomposition reaction.

2. The method for calculating the gas production from hydrate decomposition according to claim 1, characterized in that: In step S3, the equilibrium relationship of hydrate decomposition substances includes the initial gas volume, the remaining gas volume, and the decomposition gas volume; The formula for calculating the initial amount of releasable gas in the hydrate before extraction is as follows: ； in, This represents the initial amount of gaseous material that can be released from the hydrate before extraction. The radius of the outer boundary of the reservoir is . For reservoir thickness, Porosity For hydrate saturation, For the formation factor of natural gas hydrates, For standard state pressure, The standard compressibility coefficient of a gas. The gas constant is Standard temperature; The formula for calculating the amount of residual gas remaining in the undecomposed hydrate at any given time is as follows: ； in, This represents the amount of gaseous substances still remaining in the undecomposed hydrate at any given time. This represents the equivalent radius of the decomposed region or the advancing distance of the moving decomposed leading edge. The formula for calculating the amount of gas produced by the decomposition of hydrates is: 。 3. The method for calculating the gas production from hydrate decomposition according to claim 2, characterized in that: In step S3, the formula for calculating the volume of the decomposed gas is: ； in, This represents the volume of gas produced during the decomposition of hydrates. The formula for calculating the gas production rate of hydrate decomposition is: ； in, This represents the rate of gas production from hydrate decomposition under standard conditions. This refers to the mining time.

4. The method for calculating the gas production from hydrate decomposition according to claim 3, characterized in that: Step S4 also includes the following steps: S41: By combining the hydrate decomposition kinetic equations with the mass balance relationship, a control relationship for the propulsion of the moving decomposition front is established: ； S42: Construct the differential equation of the moving decomposition front as a function of time: ； in, To determine the moving decomposition leading edge propulsion velocity, The correlation coefficient is the decomposition kinetic coefficient. S43: Analytical expression for the position of the moving decomposition leading edge obtained by combining the initial conditions: ； in, This refers to the bottom hole pressure.

5. The method for calculating the gas production from hydrate decomposition according to claim 4, characterized in that: Step S5 also includes the following steps: S51: Establish radial gas flow equations within the already decomposed natural gas zone. ； in, This refers to the pressure in the natural gas zone or reservoir I. Radial distance, For mining time, The hydrodynamic diffusion constant of reservoir I region; S52: The pressure distribution expression for the natural gas zone is obtained by combining the wellbore boundary conditions and the moving decomposition front boundary conditions. The wellbore boundary conditions constrain the pressure at the wellbore radius, and the moving decomposition front boundary conditions constrain the pressure at the decomposition front. ； in, For the pressure distribution in the natural gas zone, The dimensionless variable corresponding to the radial position after the variable transformation. The dimensionless variable corresponding to the decomposition front after the variable transformation. The dimensionless variable corresponding to the wellbore boundary after variable transformation. It is an exponential integral function.

6. The method for calculating the gas production from hydrate decomposition according to claim 5, characterized in that: In step S6, the gas production from the decomposition of natural gas hydrate under standard conditions is calculated based on Darcy's law, the square form of pressure, and standard state parameters. ； in, This represents the rate of gas production from hydrate decomposition under standard conditions. For reservoir permeability, For reservoir thickness, For gas viscosity, The gas compressibility factor, For reservoir temperature, Where is the wellbore radius. is the hydrodynamic diffusion constant.

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