Hydrate-containing porous medium resistivity model construction method considering hydrate heterogeneous distribution

Abstract

The invention discloses a hydrate-containing porous medium resistivity model construction method considering hydrate heterogeneous distribution, and the method comprises the following steps: 1) manufacturing a rock core according to the rock composition of a target hydrate reservoir, and scanning the pore throat structure of the rock core through CT; 2) generating a hydrate in the core, and scanning the occurrence state of the hydrate through CT (Computed Tomography); 3) establishing a digital core model based on CT scanning data, and applying an electrostatic field; 4) changing hydrate saturation parameters in the model, and calculating the resistivity of the model through numerical simulation; 5) calculating the critical water saturation of the hydrate; and (6) fitting by taking the critical water saturation as a boundary to obtain a resistivity segmentation calculation model. According to the method, a hydrate-containing porous medium resistivity model is established based on a real rock core size, and an electric field simulation means is utilized to calculate a hydrate-containing porous medium resistivity value. In combination with CT scanning data, an undetermined coefficient of a porous medium resistivity calculation formula in a hydrate thermal decomposition experiment is obtained through electric field simulation, and a basis is provided for hydrate reservoir electric heating development.

Description

Technical Field

[0001] The invention relates to a method for constructing a resistivity model of a hydrate-containing porous medium taking into account heterogeneous distribution of hydrates, and belongs to the technical field of electric heating hydrate reservoir development. Background Art

[0002] As a clean energy source, natural gas hydrates are considered an important alternative to conventional fossil fuels in the future due to their wide distribution and large reserves. Among the methods for hydrate reservoir extraction, the pressure reduction method, as a depletion-type extraction method, leads to low hydrate reservoir recovery rates; the thermal stimulation method has low thermal efficiency. Electric heating, as a typical in-situ thermal stimulation method, places electrodes in different wells and uses the heat dissipation of the interwell formation resistance under the action of an AC electric field to provide heat supplement for hydrate decomposition, thereby achieving in-situ heat generation in the hydrate layer. This can avoid the problem of large heat loss in the wellbore of conventional thermal stimulation methods and significantly improve thermal efficiency. Among them, the resistivity change during the decomposition of hydrate reservoirs has a great influence on electric heating extraction. This paper proposes a method for constructing a resistivity model of hydrate-containing porous media considering the heterogeneous distribution of hydrates, providing a simple and efficient method for studying the resistivity change law during the thermal decomposition of hydrate reservoirs. Summary of the Invention

[0003] In view of the above problems, the present invention aims to provide a method for constructing a resistivity model of hydrate-containing porous media taking into account the heterogeneous distribution of hydrates, comprising the following steps:

[0004] Step S1: Prepare a simulated core according to the target hydrate reservoir rock composition, and obtain the core pore throat structure through CT scanning;

[0005] Step S2: Generate hydrates in the core using the gas excess method, and obtain the hydrate occurrence state through CT scanning;

[0006] Step S3: establishing a digital core model based on the core pore throat structure and hydrate occurrence state, and applying an electrostatic field with specific boundary conditions in the numerical simulation software;

[0007] Step S4: changing the hydrate saturation parameter in the numerical simulation software, obtaining the model current by importing the voltage, and calculating the model resistivity;

[0008] Step S5: Calculating the critical water saturation of hydrate based on the model resistivity and the digital core model;

[0009] Step S6: using the critical water saturation as a boundary, using a power form function to fit the ratio resistivity calculation model before and after the critical water saturation.

[0010] In step S1, a suitable core is selected according to the parameters of the target hydrate reservoir formation, and basic geological parameters of the core are collected by CT scanning, including pore throat structure and rock porosity.

[0011] In step S2, hydrates are generated in the core using the gas excess method, and then hydrate occurrence status including hydrate saturation, hydrate occurrence location, hydrate particle morphology, and reservoir permeability are collected through CT scanning.

[0012] In step S3, the electrostatic field uses a plate electrode to apply an electric field along the longest side of the model, and the specific boundary conditions of the electrostatic field are: ① the left boundary potential is u0; ② the boundary potential is 0; ③ the remaining boundaries are insulating boundaries, the normal component of the current density is continuous, and the normal derivative of the potential multiplied by the conductivity is 0.

[0013] In step S4, by changing the hydrate saturation parameters in the numerical simulation software, under different hydrate saturation conditions, the model current is obtained by introducing a voltage, and the model resistivity is calculated; the low-frequency electric heating simulation method refers to the "Basics of Reservoir Seepage in Marine Natural Gas Hydrate Exploitation" edited by Wu Nengyou et al.

[0014] In step S5, as the hydrate gradually decomposes, there is a water saturation critical value S W0 , when the water saturation is lower than the critical value, some previously disconnected areas begin to connect, and in this process, the pore connectivity and pore structure change;

[0015] Assume that the side length of the porous medium model unit is L, the average radius of the rock particles constituting the porous medium is R, and the average side length of the hydrate particles in the CT scan is l. When the water saturation of the model reaches the critical value, the geometric condition is satisfied:

[0016] L=l+2R (1)

[0017] When the model water saturation is the critical value, the expressions of the model original porosity and water saturation are:

[0018]

[0019] Where V1 is the void volume of the model, V0 is the total volume of the unit cell, S W0 is the critical value of water saturation.

[0020] In step S6, since the hydrate distribution morphology changes during the hydrate decomposition process, a new resistivity calculation model is proposed to improve the fitting accuracy of the calculation model. A power form function is used on both sides of the critical hydrate saturation value to fit the model ratio resistivity calculation value. The model formula is as follows:

[0021]

[0022] Where a, n1, and n2 are all unknown coefficients, and their values ​​are related to the pore structure, hydrate location, and particle shape. The model must satisfy the following conditions:

[0023] Since the hydrate distribution morphology changes during the hydrate decomposition process, a new resistivity calculation model is proposed to improve the fitting accuracy of the calculation model. A power form function is used on both sides of the critical hydrate saturation value to fit the model ratio resistivity calculation value. The model formula is as follows:

[0024]

[0025] According to the resistivity values ​​obtained by simulation calculation under different hydrate water saturations, the model ratio resistivity calculation formula is fitted to obtain the values ​​of the undetermined coefficients a, n1, and n2, and complete the resistivity model construction.

[0026] The beneficial effects and advantages of the present invention are:

[0027] A microscopic numerical simulation method for the resistivity of three-dimensional porous media containing hydrates was established. Electric field simulations were conducted to calculate the specific resistivity of porous media containing hydrates with heterogeneous hydrate distribution. The effect of hydrate saturation on the resistivity of porous media was studied, and a new model for calculating the resistivity of hydrate reservoirs was established. Combined with in-situ CT experiments, simulations were performed to solve the unknown coefficients in the specific resistivity calculation formula for cores during the thermal decomposition of hydrates. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a flow chart of the method for constructing a resistivity model of porous media containing hydrates based on the consideration of heterogeneous distribution of hydrates;

[0029] Figure 2 It is a schematic diagram of the electric field applied to the porous medium model;

[0030] Figure 3 is the voltage field diagram at different moments during the thermal decomposition of hydrate; DETAILED DESCRIPTION

[0031] The present invention will be further described below with reference to the accompanying drawings, but the scope of implementation of the present invention is not limited thereto.

[0032] like Figure 1 As shown, the present invention proposes a method for constructing a resistivity model of a hydrate-containing porous medium taking into account the heterogeneous distribution of hydrates, comprising the following steps:

[0033] Step S1: Prepare a simulated core according to the target hydrate reservoir rock composition, and obtain the core pore throat structure through CT scanning;

[0034] Step S2: Generate hydrates in the core using the gas excess method, and obtain the hydrate occurrence state through CT scanning;

[0035] Step S3: establishing a digital core model based on the core pore throat structure and hydrate occurrence state, and applying an electrostatic field with specific boundary conditions in the numerical simulation software;

[0036] Step S4: changing the hydrate saturation parameter in the numerical simulation software, obtaining the model current by importing the voltage, and calculating the model resistivity;

[0037] Step S5: Calculating the critical water saturation of hydrate based on the model resistivity and the digital core model;

[0038] Step S6: using the critical water saturation as a boundary, using a power form function to fit the ratio resistivity calculation model before and after the critical water saturation.

[0039] In step S1, a suitable core is selected according to the parameters of the target hydrate reservoir formation, and basic geological parameters of the core are collected by CT scanning, including pore throat structure and rock porosity.

[0040] In step S2, a gas excess method is used to generate hydrates in the core, and then CT scanning is used to collect the hydrate occurrence state, hydrate saturation, hydrate particle morphology, and reservoir permeability.

[0041] In step S3, a digital core model of the porous medium in which hydrates are present is established based on the core pore throat structure and hydrate occurrence state obtained by CT scanning, and then the digital core model is substituted into an electrostatic field; the three-dimensional porous medium model uses plate electrodes to apply an electric field along the longest side of the model, and the electric field satisfies the following boundary conditions: ① the potential of the left boundary is u0; ② the potential of the boundary is 0; ③ the remaining boundaries are insulating boundaries, the normal component of the current density is continuous, and the normal derivative of the potential multiplied by the conductivity is 0.

[0042] In step S4, by changing the hydrate saturation parameters in the numerical simulation software, under different hydrate saturation conditions, the model current is obtained by introducing voltage, and the model resistivity is calculated; the low-frequency electric heating simulation method refers to "Basics of Reservoir Seepage in Marine Natural Gas Hydrate Exploitation" edited by Wu Nengyou et al.

[0043] In step S5, as the hydrate gradually decomposes, there is a water saturation critical value S W0 , when the water saturation is lower than the critical value, some previously disconnected areas begin to connect, and in this process, the pore connectivity and pore structure change;

[0044] Assume that the side length of the porous medium model unit is L, the average radius of the rock particles constituting the porous medium is R, and the average side length of the hydrate particles in the CT scan is l. When the water saturation of the model reaches the critical value, the geometric condition is satisfied:

[0045] L=l+2R (1)

[0046] When the model water saturation is the critical value, the expressions of the model original porosity and water saturation are:

[0047]

[0048] Where R is the radius of rock particles, l is the side length of hydrate particles, V1 is the void volume of the model, V0 is the total volume of the unit cell, S W0 is the critical value of water saturation.

[0049] In step S6, since the hydrate distribution morphology changes during the hydrate decomposition process, a new resistivity calculation model is proposed to improve the fitting accuracy of the calculation model. A power form function is used on both sides of the critical hydrate saturation value to fit the model ratio resistivity calculation value. The model formula is as follows:

[0050]

[0051] Where a, n1, and n2 are all unknown coefficients, and their values ​​are related to the pore structure, hydrate location, and particle shape. The model must satisfy the following conditions:

[0052] Since the hydrate distribution morphology changes during the hydrate decomposition process, a new resistivity calculation model is proposed to improve the fitting accuracy of the calculation model. A power form function is used on both sides of the critical hydrate saturation value to fit the model ratio resistivity calculation value. The model formula is as follows:

[0053]

[0054] Based on the geometric relationship between rock particles and hydrate particles in the porous media unit, and according to the definition of porosity and hydrate saturation, the critical water saturation formula is established:

[0055]

[0056] Where V2 is the volume of the water phase in the unit cell model under critical conditions, and V3 is the volume of the hydrate phase in the unit cell model under critical conditions.

[0057] In the 3D modeling software, a porous medium unit model in which hydrate particles are located in the pore center is established according to the critical conditions. The volume of hydrate under the critical conditions is calculated using the internal statistical tools of the 3D modeling software.

[0058] Based on the geometric relationship between hydrate particles and rock particles, the critical geometric conditions of porous media units with different hydrate occurrence locations and particle morphologies are obtained. Based on the critical geometric conditions, the relationship between the critical water saturation of different porous media and the initial porosity of the rock is derived.

[0059] The CT scanning data from the hydrate thermal decomposition experiment were segmented and processed to divide the rock, hydrate and pore phases; based on the phase information after data segmentation, the pore phase was extracted to establish a three-dimensional porous medium model; an electric field was applied to the porous medium model to carry out electric field numerical simulation; the experimental and simulation results were verified and fitted to obtain the values ​​of the unknown coefficients a, n1 and n2, and the fitting calculation formula.

[0060] The following specific embodiments are given to further illustrate the content of the present invention:

[0061] Example

[0062] Assuming the target hydrate reservoir is a core sample with a diameter of 6 mm and a length of 7 mm, the experiment used 99.99% pure methane gas and a 5.0% potassium iodide solution to generate natural gas hydrates in the core at a pressure of 9 MPa. The porous medium model consists of a uniformly distributed spherical particle array with 8 × 4 × 4 minimum units distributed within a 2000 μm × 1000 μm × 1000 μm calculation area. The ratio of the radius of each rock particle to the side length of the minimum unit is 0.34. The original porosity φ0 of the model, in the absence of hydrates, is 34.15%, roughly the same as that of the core.

[0063] It can be found that when the water saturation is less than 70%, the core pore simulation data is significantly higher than the particle coverage fitting curve. The experimental data is fitted into the form of Equation 5, and the fitting calculation formula is:

[0064]

[0065] The above embodiments are only used to illustrate the present invention, and any equivalent transformations and improvements based on the technical solutions of the present invention should not be excluded from the protection scope of the present invention.

Claims

1. A method for constructing a resistivity model of hydrate-containing porous media considering heterogeneous distribution of hydrates, characterized in that: The following steps are involved: Step S1: Prepare a simulated core according to the target hydrate reservoir rock composition, and obtain the core pore throat structure through CT scanning; Step S2: Generate hydrates in the core using the gas excess method, and obtain the hydrate occurrence state through CT scanning; Step S3: establishing a digital core model based on the core pore throat structure and hydrate occurrence state, and applying an electrostatic field with specific boundary conditions in the numerical simulation software; Step S4: changing the hydrate saturation parameter in the numerical simulation software, obtaining the model current by importing the voltage, and calculating the model resistivity; Step S5: Calculating the critical water saturation of hydrate based on the model resistivity and the digital core model; Step S6: using the critical water saturation as a boundary, using a power form function to fit the ratio resistivity calculation model before and after the critical water saturation.

2. The method for constructing a resistivity model of a hydrate-containing porous medium considering heterogeneous distribution of hydrates according to claim 1, characterized in that: In step S1, a suitable core is selected according to the target hydrate reservoir rock composition, and basic geological parameters of the core are collected by CT scanning, including pore throat structure and rock porosity.

3. The method for constructing a resistivity model of a hydrate-containing porous medium considering heterogeneous distribution of hydrates according to claim 1, characterized in that: In step S2, a gas excess method is used to generate hydrates in the core, and then CT scanning is used to collect the hydrate occurrence state, hydrate saturation, hydrate particle morphology, and reservoir permeability.

4. The method for constructing a resistivity model of a hydrate-containing porous medium considering heterogeneous distribution of hydrates according to claim 1, characterized in that: In step S3, the electrostatic field uses a plate electrode to apply an electric field along the longest side of the model, and the specific boundary conditions of the electrostatic field are: ① the left boundary potential is u0; ② the boundary potential is 0; ③ the remaining boundaries are insulating boundaries, the normal component of the current density is continuous, and the normal derivative of the potential multiplied by the conductivity is 0.

5. The method for constructing a resistivity model of a hydrate-containing porous medium considering heterogeneous distribution of hydrates according to claim 1, characterized in that: In step S4, by changing the hydrate saturation parameters in the numerical simulation software, under different hydrate saturation conditions, the model current is obtained by introducing voltage, and the model resistivity is calculated; the low-frequency electric heating simulation method refers to "Basics of Reservoir Seepage in Marine Natural Gas Hydrate Exploitation" edited by Wu Nengyou et al.

6. The method for constructing a resistivity model of a hydrate-containing porous medium considering heterogeneous distribution of hydrates according to claim 1, characterized in that: In step S5, as the hydrate gradually decomposes, there is a water saturation critical value S W0 , when the water saturation is lower than the critical value, some previously disconnected areas begin to connect, and in this process, the pore connectivity and pore structure change; Assume that the side length of the porous medium model unit is L, the average radius of the rock particles constituting the porous medium is R, and the average side length of the hydrate particles in the CT scan is l. When the water saturation of the model reaches the critical value, the geometric condition is satisfied: L=l+2R (1) When the model water saturation is the critical value, the expressions of the model original porosity and water saturation are: Where R is the radius of rock particles, l is the side length of hydrate particles, V1 is the void volume of the model, V0 is the total volume of the unit cell, S W0 is the critical value of water saturation.

7. The method for constructing a resistivity model of a hydrate-containing porous medium considering heterogeneous distribution of hydrates according to claim 1, characterized in that: In step S6, since the hydrate distribution morphology changes during the hydrate decomposition process, a new resistivity calculation model is proposed to improve the fitting accuracy of the calculation model. A power form function is used on both sides of the critical hydrate saturation value to fit the model ratio resistivity calculation value. The model formula is as follows: Where a, n1, and n2 are all unknown coefficients, and their values ​​are related to the pore structure, hydrate location, and particle shape. The model must satisfy the following conditions: According to the resistivity values ​​obtained by simulation calculation under different hydrate water saturations, the model ratio resistivity calculation formula is fitted to obtain the values ​​of the undetermined coefficients a, n1, and n2, and complete the resistivity model construction.

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