A method for microcosmic evaluation of preservation conditions of shallow shale gas

Abstract

The application discloses a kind of shallow shale gas preservation condition microcosmic evaluation method, the method obtains blocky rock sample from study target layer, and carries out equal division, and is prepared into the sample of different water saturation;Through the results of several groups of different temperature and saturation conditions under the methane high-pressure adsorption experiment, and interpolation, the methane theoretical maximum adsorption amount under the condition of any formation temperature, water saturation and pressure can be obtained.Then compare with the gas content of present target layer, finally evaluate the preservation condition of the target reservoir on site.The application has a positive effect on shale gas production planning and reservoir evaluation.

Description

Technical Field

[0001] This invention relates to a microscopic evaluation method for shallow shale gas preservation conditions, belonging to the technical field of oil and gas field development engineering research. Background Technology

[0002] Shale gas is self-generated and self-storing, and its preservation conditions are crucial parameters for shale gas exploration and development, possessing significant engineering and scientific importance. The Zhaotong shale gas reservoir, represented by shallow shale gas, is shallowly buried (down to 300m), has well-developed faults (including the Tongtian Fault), and is close to erosion zones. Gas may slowly dissipate over long geological periods. Reservoirs with large amounts of dissipated gas have poor development potential, generally low reserve abundance, and poor preservation conditions. Conversely, reservoirs with small amounts of dissipated gas have high gas content, good preservation conditions, and development potential. For shallow shale gas development, in addition to conventional parameters such as gas content and reservoir pressure, it is essential to focus on the crucial parameter of reservoir preservation conditions. Especially in the complex systems of shallow shale gas with complex fault and erosion zone coupling distributions, the ability to quickly evaluate preservation conditions and locate reservoirs with favorable preservation conditions is of great significance for shale gas development.

[0003] Current methods for assessing preservation conditions primarily involve a comprehensive evaluation combining geological structure, evolutionary history, and field exploration data. However, these data often exhibit multiple interpretations, uncertainties, and are on a massive scale. For evaluating preservation conditions within the complex fracture networks and erosion systems of shallow shale gas, achieving the precision required for engineering projects is often challenging. Therefore, based on laboratory experiments and field engineering data, this paper proposes a microscopic evaluation method for shallow shale gas preservation conditions, which is of significant importance for shallow shale gas development. Summary of the Invention

[0004] The purpose of this invention is to provide a microscopic evaluation method for shallow shale gas preservation conditions. This method can start from indoor core testing, use a series of indoor experiments and calculations to quickly obtain the theoretical maximum gas content, and then compare it with the current gas content measured in the field to finally evaluate the preservation conditions of the target reservoir in the field.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] This invention provides a microscopic evaluation method for shallow shale gas preservation conditions, comprising:

[0007] Obtain massive rock samples from the target layer;

[0008] The blocky rock sample was divided into equal parts to prepare samples with different water saturation levels;

[0009] High-pressure methane adsorption experiments were conducted on the samples with different water saturation at different temperatures to obtain the absolute methane adsorption curves at different temperatures and water saturation levels.

[0010] Interpolate the absolute methane adsorption curves at different temperatures and water saturation levels to obtain absolute methane adsorption curves at arbitrary temperatures and water saturation levels.

[0011] Based on the current methane absolute adsorption curve corresponding to the current internal temperature and water saturation of the target formation in the well, as well as the current reservoir pressure and gas content of the target formation in the well, calculate the preservation condition influence coefficient η1 under static conditions and the preservation condition influence coefficient η2 under dynamic conditions.

[0012] The preservation conditions of shallow shale gas are evaluated based on η1 and η2.

[0013] Furthermore, the blocky rock sample is divided into equal parts to prepare samples with different water saturation levels, including:

[0014] The blocky rock sample was divided into n equal parts according to mass, and numbered from 1 to n.

[0015] Each rock sample was soaked in deionized water and then placed in a vacuum saturation device for 24 hours to saturate it with water.

[0016] After the rock samples were saturated with water, they were placed in an oven and heated. One rock sample was taken out and sealed at set intervals until all n samples were taken out.

[0017] Furthermore, the heating temperature of the oven is 60°C;

[0018] The heating time in the oven for the rock sample was set as follows:

[0019] The heating time for the last rock sample should be no less than 48 hours.

[0020] Furthermore, high-pressure methane adsorption experiments were conducted on the samples with different water saturation levels at different temperatures to obtain absolute methane adsorption curves at different temperatures and water saturation levels, including:

[0021] Each rock sample after saturated water influent treatment was divided into two parts. The first part of the sample was subjected to a saturation experiment to determine the water saturation of the rock, denoted as S1, S2, ... S1. n The second sample was ground into powder and then divided into m portions, numbered 1 to m.

[0022] Within the temperature range of interest for the target layer in the study, m temperature measurement points T1, T2…T are selected. m High-pressure methane adsorption experiments were conducted on m powder samples at the m temperature measuring points, and the absolute methane adsorption curves at the corresponding temperatures and water saturation levels were obtained by conversion.

[0023] The above treatment was performed on all samples with water saturation to obtain absolute methane adsorption curves at different temperatures and water saturation levels.

[0024] Furthermore, the saturation experiment to determine the water saturation of the rock includes:

[0025] Weigh the wet sample, measure the rock volume and porosity, and obtain the rock pore volume;

[0026] After drying the wet sample, calculate its mass. Divide the difference between the two mass measurements by the rock pore volume to obtain the water saturation.

[0027] The drying conditions for the wet sample were: drying at 105 degrees Celsius for more than 48 hours.

[0028] Furthermore, the particle size of the powder is 20-40 mesh.

[0029] Furthermore, the method for selecting the temperature measuring points is as follows:

[0030] Within the temperature range of interest in the target layer, m temperature measurement points are selected at equal or unequal intervals.

[0031] Furthermore, the calculation of the preservation condition influence coefficient η1 under static conditions includes:

[0032] Based on well logging or field data: the current temperature (Tt), water saturation (St), reservoir pressure (Pt), and gas content (At) of the target reservoir are obtained downhole.

[0033] From the interpolated absolute methane adsorption curve, find the absolute methane adsorption curve A = f(P,Tt,St) under conditions Tt and St. Calculate the storage condition influence coefficient η1 under static conditions, which is:

[0034]

[0035] Where P represents reservoir pressure, A is absolute methane adsorption, f is the functional relationship between A and P, M1 is the point on the A=f(P,Tt,St) curve corresponding to pressure value Pt, M3 is the point in a coordinate system with energy storage pressure as the abscissa and downhole gas content as the ordinate, with abscissa Pt and ordinate At, M5 is the abscissa position corresponding to pressure value Pt, L(M1M5) is the length from point M1 to M5, and L(M3M5) is the length from point M3 to M5.

[0036] Furthermore, the influence coefficient η2 of the storage conditions under the dynamic conditions is calculated as follows:

[0037] Based on production or well test data, after a period of time, if the reservoir pressure decreases from Pt to Pti and the gas content decreases from At to Ati, then the retention condition influence coefficient η2 under dynamic conditions is calculated as follows:

[0038]

[0039] Where M2 is the point on the curve A=f(P,Tt,St) where the pressure value is Pti; M4 is the point in a coordinate system with energy storage pressure as the abscissa and downhole gas content as the ordinate, where the abscissa is Pti and the ordinate is Ati; M6 is the abscissa position corresponding to the pressure value Pti; S(M3M4M6M5) is the area of ​​the figure formed by points M3, M4, M6 and M5; and S(M1M2M6M5) is the area of ​​the figure formed by points M1, M2, M6 and M5.

[0040] Furthermore, the evaluation of shallow shale gas preservation conditions based on η1 and η2 includes:

[0041] The closer η1 is to 1, the better the preservation conditions under static conditions; the closer it is to 0, the worse the preservation conditions under static conditions. The closer η2 is to 1, the better the preservation conditions under dynamic conditions; the closer it is to 0, the worse the preservation conditions under dynamic conditions.

[0042] The beneficial effects of the technical solution of this invention are as follows:

[0043] This invention, through high-pressure methane adsorption experiments under several different temperatures and saturation conditions, can determine the theoretical maximum methane adsorption capacity under arbitrary formation temperature, saturation, and pressure conditions, and further assess the current reservoir preservation conditions. Simultaneously, this invention provides a quantitative evaluation of the impact of preservation conditions on the dynamic processes of shale gas production during development, thus contributing positively to shale gas production planning and reservoir evaluation. The method of this invention features simple and easy-to-understand experimental steps, convenient operation, low testing cost, and fast testing speed. Attached Figure Description

[0044] Figure 1 A flowchart of a method for microscopic evaluation of shallow shale gas preservation conditions provided in an embodiment of the present invention;

[0045] Figure 2 This is a schematic diagram of the absolute methane adsorption curve constructed in the embodiments of the present invention. Detailed Implementation

[0046] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The embodiments described below with reference to the accompanying drawings are illustrative and intended to explain the present invention, and should not be construed as limiting the present invention.

[0047] It should be noted that, as used in this invention, "an embodiment" or "embodiment" refers to a specific feature, structure, or characteristic that can be included in at least one implementation of this invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that excludes other embodiments.

[0048] One embodiment of the present invention provides a microscopic evaluation method for shallow shale gas preservation conditions, see [link to relevant documentation]. Figure 1 This includes the following steps:

[0049] (1) For the target layer of the study, blocky rock samples were selected for testing.

[0050] (2) Divide the selected blocky rock samples into n parts according to their mass, number them 1 to n, and prepare them with different water saturation levels.

[0051] A preferred method is to soak each rock sample in deionized water and then place it in a vacuum saturation device for 24 hours to saturate it with water. Then, place it in an oven for heating. Take out one sample at a set time interval and seal it until all n samples are taken out.

[0052] In the above implementation method, the preferred temperature is 60°C.

[0053] It should be noted that the heating time for the last rock sample should be no less than 48 hours.

[0054] (3) Divide each of the n rock samples after saturated water treatment into two parts. Perform a saturation experiment on the first part of the sample to determine the water saturation of the rock, denoted as S1, S2, ... S1. n The second sample was ground into powder and then divided into m portions, numbered 1 to m.

[0055] One preferred method for measuring water saturation is as follows: First, weigh the wet sample. Then, measure the rock volume and porosity to obtain the rock pore volume. Finally, dry the wet sample and calculate its mass. Divide the difference between the two mass measurements by the pore volume to obtain the water saturation.

[0056] The preferred drying conditions for wet samples are: drying at 105 degrees Celsius for more than 48 hours.

[0057] One preferred sample grinding powder particle size is 20-40 mesh.

[0058] (4) Based on the scope of interest in the research target area, select m temperature measurement points T1, T2…T m High-pressure methane adsorption experiments were conducted on all powder samples at various temperature measurement points.

[0059] For example, for sample number n, its water saturation was measured to be S in step (3). n The powder samples, divided into 1 to m portions, were placed at the aforementioned locations T1, T2…T… m High-pressure methane adsorption experiments were conducted under the condition of temperature measurement points, and then the absolute methane adsorption amount curve was obtained after conversion, denoted as A. n,i =f n,i (P,T n,i ,S n,i ); where A n,i This represents the methane adsorption capacity of the i-th powder sample in sample n, where P is the pressure and T is the value of T. n,i S represents the temperature at the measuring point of the i-th powder sample of the n-th sample. n,i f represents the water saturation of the i-th powder sample of the n-th sample. n,i For A n,i The functional relationship between P and P.

[0060] It should be noted that the temperature measurement points are selected as follows: within the temperature range of interest in the target area, temperature values ​​are taken at equal or unequal intervals, but they need to be distributed within the temperature range of interest in the target area.

[0061] It should be noted that the methane absolute adsorption capacity curve obtained after conversion in the high-pressure methane adsorption experiment is achieved using techniques known to those skilled in the art. The process is as follows: The methane adsorption capacity curve directly obtained from the experiment is the relationship between the excess adsorption capacity Vex and the pressure P. It needs to be fitted according to the Langmuir adsorption formula (Vex=Vl*P / (Pl+P)*(1-ρfree / ρads)), where Vex is the measured excess adsorption capacity, P is the pressure corresponding to Vex, ρfree is the methane free phase density, and ρads is the methane adsorbed phase density. ρfree and ρads are obtained by looking up a table. The Langmuir pressure Pl and Langmuir volume Vl are obtained by fitting the above relationship. The absolute adsorption capacity Vabs is calculated using the following formula: Vabs=Vl*P / (Pl+P). Finally, the measured excess adsorption capacity Vex is converted into the absolute adsorption capacity Vabs, and the relationship curve between the absolute adsorption capacity Vabs and the pressure P is obtained.

[0062] (5) Interpolate the absolute methane adsorption curve obtained in step (4) to obtain the absolute methane adsorption curve at any temperature and water saturation.

[0063] The absolute adsorption capacity represents the theoretical maximum value of methane adsorption in the core under these conditions.

[0064] The interpolation operation is as follows: As in the experiment above, the sample is divided into m parts within the range of minimum and maximum temperature, and into n parts within the range of water saturation. Any sample within this range has its corresponding absolute methane adsorption curve A. i,j =f i,j (P,T i,j ,S i,j When the saturation and temperature values ​​are between two measuring points, the interpolation method can be used to obtain the interpolated absolute methane adsorption curve. When the saturation and temperature values ​​are outside the boundary measuring points, the extrapolation method can be used to obtain the absolute methane adsorption curve. Among the interpolation methods, there are linear interpolation, quadratic function interpolation, cubic function interpolation, Hermite interpolation, etc.

[0065] (6) Based on well logging or other field data: obtain the current temperature Tt, water saturation St, reservoir pressure Pt, and downhole gas content At within the well.

[0066] From the interpolated methane absolute adsorption curves described above, find the methane absolute adsorption curve A = f(P,Tt,St) under the conditions of Tt and St, as follows: Figure 2 As shown, the influence coefficient η1 of the storage conditions under static conditions is calculated as follows:

[0067]

[0068] Where M1 is the point on the curve A=f(P,Tt,St) where the pressure value is Pt, M3 is the point in a coordinate system with energy storage pressure as the abscissa and downhole gas content as the ordinate, where the abscissa is Pt and the ordinate is At, M5 is the abscissa position corresponding to the pressure value Pt, L(M1M5) is the length from point M1 to M5, and L(M3M5) is the length from point M3 to M5.

[0069] (7) Based on production or well test data, if the gas content decreases from At to Ati when the reservoir pressure decreases from Pt to Pti, then the retention condition influence coefficient η2 under dynamic conditions is:

[0070]

[0071] Where M2 is the point on the curve A=f(P,Tt,St) where the pressure value is Pti; M4 is the point in a coordinate system with energy storage pressure as the abscissa and downhole gas content as the ordinate, where the abscissa is Pti and the ordinate is Ati; M6 is the abscissa position corresponding to the pressure value Pti; S(M3M4M6M5) is the area of ​​the figure formed by points M3, M4, M6 and M5; and S(M1M2M6M5) is the area of ​​the figure formed by points M1, M2, M6 and M5.

[0072] It should be noted that Pti is the reservoir pressure obtained from production data or well test interpretation after a period of time, and Ati is the gas content obtained from production data or well test interpretation.

[0073] (8) Determine the storage conditions based on η1 and η2.

[0074] The closer η1 is to 1, the better the preservation conditions under static conditions; the closer it is to 0, the worse the preservation conditions under static conditions. The closer η2 is to 1, the better the preservation conditions under dynamic conditions; the closer it is to 0, the worse the preservation conditions under dynamic conditions.

[0075] This allows for the quantitative characterization of the reservoir's preservation conditions and the determination of whether the reservoir's preservation conditions are favorable, providing guidance and assistance for scientific production planning.

[0076] Because the methane adsorption capacity of shale is affected by multiple parameters such as temperature, pressure, and water saturation, current tests typically only measure the methane adsorption curve under a single temperature condition, without considering the influence of water content at different temperatures on methane adsorption. Therefore, this invention, for the target layer sample, creates methane adsorption curves at different temperatures and water saturations, and converts the excess adsorption curves into absolute adsorption curves. Further, interpolation is used to obtain the absolute methane adsorption curves under arbitrary temperature and water saturation conditions, thereby calculating the theoretical maximum methane content of the sample. Then, based on field data, the current methane content under the same temperature and water saturation conditions is obtained. If the current methane content is not far from the theoretical maximum methane content, the preservation condition coefficient is close to 1, indicating good preservation conditions; conversely, if the current methane content is much lower than the theoretical maximum methane content, it indicates that the reservoir may not have generated enough hydrocarbons, failing to reach the theoretical maximum methane content, or that the maximum methane content was originally reached but dissipated due to poor preservation conditions. However, since this problem involves the same reservoir with essentially the same hydrocarbon generation process, it's impossible for some shallow shale gas formations in near-surface complex fault fractures and erosion zones to generate hydrocarbons while others don't. The only possibility is poor preservation conditions leading to hydrocarbon escape. This preservation condition coefficient can be used to determine whether the preservation conditions of the target layer are good.

[0077] Furthermore, during the production process, the gas content and pressure of the reservoir decrease, and the theoretical maximum gas content of the reservoir also decreases. Therefore, preservation conditions also affect dynamic production. In this embodiment of the invention, a preservation condition influence coefficient η2 under dynamic conditions is further constructed to quantitatively reflect the magnitude of the influence of preservation conditions on the production process during development. If η2 is closer to 1, the influence of preservation conditions on the production process is small, which is beneficial to development; if η2 is closer to 0, the influence of preservation conditions on the production process is large, which is detrimental to development.

[0078] Combining η1 and η2 allows for a quantitative evaluation of shallow shale gas preservation conditions and their impact on shale gas production.

[0079] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A microscopic evaluation method for shallow shale gas preservation conditions, characterized in that, include: Obtain massive rock samples from the target layer; The blocky rock sample was divided into equal parts to prepare samples with different water saturation levels; High-pressure methane adsorption experiments were conducted on the samples with different water saturation at different temperatures to obtain the absolute methane adsorption curves at different temperatures and water saturation levels. Interpolate the absolute methane adsorption curves at different temperatures and water saturation levels to obtain absolute methane adsorption curves at arbitrary temperatures and water saturation levels. Based on the current methane absolute adsorption curve corresponding to the current internal temperature and water saturation of the target formation in the well, as well as the current reservoir pressure and gas content of the target formation in the well, calculate the preservation condition influence coefficient η1 under static conditions and the preservation condition influence coefficient η2 under dynamic conditions. The preservation conditions of shallow shale gas are evaluated based on η1 and η2.

2. The method for microscopic evaluation of shallow shale gas preservation conditions according to claim 1, characterized in that, The blocky rock sample was divided into equal portions to prepare samples with different water saturation levels, including: The blocky rock sample was divided into n equal parts according to mass, and numbered from 1 to n. Each rock sample was soaked in deionized water and then placed in a vacuum saturation device for 24 hours to saturate it with water. After the rock samples were saturated with water, they were placed in an oven and heated. One rock sample was taken out and sealed at set intervals until all n samples were taken out.

3. The method for microscopic evaluation of shallow shale gas preservation conditions according to claim 2, characterized in that, The heating temperature of the oven is 60°C; The heating time in the oven for the rock sample was set as follows: The heating time for the last rock sample should be no less than 48 hours.

4. The method for microscopic evaluation of shallow shale gas preservation conditions according to claim 2, characterized in that, High-pressure methane adsorption experiments were conducted on the samples with different water saturation levels at different temperatures to obtain absolute methane adsorption curves at different temperatures and water saturation levels, including: Each rock sample after saturated water influent treatment was divided into two parts. The first part of the sample was subjected to a saturation experiment to determine the water saturation of the rock, denoted as S1, S2, ... S1. n The second sample was ground into powder and then divided into m portions, numbered 1 to m. Within the temperature range of interest for the target layer in the study, m temperature measurement points T1, T2…T are selected. m High-pressure methane adsorption experiments were conducted on m powder samples at the m temperature measuring points, and the absolute methane adsorption curves at the corresponding temperatures and water saturation levels were obtained by conversion. The above treatment was performed on all samples with water saturation to obtain absolute methane adsorption curves at different temperatures and water saturation levels.

5. The method for microscopic evaluation of shallow shale gas preservation conditions according to claim 4, characterized in that, The process of conducting a saturation experiment to determine the water saturation of the rock includes: Weigh the wet sample, measure the rock volume and porosity, and obtain the rock pore volume; After drying the wet sample, calculate its mass. Divide the difference between the two mass measurements by the rock pore volume to obtain the water saturation. The drying conditions for the wet sample were: drying at 105 degrees Celsius for more than 48 hours.

6. The method for microscopic evaluation of shallow shale gas preservation conditions according to claim 4, characterized in that, The particle size of the powder is 20-40 mesh.

7. The method for microscopic evaluation of shallow shale gas preservation conditions according to claim 4, characterized in that, The method for selecting the temperature measuring points is as follows: Within the temperature range of interest in the target layer, m temperature measurement points are selected at equal or unequal intervals.

8. The method for microscopic evaluation of shallow shale gas preservation conditions according to claim 4, characterized in that, The calculation of the preservation condition influence coefficient η1 under static conditions includes: Based on well logging or field data: the current temperature (Tt), water saturation (St), reservoir pressure (Pt), and gas content (At) of the target reservoir are obtained downhole. From the interpolated absolute methane adsorption curve, find the absolute methane adsorption curve A = f(P,Tt,St) under conditions Tt and St. Calculate the storage condition influence coefficient η1 under static conditions, which is: Where P represents reservoir pressure, A is absolute methane adsorption, f is the functional relationship between A and P, M1 is the point on the A=f(P,Tt,St) curve corresponding to pressure value Pt, M3 is the point in a coordinate system with energy storage pressure as the abscissa and downhole gas content as the ordinate, with abscissa Pt and ordinate At, M5 is the abscissa position corresponding to pressure value Pt, L(M1M5) is the length from point M1 to M5, and L(M3M5) is the length from point M3 to M5.

9. A microscopic evaluation method for shallow shale gas preservation conditions according to claim 8, characterized in that, The influence coefficient η2 of the storage conditions under the dynamic conditions is calculated as follows: Based on production or well test data, after a period of time, if the reservoir pressure decreases from Pt to Pti and the gas content decreases from At to Ati, then the retention condition influence coefficient η2 under dynamic conditions is calculated as follows: Where M2 is the point on the curve A=f(P,Tt,St) where the pressure value is Pti; M4 is the point in a coordinate system with energy storage pressure as the abscissa and downhole gas content as the ordinate, where the abscissa is Pti and the ordinate is Ati; M6 is the abscissa position corresponding to the pressure value Pti; S(M3M4M6M5) is the area of ​​the figure formed by points M3, M4, M6 and M5; and S(M1M2M6M5) is the area of ​​the figure formed by points M1, M2, M6 and M5.

10. A microscopic evaluation method for shallow shale gas preservation conditions according to claim 9, characterized in that, The evaluation of shallow shale gas preservation conditions based on η1 and η2 includes: The closer η1 is to 1, the better the preservation conditions under static conditions; the closer it is to 0, the worse the preservation conditions under static conditions. The closer η2 is to 1, the better the preservation conditions under dynamic conditions; the closer it is to 0, the worse the preservation conditions under dynamic conditions.

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