Shale conductivity and mechanical damage coupling correlation test method

Abstract

The invention belongs to the technical field of unconventional oil and gas development, relates to a shale conductivity and mechanical damage coupling correlation testing method, and aims to solve the problem that dynamic correlation of shale conductivity and mechanical damage in a complex reservoir environment cannot be accurately captured in the prior art. According to the method, synchronous monitoring of shale conductivity and mechanical damage under the combined action of hydration, temperature, confining pressure and stress loading is realized through standardized rock sample preparation, hydration treatment, multi-field parameter collaborative setting, synchronous test system building and whole-process data acquisition and analysis. According to the method, the corresponding rule of shale electrical characteristics and damage evolution under different development working conditions can be determined, a reliable experimental method support is provided for parameter optimization and reservoir stability monitoring of technologies such as underground dry distillation and hydraulic fracturing, and the method is suitable for development technology research of various shale gas reservoirs.

Description

Technical Field

[0001] This invention belongs to the field of unconventional oil and gas development technology, and in particular, it is a method for testing the coupling correlation between electrical conductivity and mechanical damage in shale. Background Technology

[0002] Shale gas, as a core representative of unconventional oil and gas resources, has become a key strategic resource for ensuring energy security in the global energy transition process due to its vast reserves and clean, environmentally friendly characteristics. my country possesses superior shale gas resources, widely distributed in regions such as the Sichuan Basin and the Ordos Basin, with enormous development potential. However, shale reservoirs have inherent geological characteristics of "low porosity, low permeability, and strong heterogeneity," which is drastically different from the "layered distribution of hydrocarbon generation, storage, and capping" structure of conventional oil and gas reservoirs. Shale gas's generation, storage, and capping functions are concentrated in the same stratum, which necessitates targeted reservoir stimulation technologies such as hydraulic fracturing and underground retorting for shale gas extraction.

[0003] Hydraulic fracturing is a fundamental modification method for shale gas extraction. It involves injecting fracturing fluid under high pressure to create an artificial network of fractures within the shale, thereby improving oil and gas flow channels. However, in actual engineering, fracturing fluid flowback rates are generally low, with large amounts of fluid remaining in the reservoir. Some gas wells even employ a "well-steaming" technique, intentionally prolonging the residence time of the fracturing fluid in the reservoir to further expand the fractures and enhance the fracturing effect by utilizing the hydration and expansion effect of clay minerals in the shale. Therefore, in actual development scenarios, shale reservoir rocks are in a state of hydration to varying degrees for extended periods.

[0004] Electrical conductivity, as a sensitive parameter characterizing the internal structure and damage state of shale, is closely related to hydration expansion, thermal stress, fracture development, and pore fluid distribution. Hydration alters the electrical conductivity of clay minerals, pyrolysis reconstructs pore electrical pathways, and stress loading affects fracture connectivity; all these processes are reflected in the dynamic changes of electrical conductivity. Therefore, electrical conductivity can serve as a "real-time probe" for monitoring shale conditions, providing crucial information for parameter optimization in technologies such as underground pyrolysis and hydraulic fracturing. Summary of the Invention

[0005] To address the aforementioned issues, this invention proposes a coupled correlation testing method for shale electrical conductivity and mechanical damage, enabling the monitoring of the electrical-mechanical properties of shale under complex working conditions.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for testing the coupled correlation between electrical conductivity and mechanical damage in shale, including... Shale sample preparation: Select shale cores from the target reservoir and process them into standard cylindrical samples; Shale hydration treatment: Shale samples in different hydration states were obtained by soaking them in different types of solutions or setting different hydration times. Multiple parameter settings: For different groups of rock samples, temperature gradient, confining pressure range and mechanical loading mode are set to construct a complex environment that simulates the actual reservoir; Synchronous testing system setup: Electrodes are placed on a standard cylindrical rock sample. Environmental conditions and mechanical loads are applied to the standard cylindrical rock sample. The instrument collects conductivity data in real time and records stress-strain curves synchronously. Coupled experiment implementation: Conduct several coupled experiments according to the set parameters, keep the environmental parameters stable, and collect data synchronously throughout the process; Data acquisition and damage analysis: Real-time recording of electrical conductivity, stress, strain, temperature and confining pressure data; quantification of shale mechanical damage degree after experiment using methods such as fracture observation and signal analysis.

[0007] As a preferred method, in the preparation of shale rock samples, standard cylindrical rock samples are de-cracked and impurities are removed, and after drying to constant weight, they are divided into a dried group, a naturally hydrated group, and a hydration-treated group.

[0008] As a preferred option, in the construction of the synchronous testing system, a four-electrode method is used to arrange high-temperature and corrosion-resistant electrodes, and environmental conditions and mechanical loads are applied through a high-temperature and high-pressure rock mechanics experimental system.

[0009] As a preferred option, in the shale hydration treatment, the hydration solution is selected from the target reservoir formation water or fracturing fluid used in fracturing engineering. The hydration time is controlled by the change in rock sample quality, and the quality is measured every 12 hours until the set water content is reached or the quality is stable.

[0010] Preferably, in the multi-field parameter settings, the temperature gradient is 25-300℃, the confining pressure range is 0-50MPa, and the mechanical loading modes include uniaxial compression, triaxial compression, and cyclic loading and unloading.

[0011] As a preferred option, in the construction of the synchronous testing system, the electrodes are made of platinum, and the contact points between the electrodes and the rock samples are filled with conductive paste to reduce contact resistance. The experimental system uses insulating materials to isolate the mechanical loading device from the electrodes to avoid electromagnetic interference.

[0012] Preferably, in the coupling experiment, the temperature is programmed to rise at a rate of 1-5℃ / min. After reaching the set temperature, the temperature is held constant for 30-60 minutes before mechanical loading is applied, with the loading rate controlled at 0.001-0.005mm / s.

[0013] As a preferred approach, in data acquisition and damage analysis, the characteristics of crack development are observed by scanning electron microscopy, the damage evolution process is analyzed by combining acoustic emission signals, and the degree of damage is quantified based on the magnitude of conductivity change and the characteristics of stress-strain curves.

[0014] The beneficial effects of using this invention are: A great deal of information can be obtained through this method: 1. This invention can reveal the correlation between electrical conductivity and mechanical damage in shale under different hydration states; 2. The present invention can obtain the dynamic response characteristics of shale electrical conductivity under different temperature-confining pressure coupling conditions; 3. This invention can obtain the correlation between the change in electrical conductivity of shale and fatigue damage during cyclic loading; 4. This invention can clarify the key factors affecting the correlation between shale electrical conductivity and damage, providing a basis for setting reservoir monitoring indicators.

[0015] The information obtained through this experimental method can describe in detail the electrical and mechanical properties of shale under complex environments. This information is crucial for evaluating the fracturing effect of shale gas reservoirs and optimizing underground pyrolysis technology. The experimental method provided in this invention can provide a theoretical and data basis for unconventional oil and gas development. Attached Figure Description

[0016] Figure 1 This is a flowchart of the coupling correlation test method for shale electrical conductivity and mechanical damage according to the present invention.

[0017] Figure 2 This is an experimental diagram showing the coupling of shale electrical conductivity and mechanical properties according to the present invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this technical solution clearer, the following detailed description, in conjunction with specific embodiments, further illustrates this technical solution. It should be understood that these descriptions are merely exemplary and not intended to limit the scope of this technical solution.

[0019] like Figure 1 As shown in this embodiment, a coupled experimental study method for the electrical conductivity and mechanical damage of shale under different states is proposed, which includes the following steps: Step 1: Process the mudstone and shale samples according to requirements; Step 2: Hydrate the shale. In this process, different solutions can be used for soaking to obtain the effect of different solutions on conductivity and mechanical damage; or different hydration times can be used to obtain the effect of the degree of hydration on conductivity. Step 3: Dry the shale. Drying is performed by placing the sample in a designated high-temperature oven at 105°C for 12 to 48 hours to ensure complete removal of moisture from the sample.

[0020] Step 4: Real-time data acquisition and recording, which can simultaneously obtain data on electrical conductivity, stress, strain, temperature and confining pressure; After the experiment, the mechanical damage state of shale is evaluated through methods such as fracture observation and acoustic emission signal analysis.

[0021] Specifically, in step 1, the sample is processed according to the high temperature and high pressure rock mechanics test standard: Longmaxi Formation shale core is selected and processed into a cylindrical rock sample with a diameter of 25mm and a height of 50mm using a diamond cutting machine. The end face flatness error is ≤0.02mm by sanding.

[0022] In step 2, there are two controllable factors: the type of hydration solution and the hydration time. The hydration solution can be water or the fracturing fluid used in hydraulic fracturing of the studied formation, to simulate the underground rock conditions. The hydration time is a parameter reflecting the degree of hydration of the shale. By changing the hydration time, electrical conductivity and mechanical damage experiments can be conducted on shale with different degrees of hydration. Alternatively, rock samples can be hydrated according to the degree of hydration of the formation rocks.

[0023] In step 3, the drying time needs to be adjusted to match the experimental requirements: For rock samples that need to be completely dried, place them in a 105℃ oven for 48 hours, and weigh them every 12 hours until the difference between two consecutive weighings is ≤0.01g, which is considered as complete removal of moisture; For rock samples that need to retain some moisture, the drying time can be shortened to 12-24 hours, and the moisture content can be calculated by weighing to accurately control the degree of drying.

[0024] In step 4, the electrical conductivity of the shale is precisely measured using the four-electrode method (or two-electrode method) throughout the experiment. During the test, a small current is applied through the electrodes and the voltage difference is measured to calculate the conductivity.

[0025] Example 1

[0026] The selected sample was a shale from Sichuan, which was processed into three core columns, each 50 mm high and 25 mm in diameter.

[0027] Experimental procedure: 1. Take 3 intact rock samples with no visible cracks on the surface, measure their size and weight, and number them S1-S3.

[0028] 2. Sample S1 was subjected to hydration treatment, S2 was in its natural state, and S3 was in its dry state.

[0029] 3. Place the hydrated sample into a uniaxial / triaxial compressor and simultaneously measure the shale resistivity. Place the natural sample into a uniaxial / triaxial compressor and simultaneously measure the shale resistivity. Place the dried sample into a uniaxial / triaxial compressor and simultaneously measure the shale resistivity.

[0030] 4. The computer synchronously records the resistance data, and subsequently calculates and analyzes the conductivity value based on the resistance value. Experimental results: like Figure 2As shown, observations of shale samples after electrical conductivity and mechanical tests revealed that sample S1 exhibited obvious cracks on its surface, a significant increase in conductivity, and large fluctuations in conductivity under high stress conditions, indicating significant brittleness. Hydration enhanced crack development, leading to changes in the conductive path. Sample S2 showed no obvious cracks on its surface, had low conductivity, and maintained stable mechanical properties without significant changes. Sample S3 showed a few cracks on its surface, low conductivity, and small fluctuations in conductivity during compressive loading, indicating relatively hard mechanical properties and slow crack propagation.

[0031] The experimental results and graphical data show that hydration treatment significantly improves the electrical conductivity of shale, especially under high stress conditions. Compared with dry samples, hydrated and natural samples exhibit significant differences in mechanical properties and changes in electrical conductivity. Hydration significantly alters the electrical conductivity pathway by increasing porosity and fracture development, thereby enhancing the responsiveness of electrical conductivity. This provides important data support for the coupled study of the electrical and mechanical properties of shale.

[0032] The above content is only a preferred embodiment of the present invention. For those skilled in the art, many changes can be made in the specific implementation and application scope based on the ideas of the present invention. As long as these changes do not depart from the concept of the present invention, they all fall within the protection scope of this patent.

Claims

1. A method for testing the coupled correlation between electrical conductivity and mechanical damage in shale, characterized in that: include Shale sample preparation: Select shale cores from the target reservoir and process them into standard cylindrical samples; Shale hydration treatment: Shale samples in different hydration states were obtained by soaking them in different types of solutions or setting different hydration times. Multiple parameter settings: For different groups of rock samples, temperature gradient, confining pressure range and mechanical loading mode are set to construct a complex environment that simulates the actual reservoir; Synchronous testing system setup: Electrodes are arranged on a standard cylindrical rock sample. Environmental conditions and mechanical loads are applied to the standard cylindrical rock sample. The instrument collects conductivity data in real time and records stress-strain curves synchronously. Coupled experiment implementation: Conduct several coupled experiments according to the set parameters, keep the environmental parameters stable, and collect data synchronously throughout the process; Data acquisition and damage analysis: Real-time recording of electrical conductivity, stress, strain, temperature and confining pressure data; quantification of shale mechanical damage degree after experiment using methods such as fracture observation and signal analysis.

2. The method for testing the coupling correlation between electrical conductivity and mechanical damage in shale according to claim 1, characterized in that: In the preparation of shale rock samples, standard cylindrical rock samples are de-fired and impurities are removed from their surface. After drying to constant weight, they are divided into a dried group, a naturally hydrated group, and a hydration-treated group.

3. The method for testing the coupling correlation between electrical conductivity and mechanical damage in shale according to claim 1, characterized in that: In the construction of the synchronous testing system, a four-electrode method was used to arrange high-temperature and corrosion-resistant electrodes, and environmental conditions and mechanical loads were applied through a high-temperature and high-pressure rock mechanics experimental system.

4. The method for testing the coupling correlation between shale electrical conductivity and mechanical damage according to claim 1, characterized in that: In shale hydration treatment, the hydration solution is selected from the target reservoir formation water or fracturing fluid used in fracturing engineering. The hydration time is controlled by the change in rock sample quality, and the quality is measured every 12 hours until the set water content is reached or the quality is stable.

5. The method for testing the coupling correlation between electrical conductivity and mechanical damage in shale according to claim 1, characterized in that: In the multi-field parameter settings, the temperature gradient is 25-300℃, the confining pressure range is 0-50MPa, and the mechanical loading modes include uniaxial compression, triaxial compression, and cyclic loading and unloading.

6. The method for testing the coupling correlation between electrical conductivity and mechanical damage in shale according to claim 1, characterized in that: In the construction of the synchronous testing system, the electrodes are made of platinum, and conductive paste is filled at the contact point between the electrodes and the rock sample to reduce the contact resistance. The experimental system uses insulating materials to isolate the mechanical loading device from the electrodes to avoid electromagnetic interference.

7. The method for testing the coupling correlation between electrical conductivity and mechanical damage in shale according to claim 1, characterized in that: In the coupling experiment, the temperature was programmed to rise at a rate of 1-5℃ / min. After reaching the set temperature, the temperature was held for 30-60 minutes before mechanical loading was applied, with the loading rate controlled at 0.001-0.005mm / s.

8. The method for testing the coupling correlation between electrical conductivity and mechanical damage in shale according to claim 1, characterized in that: In data acquisition and damage analysis, the characteristics of crack development are observed by scanning electron microscopy, and the damage evolution process is analyzed by combining acoustic emission signals. The degree of damage is quantified based on the magnitude of conductivity change and the characteristics of stress-strain curves.

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