Measurement of granite resistivity under deep underground conditions based on van der Pauw method

With the growing demand for sustainable energy and infrastructure development in the country, the utilization of deep geothermal resources and the development of deep underground space engineering are becoming more and more important. Under the background that the country vigorously promotes the development of deep rock and soil underground space and the utilization of resources. The experiment will be performed on granite, one of the deep subsurface rocks, a rock type widely used in geotechnical research projects because of its high strength, low permeability and thermal stability. However, its properties under the extreme temperature and chemical conditions deep in the earth are unknown, and how to efficiently convert deep rocks into geothermal resources for utilization is still unknown. Therefore, studying the influence of temperature and chemical corrosion on the resistivity of granite is very important to ensure the safety and durability of underground structures and effectively utilize thermal energy resources in deep underground. This experiment will control the influence of temperature, pressure, chemical corrosion and the comprehensive effect of the three on the resistivity of granite, and indirectly measure the amount of heat absorbed in the granite rock mass. Simulate the deep underground granite under high confining pressure, high ground temperature and corrosion conditions the nature of the performance. Record, count and analyze the data, comprehensively judge the environmental interference of the granite rock formation under actual conditions and compare the destructive effects of temperature and chemical corrosion and the change of the resistivity of the rock mass. Help scientifically judge and accurately analyze the underground construction problems that may be encountered in future projects and the effective utilization of geothermal energy and provide a basis for the application of key technologies and methods.


Introduction
With the acceleration of urbanization, the regional population is saturated, and the ground surface buildings are more concentrated.The problem of traffic congestion is becoming more and more serious, the surface environment is greatly affected, and the development and utilization value of deep underground spaces are highly valued.Deep underground projects including geothermal resource utilization, tunnelling, and power generation are becoming more and more important.Important, for example: during the excavation of the Sichuan-Tibet tunnel, the rock stability engineering problems encountered in the Himalayan structural section and the construction of the world's largest deep-buried water diversion tunnel project during the construction of Jinping Power Station [1][2][3][4].At the same time, the development of deep underground space is also an important way for my country to build a resourcesaving and environment-friendly society [5,6].In the construction of deep rock and soil, many factors such as poor rock properties will cause certain risk factors in the actual project.Granite is a widely used material in architecture and engineering because of its excellent physical and mechanical properties, such as high compressive strength, low porosity, and weather resistance [7].In recent years, there has been increasing interest in exploring the potential of deep subsurface granite for the geological storage of hazardous waste, extraction of geothermal energy, and carbon capture and storage.However, the resistivity properties of deep subsurface granites under temperature and chemical attack are unknown, and their performance in these environments is a key factor in evaluating their suitability for these applications.Deep subsurface granite is subjected to extremes of temperature and pressure, and its resistivity properties can be affected by factors such as the presence of caustics, pH, temperature, and pressure that cause changes in the resistivity of the granite.Understanding the impact of these factors on the resistivity properties of deep subsurface granite is critical for predicting its long-term performance and designing effective utilization strategies.This paper aims to review and summarize the research status of the resistivity characteristics of deep underground granite under the action of temperature and chemical corrosion.Specifically, this study will discuss the experimental methods used to measure the resistivity properties of deep subsurface granites and the factors affecting the resistivity of deep subsurface granites.The paper also discusses the implications of these findings for the design and maintenance of deep subsurface granite structures in the context of geothermal energy extraction [6].

Measuring resistivity by the van der Pauw method
The traditional methods for measuring rock resistivity mainly include the two-electrode method and the four-electrode method.Although the two-electrode method is simple to operate, due to the contact resistance, the measurement accuracy is low, the result is too large, and there is polarization [8].It was first proposed by Valdes in 1954 to be applied to the measurement of semiconductor resistivity.Although the four-electrode method can eliminate the influence of contact resistance and improve certain measurement defects of the two-electrode method, it also has problems in terms of accuracy and stability [9].The van der Pauw method was chosen as the measurement method of the experimental resistivity under the purpose of integrating various experimental conditions and the accuracy of the experimental results.The van der Pauw method is widely used to measure the resistivity of four electrodes placed on the edge of an isotropic thin sample.This method effectively solves the influence of the traditional two-phase electrode method on the contact resistance and breaks through the fourelectrode method on the shape of the sample.limitations of [10,11].Some researchers applied the test principle of the Vanderbilt method to the measurement of soil resistivity and proved that this method has the advantages of high accuracy, stable test, and no interference to soil samples when measuring soil resistivity [12,13].This is also the main reason why this experiment chooses the Vanderbilt method as the method of measuring resistance.The measurement under the premise of reducing the disturbance of deep soil makes the numerical results of the experiment closer to the real soil state.

Triaxial experiment
Select a granite thin slice sample with a certain degree of corrosion, and carefully remove the dirt or debris on the surface of the sample.Afterward, assemble the triaxial battery, place the granite on the top of the porous disc to fit the two solid cylindrical indenters, connect the corresponding metal wires, use a certain length of the heat-shrinkable tube to seal the granite thin-schist sample, and connect the external wires to the van der Pauw resistivity measuring instrument.After assembly, place the entire triaxial battery inside the loading frame.In this experiment, hydraulic oil was used as the pressure transmission medium, and a hydraulic pump was used to generate confining pressure around the granite thin slice sample.When the confining pressure is stable, the axial load will be transmitted to the surface of the granite thin slice sample through the piston.During the loading process, when the pressure reaches a steady state, record the value of the resistivity, and increase the pressure to a maximum of 25 MPa in turn.Disassemble the triaxial battery at the end of the experiment.If the surface of the sample is found to have oil stains or the packaging is not dense, the experimental data will be invalidated, and the experiment will be repeated after the same pre-sample treatment.

Acid-based corrosion and saturated salt solution corrosion
Granite slices with a diameter of 50 mm and a height of 10 mm were used as thin-section granite samples.In order to simulate the complex chemical environment in the deep underground and the environmental characteristics of the comprehensive action of high ground pressure, high ground temperature and chemical corrosion in deep engineering, a series of gradient pH conditions and different acid-based chemistry were tested for the number of corrosion times on the granite samples at the beginning of the experiment [14].After the rock sample is corroded by acid and alkali, it will be corroded by salt.And use the vacuum method to saturate the test piece: mark the samples with different corrosion degrees, put them into the vacuum saturation device for saturation, keep the Na2SO4 solution in the saturation device container higher than the upper surface of the rock sample, and control the vacuum pressure gauge to read 100kPa until there is no air bubbles If it overflows, the pumping time should not be less than 4 hours, take out the vacuum saturation device, and soak it in saturated sodium sulfate solution for a period of time, then take out the samples with different marks, cover them tightly with plastic wrap, and place them in a cool place.Take out piece by piece, remove the plastic wrap, put a series of saturated original samples with different degrees of corrosion into the pressure and temperature apparatus and conduct experiments [15].

Statistical analysis of experimental data
Take the resistivity curve of an uncorroded rock sample with temperature and pressure as an example.Under certain other conditions, the influence of temperature on the resistivity of granite is more obvious than that of pressure, and the resistivity of granite decreases significantly with the increase in temperature.At the same temperature, loading further destroys the original fractures of the rock sample, and the saturated solution enters the cracks of the rock sample to enhance the conductivity, that is, the resistivity gradually decreases with the increase of pressure.However, resistivity is negatively correlated with confining pressure as a whole.Under the same corrosion degree and temperature, the overall resistivity of granite decreases by about 0.1%~0.6%for every 5Mpa increase in pressure; under high confining pressure conditions, the decrease in resistivity is more obvious.Due to the condition of high confining pressure, some cracks, defects, or original cracks will appear in the granite thin slice samples after acid-base corrosion and salt corrosion, and the gaps will be filled by the solution medium, resulting in a decrease in resistivity.Under the same temperature conditions, as the corrosion time increases, the internal pores of the rock sample also increase, the conductive solution in the saturated sample enters the pores, and the overall resistivity of the granite shows a significant decrease.With the prolongation of corrosion time, the weakening of chemical solution corrosion ability, and other factors, the decline rate of granite resistivity decreases from the initial 16% to about 10%.In general, granite often has high resistivity due to its mineral composition, which includes minerals that are highly resistant to electrical conductivity, such as quartz and feldspar.However, the resistivity of granite can be affected by the presence of cracks or other imperfections in the rock, since these imperfections provide pathways for electrical current and increase the possibility of corrosion, leading to a further decrease in resistivity [16,17].

Error sources and analysis of experimental data
During the experiment, to provide stable data when the granite rock sample is pressurized in the triaxial experimental equipment, the pressure will always increase; to prevent the influence of repeated loading and unloading on the granite thin-section sample and cause errors in the experimental data.During the experiment, the pressure can be fixed first, and the temperature can be raised to measure the resistance value corresponding to each temperature gradient, then the pressure can be increased, the temperature can be lowered, and the resistance value of the rock sample under different pressure states can be measured as the temperature decreases.By repeating, the resistivity corresponding to rock samples with different temperatures and different loads can be obtained.Granite thin-slice samples should be placed in a relatively dry and cool curing environment during corrosion and storage, and maintain stable physical properties before the formal loading pressure test.

Conclusion
Studies on the resistivity characteristics of deep geotechnical granite under temperature and chemical corrosion have shown that the resistivity of these materials is significantly affected by changes in temperature and chemical composition.The findings suggest that the resistivity of granite decreases with increasing temperature and exposure to aggressive chemical solutions, which can affect certain reliability and performance of structures built on or through such materials.The results of this study have important implications for the design and construction of infrastructure based on deep rock and soil granite.Engineers and other professionals involved in such engineering design and maintenance need to understand the influence of temperature and chemical corrosion on the resistivity properties of such materials to ensure the reliability and safety of actual deep science and deep engineering construction.This study contributes to our understanding of the resistivity characteristics of deep rock and soil granite and provides valuable insights into how temperature and chemical corrosion affect these materials, enabling further in-depth research in this field to develop to be more resistant to deep geothermal temperatures.Chemically corroded new materials or subterranean space construction techniques to improve the durability and reliability of future deep-ground infrastructure.When subjected to pressure, the influence of pressure on resistivity is mainly reflected in three aspects: the rock will produce cracks or even break under the action of pressure; the action of pressure will close the pores in the rock; the chemical composition of the rock will change under high pressure.Variety.Some studies have studied the resistivity change of quartz sandstone and limestone under uniaxial pressure, and believe that with the increase of stress, the rock resistivity gradually increases [12].Similarly, some researchers have tested the resistivity of granite, quartz sandstone, and vanadium-titanium magnetite under uniaxial stress and believe that the general trend of resistivity change is decreasing [18,19].The contradictory conclusions show that the mechanism of the effect of pressure on the resistivity of rock mass is very complicated, and should be considered in future research: (1) Expand the loading gradient range to obtain a more comprehensive rock stress-resistivity relationship.
(2) Optimize the method of measuring resistivity, improve the accuracy of van der Pauw's method for measuring the resistivity of thin slice rock samples, establish the theoretical relationship between stress and resistivity, and quantitatively study the law of rock resistivity changing with stress.
In the process of using granite thin-section samples, the special rock resistivity properties of granite in the deep underground environment were explored.This research is beneficial to the development of deep underground geothermal energy.Granite is corroded, under the multiple effects of temperature and pressure, the surface of the rock has a higher absorption efficiency for underground heat due to its increased contact area, and special equipment can be used to take out the heat stored in the rock, and then it can be directly used in production, life and industrial activities, it has important development potential value.The granite thin-layer sample used in this experiment is a rock sample used as a deep underground heat source to explore that most rocks undergo pressure changes in the deep state.As the temperature rises, the heat absorption efficiency under chemical corrosion, because the amount of heat energy absorbed by it cannot be directly measured, the resistivity of the rock can be indirectly measured to determine the energy contained in it, which is the core of this experiment Target.

Figure 2 .
Figure 2. Variation curve of resistivity of uncorroded rock sample with temperature and pressure.

Figure 3 .
Figure 3.The resistivity curve of the sample corroded for 90d with the pressure (20℃).

Table 1 .
Variation of resistivity of uncorroded granite samples with temperature and pressure.

Table 2 .
Variation of resistivity with temperature and pressure of 30-day-corroded granite samples.

Table 3 .
Variation of resistivity with temperature and pressure of 60-day-corroded granite samples.

Table 4 .
Variation of resistivity with temperature and pressure of 90-day-corroded granite samples.