Coupling effect of moisture desorption and matrix contraction on resistivity of water-bearing coal under high geothermal environment

Abstract

Resistivity is a key method for geophysical exploration of underground coal seams. However, the deep, high geothermal environment poses significant challenges to this approach, mainly due to moisture desorption and matrix contraction effects induced by high temperatures. In this study, experiments were conducted to assess the resistivity of water-bearing coal at varying temperatures between 30 °C and 70 °C. In addition, Nuclear Magnetic Resonance (NMR) technology was used to analyze the moisture distribution within the coal under high temperature conditions. The results indicate that moisture desorption in coal at elevated temperatures occurs in two distinct stages: a rapid desorption stage from seepage pores and a slower desorption stage from adsorption pores. As the temperature increased from 30 °C to 70 °C, the amount of moisture desorbed increased by 117 %, while the matrix contraction strain increased by 130 %. Furthermore, the variation of coal resistivity under high temperature conditions can be categorized into three stages: a transient decreasing stage due to the Soret effect, a significant increasing stage caused by moisture desorption, and a continuous decreasing stage due to coal matrix contraction. Finally, a theoretical model was developed to characterize the coupled effects of moisture desorption and matrix contraction on coal resistivity. This model provides a basis for the application of resistivity methods in deep, high-geothermal environments.