Gas production decline trends for Longmaxi shale under thermally stimulated conditions

Abstract

Recently, thermal treatment has emerged as a promising technology to enhance shale gas recovery efficiency by promoting gas desorption. However, the understanding of gas desorption characteristics from heterogeneous shale matrix under thermal stimulation remains inadequate, which constrains the development of thermal recovery technology. In this study, on-site canister desorption tests were employed to investigate the shale gas desorption patterns of four distinct shale cores, sourced from four various production zones, under two heating stages (50 °C and 110 °C). Additionally, low-temperature N2 adsorption experiments and infrared examination were used to study the microstructure characteristics of shale before and after thermal treatments. The canister desorption tests revealed that the gas production rates increased for all shales under heating conditions, particularly when transitioning from 50 °C to 110 °C. However, the peak gas production rates at the two heating stages exhibited different trends: nearly identical for Shale-1 and Shale-2, decreasing for Shale-3 and increasing for Shale-4. This can be attributed to the different quantities of adsorbed gas present in shale samples, with Shale-4 containing approximately 0.0088 m3, compared to the markedly lower volumes in Shale-1 (0.0053 m3), Shale-2 (0.0040 m3), and Shale-3 (0.0041 m3). Furthermore, the gas production rates followed distinct decline trends at different temperatures: exponentially at 50 °C and following a power law at 110 °C, indicating varying gas desorption dynamics under different thermal conditions. Moreover, the findings from low-temperature N2 adsorption experiments and infrared examination indicated that combustion increased the pore volume and induced thermal cracks and fractures, thereby enhancing shale permeability. These insights highlight the potential of combustion fracturing as a technology to improve gas recovery efficiency.