Experimental study on seepage characteristics and evolution of granite after alternating cooling and heating

Deep geothermal resources in China are widely distributed, with large reserves, and have broad prospects for development and utilization. In order to investigate the seepage characteristics and evolution laws of deep high-temperature reservoirs under the condition of intermittent water injection geothermal exploitation. Using the combines method of laboratory testing and microscopic analysis, a seepage test was carried out on granite after alternating thermal and cold cycles at different temperatures, and the variation of permeability with temperature under different confining pressure were analyzed. Combining microscopic characteristics and CT scaning reconstruction results, the evolution of the microscopic pore and crack structure with temperature and cycle times was studied. The results indicated that: (1)The permeability of granite is related to temperature and cycle numbers. Under the two cooling methods, the permeability of granite gradually increases with the increase of temperature, and the permeability increases sharply when the temperature exceeds 500℃. In addition, the permeability increase under water-cooled conditions is significantly greater than that under natural cooling conditions, and the parameter changes after 10 water-cooled cycles are more significant. (2)The stress state has a significant influence on the permeability. Under the confining pressure, the internal pore and fracture structure of the specimen is closed. With the increase of confining pressure, the permeability of the granite decreases after the alternating hot and cold cycles at different temperatures, and gradually increases with the increase of osmotic pressure. (3)Under the action of high temperature cooling cycle, micro-pore cracks in granite gradually develop, temperature gradient stress causes non-uniform deformation among mineral particles, and degradation damage such as intergranular cracks, transgranular cracks and particle spalling occurs under the combined action of physical and chemical reactions such as dehydration and phase transformation. With the increase of cycle times, the number, size and connectivity of cracks further increase. The research results can provide theoretical support and technical reference for geothermal development and reservoir construction in deep dry hot rock.