Comparative experimental investigation on seepage characteristic of limestone after thermal-water cooling circulation and thermal-air cooling circulation treatments

For both subsurface dry and heat rock and geothermal energy extraction process will involve thermal-cooling cycle effect. In order to study the permeability characteristics of limestone after the action of different temperatures and different cooling cycles, the evolution mechanism of permeability of limestone under the conditions of thermal-air cooling cycle and thermal-water cooling cycle in the temperature range of 20℃~800℃ was studied by indoor experiments, and the change pattern of relevant physical parameters was analysed. Meanwhile, the development of microcracks and pores inside the specimens with temperature and number of cycles was obtained using SEM scans. After comparison, it was found that the variation of P-wave velocity, the average reductions of wave velocity for limestone specimens at 200°C to 800°C were 12.75%, 20.01%, 32.15%, and 64.58% after water cooling cycles and 10.24%, 18.46%, 29.36%, and 54.26% after air cooling cycles under 1 cycle, respectively. Furthermore, under the same number of cycles and the same seepage pressure action, the thermal-water cooling cycle action has a greater effect on the permeability of limestone than the thermal-air cooling cycle action, which is mainly due to the fact that the thermal-water cooling cycle action causes more microfractures to be produced and expanded in limestone.