Evolutionary characteristics of sandstone pore-fracture structure under the action of high and low temperature cyclic impact

China is rich in coal-measure gas reserves. However, the complex geological conditions of coal-measure gas reservoirs make it costly and inefficient to mine a single gas-bearing system, and difficult to co-mine multiple gas-bearing systems. How to improve the permeability and compatibility of coal reservoirs is the key issue in development of coal-measure gas. In order to explore effective methods to improve the structure of coal reservoirs, the evolution characteristics of sandstone pore-fracture in the top plate of coal seams under the cyclic impacts of high temper-ature (200 ℃) and low temperature (−196 ℃) were investigated. The development of sandstone pore-fracture before and after high- and low-temperature impacts was characterized using industrial micro-CT. The CT data of the specimens before and after impact were filtered and homogenized using digital image and three-dimensional reconstruction techniques. The changes of volume, surface area, porosity and fractal dimension of distribution complexity of sandstone pore-fracture after 0, 1, 5, 10 and 15 high- and low-temperature impacts were qualitatively and quantitatively analyzed. The results shown that: ① High- and low-temperature cyclic impacts promoted the development, expansion and connectivity of pore-fracture, and the sprouting of secondary fracture in sandstone. The impacts contributed to the expansion of micro pore-fracture to form larger macro fracture; ② The volume, surface area, porosity and fractal dimension of distribution complexity of sandstone pore-fracture varied logarithmically with the increase of the number of temperature impacts, and the growth tended to increase first and then decrease. The increase of its area and porosity reached a maximum of 15.48% and 20.98%, respectively, after the 5th impact effect; ③ High- and low-temperature cyclic impacts weakened the binding force between the sandstone matrix and broke the original static equilibrium. The variation curve of sandstone porosity with the number of scanned layers evolved into a “C-like” shape, which indicated that the binding force at the ends of the sandstone was more attenuated.