Pore Structure Evolution of Mudstone and Sandstone under Thermal Action and Its Effect on Closure

This study explores the evolutionary characteristics of microstructure, porosity-permeability, and sealing properties in caprocks (mudstone and sandstone) under thermal effects during underground coal gasification. Using mudstone and sandstone samples from the Xishanyao formation in the Baikouquan area of the Junggar Basin, we observed microstructural changes in rocks at different temperatures (25~1 200 ℃) through scanning electron microscopy. Combined with overburden pore penetration and breakthrough pressure data, we analyzed the thermal effects on rock pore networks and sealing capacity. The results show that: (1) Mudstone is rich in clay minerals, and its thermal response is mainly characterized by dehydration, cracking and melting of clay minerals, which leads to the expansion of mudstone microfractures and the growth of nascent fissures. The sandstone is rich in quartz and feldspar, and when heated, it is mainly characterized by expansion and mineral melting, fissure expansion and increased connectivity. (2) Overburden pore penetration experiments showed that mudstone porosity and permeability decreased rapidly in the range of 1~12 MPa and then stabilized, whereas the corresponding intervals for sandstone are 1~16 MPa and 16~36 MPa. The porosity and permeability of both types of rocks peaked at 900~1 200 ℃, and then decreased due to the clogging of mineral melts. (3) At 25~300 ℃, mudstone breakthrough pressure plummets due to dehydration of clay minerals. Sandstone moderately increased due to quartz thermal expansion. At 300~1 200 ℃, both dropped dramatically with the temperature increasing. After 600 ℃, caprocks completely loss their closure ability.