Experimental deformation of shales at elevated temperature and pressure: Pore-crack system evolution and its effects on shale gas reservoirs

Although many studies based on naturally deformed samples have been carried out to investigate the pore-crack characteristics of shales, studies based on high temperature (T) and high pressure (P) deformation experiments, which can exclude sample heterogeneity factors, simulate deep T-P conditions, and generate a continuous deformation sequence, are still rare. In this study, shales with different deformation levels are generated by triaxial compression experiments, and methods including scanning electron microscopy, mercury injection, and gas sorption are utilized to characterize their influence factors and pore-crack characteristics. Results indicate that T is the primary factor influencing shale deformation when P is low, while P is dominant under high P conditions. At T < 90 °C and P < 60 MPa, shales undergo brittle deformation and their macropores decrease due to the compaction of primary pores, while mesopores increase because of the interconnection of micropores. At 90 °C < T < 200 °C and 60 MPa< P < 110 MPa, shales experience brittle-ductile transitional deformation, and their macro- and micropores increase because of the extension of open cracks and the plastic deformation of clay flakes respectively, while mesopores decrease dramatically. At T > 200 °C and P > 110 MPa, shales are subjected to ductile deformation, and their micro- and mesopores drop significantly due to the intense compaction in the matrix while macropores continuously increase with crack expansion. The permeability of shale increases with the degree of deformation and ductile material contents are predicted to be a key factor determining whether open microcracks can be preserved after ductile deformation. To account for these experimental results, an ideal model of micro pore-crack system evolution in deformed shales is further proposed, which can provide guidance for the exploration of shale gas resources in the deep or structurally complex zones.