Impact of cyclic liquid nitrogen treatment on the pore and fracture structures of shale at different scales and its effect on permeability enhancement

The ultra-low permeability of shale reservoirs limits shale gas production. Liquid nitrogen (LN2) fracturing is a promising reservoir stimulation technology that has attracted considerable attention in the unconventional natural gas industry. However, the evolution of pore and fracture structures in shale subjected to cyclic LN2 treatment and its effect on permeability enhancement have not been fully elucidated, especially with the pres-ence of pore or fracture water. In this study, the physical response of Longmaxi Formation shale to cyclic LN2 treatment was carefully investigated. Scanning electron microscopy (SEM) and computed tomography (CT) were used to investigate the alteration of the pore and fracture structures in water-saturated and dry shale samples at different scales before and after cyclic LN2 treatment. Based on digital image processing and the fractal theory of pore structure, the variation in pore and fracture structures in shale matrices at the microscale was quantitatively analyzed. The experimental results indicate that cyclic LN2 treatment can effectively enhance pore volume and induce microfracture initiation and propagation. These phenomena were more significant in the water-saturated shale samples. Different correlations between the areal porosity of matrices and overall porosity/permeability in dry and water-saturated samples indicate that thermal stress and frost-heave force have different effects on shale pore and fracture structures. Owing to the different generation mechanisms, thermal stress was mostly applied to shale matrices, whereas frost-heavy force only existed near water-rich minerals, which were mainly clay in shale natural fractures. These two driving forces also had different responses to cyclic LN2 treatment. Because the new generated pores and microfractures have increased the distance among mineral particles, thermal stress grad-ually decreased with increasing LN2 treatment cycles. However, frost-heavy force could still induce macro -fractures along the bedding face of shale samples after several LN2 treatment cycles, owing to water migra-tion during water-ice phase change.