Research on evolution characteristics of shale pore-fracture and permeability under freeze-thaw cycles

Shale gas reservoirs have ultra-low permeability, and liquid nitrogen fracturing has attracted much attention as a promising reservoir permeability enhancement technology. This paper took the Longmaxi Formation shale in southern Sichuan as the research object, and studied the physical response of the water-bearing shale core under the freezing-thawing cycle of liquid nitrogen (LN₂), which was a cryogenic fluid. The LN₂ freeze-thaw cycle treatment was carried out for the shale in the water-bearing state, scanning electron microscope (SEM) was used to observe the microscopic pore and fracture structure of shale samples before and after the LN₂ freeze-thaw cycle, digital image processing technology and fractal theory were used to quantitatively analyze the pore-fracture changes at the same location, and then porosity and permeability tests were performed, computer tomography (CT) was used to show the macroscopic fracture failure process of shale samples with the LN₂ freeze-thaw cycle, finally discussed the cracking mechanism of liquid nitrogen freezing and thawing. The results showed that the liquid nitrogen freeze-thaw cycle treatment could effectively promote the initiation and expansion of pores and cracks. When the liquid nitrogen froze and thawed, the shale generated new cracks under the action of thermal stress and frost heave force, and the pores and cracks developed steadily increased with the number of freeze-thaw cycles. The cumulative increase in shale porosity under the freeze-thaw cycle was 54.6%, and the increase in permeability was very significant (up to 3 orders of magnitude).