Experimental Study on Improving Permeability of Shale by CO₂ Huff and Puff Assisted by Ultra-Low Temperature Liquid Nitrogen

The shale reservoir has tight rock, small pore throat, and very low permeability The knowledge of flow mechanism in shale is the key to improving the recovery of shale oil and gas. By conducting low-temperature liquid nitrogen (LN₂) treatment experiments on shale cores and experiments of cyclic injection of CO₂, the effects of cyclic gas injection at different injection pressures after low-temperature LN₂ treatment on the recovery of shale oil, physical properties of cores, and relative permeability of oil and gas were studied, and the changes in the microscopic pore structure before and after the treatment were identified. The experimental results show that after LN₂ injection, the shale can generate a thermal stress of 313.5 MPa, inducing the formation of micro-fractures. The volume expansion effect of LN₂ vaporization and cyclic injection for CO₂ can form a re-pressurization mechanism in the core after the formation of micro-fractures, expanding the induced fractures and improving the permeability. The recovery by CO₂ huff and puff is proportional to the injection pressure. The cumulative recovery by the first and third rounds of supercritical CO₂ huff and puff are 32.4% and 34.9% higher than those of subcritical CO₂ , with an increase of 154.6% and 101.7%, respectively. The number of required huff and puff cycles for high-pressure CO₂ injection is reduced, and the produced oil mainly comes from the first two rounds of huff and puff. Compared with that of the initial shale core, after supercritical cyclic CO₂ huff and puff, the average pore size increases by 176%, and the maximum relative permeability of oil and gas increase by 1.8 and 2.3 times, respectively. The research results provide a reference for the production increase of shale oil and gas.