Study on Stress Sensitivity of Different Lithofacies Shale Under Dynamic Stress Conditions

Shale oil and gas play an important role in unconventional oil and gas resources, which usually rely on elastic capacity for development. With the production of oil and gas, the formation pressure continues to decline, the matrix skeleton deforms under the overburden pressure, and nanoscale pores are easily closed, resulting in a decrease in the permeability of shale. In order to investigate the stress sensitivity of different lithofacies shale under variable stress conditions, eight matrix shale cores and eight mixed shale cores were selected from the JY depression. Online nuclear magnetic resonance equipment, CMS400 overexposed pressure porosity and permeability testing equipment, and shale seepage capacity testing device were used to conduct tests on the pore structure, porosity, and permeability of different lithofacies shale, revealing the effects of lithofacies, confining pressure, and injection medium on shale stress sensitivity. The results indicate that the porosity and permeability of mixed shale are higher than those of matrix shale. The proportion of macropores in mixed shale is generally 5% higher than that in matrix shale, indicating greater potential for resource utilization. Under nitrogen and water injection conditions, the presence of sand bars in mixed shale can significantly improve its permeability, but as pressure increases, the permeability of shale containing sand bars rapidly decreases, while shale without sand bars still has permeability. Under the condition of supercritical CO2 injection, the permeability of mixed shale is better than that of matrix shale, and the permeability of shale containing laminations in matrix shale is significantly stronger than that of layered shale.