Experimental investigation of the relationship between permeability and effective stress for low-permeability sandstone with micro-fractures under high pressure

Effective stress is known to be a key factor affecting permeability measurements under geological conditions. As effective stress increases, the permeability of rock containing micro-fractures will decrease significantly. Based on laboratory measurement data, several scholars have come up with empirical equations to describe permeability changes with effective stress and found that there generally exists an exponential or power-law relationships. In this study, the experimental sample is a tight sandstone formation containing microfractures from Kuche Depression in Tarim Basin, China, where gas is produced from deep reservoirs of over 6000 m. Permeability was measured using the conventional pulse-decay method using an in-house true triaxial stress cell with maximum confining pressure of 120 MPa, pore pressure of 100 MPa and axial pressure of 250 MPa. The tight sandstone contains micro-fracture and an ambient porosity of 5%. Under the condition of high pore pressure (up to 80 MPa), the Knudsen number K_n＜0.01, and the gas slippage effect appears to have little impact on the permeability, characteristic in the Darcy flow state. As the confining pressure increases, the gas permeability decreases significantly, whereas as the pore pressure increases, the gas permeability increases. It has been shown that as the effective stress increases, the gas permeability decreases, and ln(K/K₀) shows an exponential relationship with (δ - δ₀) (subscript 0 represents the initial state). As the effective stress decreases, ln(K/K₀) shows a logarithmic relationship with (δ - δ₀). Under the condition of equal effective stress, ln(K/K₀) shows a linear relationship with pore pressure. In addition, we have also noticed a strong anisotropy in the permeability when differential axial stress was applied during the permeability measurement, reflecting a preferential distribution of microfractures in the tight sandstone measured.