Experimental methods of shale permeability

This thesis explores the permeability of shale caprock (Green River Shale). Dynamic coupling between fluid transport and mechanical response have been observed in this unconstrained sorbing and swelling shale. We report laboratory experiments that investigate the permeability evolution of shale sample as a function of applied stress and pore pressure at room temperature and gas transport between fracture and matrix. Experiments were conducted on 2.5 cm diameter by 1.2cm long cylindrical samples at confining and axial stresses of 7 MPa. Permeability and sorption characteristics were measured using a pulse transient method that was also used to measure volumetric strains for both helium (He) and carbon dioxide (CO₂). From these experiments it was found that gas adsorption is closely linked with the apparent permeability of the sample. As pore pressure increased, the permeability to He increased from 6.01×10^-19 m² to 9.74×10^-19 m² while the permeability to CO₂ which is a larger molecule with higher absorbability increased from 3.03×10^-19 to 4.32×10^-19 m²,. Gas transport between fracture and matrix also played a significant role in permeability evolution. During gas transport, gas diffused gradually from the fractures to the matrix and the area affected by gas transport expanded dynamically from the fractures towards to the matrix. This suggests permeability evolution is dynamic during gas transport and can be divided into two stages. In the first stage, the gas-influenced area is limited to fracture surfaces, which causes the fracture apertures to decrease, thereby reducing permeability. In the second stage, the gas-influenced area penetrates further into the matrix which causes the fracture apertures to increase, resulting in permeability recovery. This hypothesis is supported by our experimental observations. Between two stages of the gas transport process, where local swelling dominated, the permeability of both He and CO₂ was lower than at the final equilibrium state. At the final equilibrium state, where uniform macro swelling occurred, permeability of both He and CO₂ increased as pore pressure increased. Our experiments reveal that not only adsorption-induced swelling and effective stress but also gas transport has an important impact on permeability of shale caprock.