Abstract
Permeability is one of the most important parameters to evaluate gas production in shale reservoirs. Because shale permeability is extremely low, gas is often used in the laboratory to measure permeability. However, the measured apparent gas permeability is higher than the intrinsic permeability due to the gas slippage effect, which could be even more dominant for materials with nanopores. Increasing gas pressure during tests reduces gas slippage effect, but it also decreases the effective stress which in turn influences the permeability. The coupled effect of gas slippage and effective stress on shale permeability remains unclear. Here we perform laboratory experiments on Longmaxi shale specimens to explore the coupled effect. We use the pressure transient method to measure permeability under different stress and pressure conditions. Our results reveal that the apparent measured permeability is controlled by these two competing effects. With increasing gas pressure, there exists a pressure threshold at which the dominant effect on permeability switches from gas slippage to effective stress. Based on the Klinkenberg model, we propose a new conceptual model that incorporates both competing effects. Combining microstructure analysis, we further discuss the roles of stress, gas pressure and water contents on gas permeability of shale.
Highlights
Permeability is one of the most important parameters to evaluate gas production in shale reservoirs
Shale gas has been considered as an attractive low-carbon solution for the transition period to a future power by renewable energy[1]
The average burial depth of shale gas in the US is about 800–2000 m, while the average depth in China is over 3500 m where the stress is significantly high (>50 MPa)[2]
Summary
Based on the principle of pressure transient method, a specific setup of gas permeability measurements has been designed and developed in the Institute of Rock and Soil Mechanics of Wuhan, CAS. In this system, a triaxial core holder, capable of accepting membrane-sheathed cylindrical samples (2.5 cm diameter) and of applying independent loading in the radial and axial directions, is connected to two gas reservoirs. At the end of the mechanical unloading, a hydrostatic stress of 16 MPa was applied on the sample No.[1] and a series of gas permeability measurements were carried out with different gas pressures (1 MPa, 2 MPa, 3 MPa, 4 MPa) to study the gas slippage effect. Nitrogen was chosen to measure gas permeability and the temperature of the whole system was kept at 30 °C
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
