The advances in hydraulic fracturing and horizontal well technology have unlocked considerable natural gas reserves contained in the shale formations. Reliable values of the shale key petrophysical properties including permeability and porosity are necessary to estimate the original gas-in-place, predict the production rates, and optimize the hydraulic fracturing treatments. The quantification of the key shale petrophysical properties however remain challenging due to complex nature of the shale foramtions. Unsteady state techniques are commonly used to estimate permeability of the shale samples because the shales typically have permeability values in nano-Darcy range. The measured permeability values by these techniques however suffer from a large margin of uncertainty and reproducibility problems. Furthermore, the unsteady state measurements cannot be performed under the reservoir stress and temperature conditions.
In this study, a fully automated laboratory set-up, which has been designed and constructed for the evaluation of the ultra-low permeability petrophysical properties under the reservoir conditions, was utilized to measure the porosity and permeability of the Marcellus shale core plugs. The core plugs were obtained from a vertical well drilled specifically for the laboratory research and other scientific purposes (science well) on the site of the Marcellus Shale Energy and Environment Laboratory (MSEEL). MSEEL is a field site and dedicated laboratory in the Marcellus Shale unconventional production region of north-central West Virginia. The filed site is owned and operated by Northeast Natural Energy, LLC and contains several horizontal Marcellus Shale wells. MSEEL provides a unique opportunity to undertake field and laboratory research to advance and demonstrate new subsurface technologies and to enable surface environmental studies related to unconventional energy development.
One of the core plugs obtained from the science well was used in this study for the evaluation of reliable Marcellus Shale petrophysical properties. The permeability of the core plug was measured under different gas pressures at constant net stress. The absolute permeability was then determined by applying the appropriate gas slippage correction. The porosity and the permeability of the core plug were then measured under a wide range of net stress. The measured porosity and permeability values were found to be sensitive to the stress. The permeability measurement results exhibited two distinctive behaviors with respect to the net stress that can be attributed to the natural fracture and matrix properties. The experimental results were then utilized to determine the natural fracture closure stress. The measurements also revealed that gas adsorption, when an adsorbent gas was used for the mesurements, resulted in a reduction in the absolute permeability of the sample.