Abstract
The advances in hydraulic fracturing and horizontal well technology have unlocked considerable reserves of hydrocarbon contained in shale formations. However, quantification of the key shale petrophysical properties remain challenging. It is not practical to measure the permeability of the unconventional formations such as shales by standard steady state techniques because shales typically have permeability values in nano-Darcy range. Therefore, unsteady state methods have been extensively used to estimate permeability of the shale samples. However, the measured permeability values by these techniques suffer from large margin of uncertainty and reproducibility problems. These problems are attributed to the lack of consistent experimental protocols and the interpretations of the transient data. Another limitation of the unsteady-state measurements is that the experiments cannot be performed under the reservoir stress and temperature conditions.
This paper provides the results of the porosity and permeability measurements on Marcellus shale core plugs which were performed using a fully automated laboratory set-up for evaluation of the ultra-low permeability petrophysical properties under the confining pressure. The permeability of the core plug were first measured under different gas pressures at constant net stress. The absolute permeability was then determined by applying the gas double-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 stress. Two distinctive behaviors with net stress, for both porosity and permeability, were observed that can be related to the natural fracture and matrix properties. The experimental results were then utilized to determine the natural fracture closure pressure. The permeability measurements with carbon dioxide revealed that permeability is impacted by adsorption. The results of the measurements with were carbon dioxide also provided information for determination of the sorption characteristics that were found to be in agreement with the published values.
