The interaction of different gases, effective stress, and complex pore structures impacts gas flow in unconventional reservoirs. However, our understanding of these petrophysical properties in organic-rich shale with fine pores and high matrix compressibility is limited, especially in depleted reservoir conditions. This study examined the effects of various gases (N2, CO2, He, Ar) on slippage (b), intrinsic permeability (k∞), and effective stress coefficient (χ) in anisotropic shale samples from the Krishna-Godavari basin, India. We also compared these results with nanopore characterization using low-pressure sorption (N2, CO2) and He pycnometer analysis. The apparent permeability (kapp) was estimated using a pulse decay permeameter at low pore pressure (Pm: 0.72 to 3.07 MPa) with intervals of 0.34 MPa, at different confining pressure (Pc: 5, 10, 15, and 20 MPa). It was observed that kapp decreases with increasing Pm, and conversely, increases with Terzaghi effective stress (Pc - Pm), indicating an increase in gas-wall collisions due to the reduced width of pore conduits at higher effective stress levels.
This comprehensive study provides valuable insights into various aspects of shale formations, including anisotropy, permeability, gas influence, pore width, stress-dependent behavior, and their implications for reservoir engineering and hydrocarbon production. The key findings reveal that permeability anisotropy in shale is direction-dependent, with higher permeability observed parallel to bedding than perpendicular. However, increased effective stress leads to a shift, resulting in higher permeability perpendicular to bedding. Different gases exhibit varying permeabilities in shale, with helium demonstrating the highest permeability, followed by argon, nitrogen, and carbon dioxide. This difference is attributed to variations in adsorption affinity, with noble gases showing higher permeability than sorbing gases. Larger pore widths in shale lead to reduced permeability due to decreased pore throat connectivity. Additionally, gas type affects permeability due to differing adsorption affinities. Increased effective stress reduces permeability in shale, with the exponential function offering a better fit to describe this behavior compared to the power law function. The study also determines the stress sensitivity coefficient and permeability at zero effective stress for different flow directions and sample types, highlighting the higher sensitivity of permeability to effective stress in parallel flow samples compared to perpendicular flow samples. Finally, based on the findings, supercritical CO2 shows potential as an alternative fracking technique to enhance permeability. It could serve as a long-term carbon capture storage (CCS) solution in depleted reservoirs, utilizing their ample space for carbon storage.
