The dependence of shale permeability on confining stress and pore pressure

Abstract

We examined the permeability behavior of two Marcellus Shale samples and two Eagle Ford Shale samples using argon gas under multiple confining stresses (PC = 13.8–68.9 MPa) and pore pressures (PP = 1.6–28.2 MPa). We corrected our measured permeability values for the Klinkenberg effect and contoured permeability as a function of PP and PC to obtain the effective stress coefficient for permeability (nk), which scales the influence of PP on permeability relative to PC. We explored the underlying mechanisms controlling nk using two idealized models of a cylindrical capillary tube consisting of low and high compressibility material to model the unique pore structure present in our samples. Finally, we investigated the impact of PP reduction on permeability during production. We show that the permeability is stress-dependent and that nk is formation-dependent. Marcellus Shale samples, with predominantly organic matter hosted pores, have a Klinkenberg-corrected permeability of 11.0 to 1.9 nD (PC - PP = 5.3–33.8 MPa) and nk value of 1.11 and 1.60, respectively. In contrast, Eagle Ford Shale samples, with predominantly inter-particle pores, have Klinkenberg-corrected permeability of 33.6 to 2.0 nD (PC - PP = 13.3–40.8 MPa) and nk value of 0.96 and 0.84, respectively. We infer that the PP change produces considerable pore wall strain relative to the PC change in the Marcellus Shale organic matter pores resulting in nk > 1. In contrast, we infer a smaller pore wall strain for the same PP and PC changes in the Eagle Ford Shale inter-particle mineral pores giving in nk < 1. Our findings will inform models to predict production behavior of reservoirs and may be used to improve recovery efficiency by adopting the best production strategy in unconventional shale reservoirs.