The Impact of Gas Adsorption and Composition on Unconventional Shale Permeability Measurement

Abstract

Permeability determination of organic-rich shales is still a major challenge. Uncertainty in this estimate involves several factors. Two significant ones are the occurrence of gas adsorption which can severely limit gas transport in the pores and understanding the physical chemistry issues of the pore's surface area estimation when using various gases. In this study, we reported our experimental results of permeability measurement on several unconventional shale samples, and investigated the effect of gas type, pore pressure, effective stress and sample orientation on the measured permeabilities.Permeability of shale samples is measured using the complex pressure transient technique. Three different gases, argon, nitrogen, and carbon dioxide, are used as permeating fluid through the samples. Experiments are conducted isothermally at various pore and confining pressures that maintain a constant net effective stress. Generally, samples have higher measured permeabilities when using nitrogen as pore fluid rather than using argon. The discrepancy was attributed to different adsorption potentials between argon and nitrogen: Argon has a similar sorption potential to methane while nitrogen's sorption potential is relatively weak. As expected, the measured permeability of all samples decreases when the pore pressure increases reflecting the reduction in the gas slippage effect. Samples from the same whole core display permeability anisotropy: Horizontal plugs cut parallel to bedding have a higher measured permeability, which is in the range of microdarcy, while the permeability of vertical plugs cut perpendicular to bedding is in the range of nanodarcy. This anisotropy behavior is believed to be caused by the fractures contained within the horizontal samples. The measured permeability is observed to decease with increasing effective stress acting on the samples. This reduction behaves differently: Permeability decreases very slowly when the increasing effective stress is resulted from the decrease of the pore pressure. The enhanced Klinkenberg effect due to the decreasing pore pressure compensates at least partly the permeability reduction resulting from increasing effective stress. However, permeability reduces dramatically when the effective stress increases because of the increasing confining pressure. In this case, the flow channels may be reduced or even closed, thus blocking the flow of gas.

Analytical and Experimental Investigations of Gas-Flow Regimes in Shales Considering the Influence of Mean Effective Stress

“…Several correlations were proposed for b in Eq. 5 on the basis of laboratory experiments (Heid et al 1950;Jones and Owens 1980;Sampath and Keighin 1982;Florence et al 2007;Tanikawa and Shimamoto 2009); b is generally taken to be a function of absolute permeability (and in some cases, of porosity) and, to a lesser extent, of gas type. Throughout the literature, there is evidence of deviations from Eq.…”

“…5 (Klinkenberg 1941;Jones and Owens 1980;Sampath and Keighin 1982;Noman and Kalam 1990;Wu et al 1998;Rushing et al 2004;Li et al 2009;Macini et al 2011;Fathi et al 2012;Dong et al 2012), with most reporting a deviation from the Klinkenberg straight-line plot, especially in low-permeability cores. Jones and Owens (1980) conducted a series of experiments on low-permeability gas sands. Even though their results yielded reasonably straight lines in Klinkenberg plots, they noticed an overestimation of absolute permeability of up to 25%.…”

“…However, in his experiments, permeability was high enough for the assumption to hold. Jones and Owens (1980) acknowledged that the deviations were caused by the departure from Warburg's (Kundt and Warburg 1875) model, resulting from the constant-velocity-gradient assumption. Li et al (2009) stated that deviations emerge because, unlike Klinkenberg's assumption, the mean free path is not strictly inversely proportional to pressure.…”

Evolution of Shale Permeability under the Influence of Gas Diffusion from the Fracture Wall into the Matrix

“…Another essential influence on permeability evolution of sorbing porous media, such as shale and coal rocks, comes from sorptioninduced volumetric strains. 30 Jin et al 31 measured shale permeability by using different gas types and reported that the nitrogen permeability is higher than the argon permeability due to their different adsorption potentials. Argon's sorption potential is larger than that of nitrogen.…”

A model for stress‐dependence of apparent permeability in nanopores of shale gas reservoirs

“…Similar to CO 2 , this two‐stage evolution of permeability with gas pressure was consistently observed for N 2 . Jin et al 16 measured the change of permeability of N 2 and Ar in North America shale during gas desorption. The permeability was measured under a CCP of 11 MPa while the gas pressure decreased from 9 to 4 MPa.…”