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

Abstract

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.