A New Technique for Simultaneous Measurement of Nanodarcy-Range Permeability and Adsorption Isotherms of Tight Rocks Using Magnetic Suspension Balance

Summary Flow and adsorptive characteristics of methane and other natural gases onto tight rock formations are of economic interest for proper engineering evaluation and reservoir performance management. This work presents a novel technique to enable the simultaneous determination of nanodarcy permeability and adsorption isotherms of gas in such formations through the transient analysis of experimental data from magnetic suspension balance (MSB). MSB has primarily been shown to be an effective tool for evaluating the amount of gases adsorbed onto tight shales/coals, especially when the adsorbed amount is small. In addition to the classical usage of the measured data using MSB, a new mathematical model based on volume averaging has been developed to describe the transient behavior of the adsorption phenomenon and to obtain the nominal or apparent permeability of shale samples from experimental data. Historically, the permeability of nanoporous materials is measured using two leading methods: the Gas Research Institute method and the pressure–pulse–decay method; however, neither of these methods yields information about the adsorptive behavior of the porous medium or considers such phenomena. In this study, we developed a simple theoretical framework to obtain the isotherm of gas adsorption onto a tight shale (or other tight materials such as coal) sample and the permeability of the sample, simultaneously. The results show that the permeability vs. pressure plot follows the Klinkenberg effect at lower pressures, as expected. The overall methodology developed here can be applied to any type of adsorbing gases and shale/coal samples. The utility and validity of the methodology are demonstrated by applying the developed methodology in experiments performed on three tight shale samples (unconventionals) using two different gases: methane and CO2.