Adsorption Isotherms and Permeability of Tight Shales Using Magnetic Suspension Balance Experiments

Abstract

Abstract Adsorption and flow characteristics of methane and other natural gases onto tight rock formations is of economic interest for the proper evaluation and production of the field. In this work, a novel technique to enable the determination of adsorption isotherms and permeability of a gas in such formations through the transient analysis of experimental data from magnetic suspension balance is presented. Adsorption of gases onto the tight shale samples were measured using the magnetic suspension balance (MSB) apparatus at a constant pressure and temperature conditions. The current work presents the experimental technique in sufficient detail to allow others to replicate and evaluate the results. MSB has been proved a very effective tool for evaluating the adsorbed amount of gases onto tight shales/coals, especially when the adsorbed amount is very small. A mathematical model based on volume averaging is developed to describe the transient behavior of the MSB-set up, which was utilized to obtain the permeability of shale sample from experimental data. A simple methodology, developed with a theoretical framework, is presented to obtain adsorption isotherm of a gas onto tight shale/coal sample and permeability of the sample. The methodology can be utilized to any type of adsorbing gases and shale/coal samples. In particular, Experimental data for adsorption of methane onto a shale sample is presented at 50C and in the pressure up to 25bar using the MSB apparatus. The density of methane was also measured with MSB which shows very good match with the data available in literature, demonstrating that MSB can be an excellent tool for density measurement of gases. A simple method is developed to decouple the buoyancy effect and obtain the absolute adsorption isotherms. Most-importantly, a low-dimensional mathematical model based on volume averaging is developed to describe the transient adsorption process in MSB experiments, which was used to regress the experimental data and obtain the permeability of the shale sample. The permeability versus pressure plot shows the Klinkenberg effect, as expected. Further, the validity and accuracy of the model was verified by comparing the absolute permeability obtained from our method with an independent measurement using pulse-decay experiments. The novelty of the work includes the development of a mathematical model describing the transient adsorption process in MSB experiments that can be utilized to obtain permeability of tight shale samples. In addition, the current work presents how to decouple the buoyancy effects to determine absolute adsorption isotherm. The methodology was tested and verified against the experimental measurements and can be utilized for industrial applications.