Supercritical Methane Adsorption in Shale: Isothermal Adsorption and Desorption of Eagle Ford Shale Gas

Abstract

Gas adsorption-desorption highly affect gas storage and production behaviour in shale nanopores. The study of methane adsorption isotherm in shale has been extensively conducted experimentally. The shale compositions and reservoir conditions prominently control the adsorption capacity of methane. However, to date, there is a lack of discussion on the effect of heterogeneous TOC towards the adsorption isotherm and comparison with adsorption isotherm modelling. This study used the gravimetric method for supercritical methane adsorptions - desorption isotherms measurements. Isotherms measurements were conducted with three shale samples with various TOC values (9.67, 13.9, and 15.4 wt.%) from the Eagle Ford formation at pressure up to 10 MPa and temperature at 120 °C. The isotherms gathered were fitted with standard adsorption-desorption isotherm models, Langmuir, Freundlich and extended Sips to test the applicability of these models depicted the adsorption of supercritical methane. The results show that EF C with the highest TOC content (15.4 wt.%) has the highest adsorption-desorption methane capacity, more than 0.7 mmol/g, compared to other samples. The composition differences between these samples indicate that the organic contents were likely a major controlling factor of the adsorption capacities obtained. The TOC provides a higher surface area for adsorption to occur. Thus, a higher adsorption-desorption capacity was observed through this study. On the other hand, the adsorption and desorption curves did not intercept due to the hysteresis caused by the capillary condensation. The significant binding capacity of the shale surface for methane gas molecules leads to the hysteresis observed during methane desorption. It was observed that the Freundlich model was the most accurate adsorption model in describing the adsorption-desorption behaviour with tested shales with average R2 more than 0.90 and ARE (%) less than 10 % compared to other models with 15.8 % (Langmuir) and 18.9 % (Sips). This study also proved the influence of organic matter on predicting the adsorption-desorption capacity with adsorption isotherms highlighting the importance of modelling the TOC of shale with adsorption isotherm to determine the adsorption-desorption properties.