Porosity at the interface of organic matter and mineral components contribute significantly to gas adsorption on shales

Abstract

Using CO2 as a shale fracturing fluid was proposed recently as an alternative to H2O fracturing, one of its advantages is enhanced recovery of CH4, which is based on the competitive adsorption of CO2 and CH4 on shales. Therefore, investigations on gas adsorption in shales are of great importance. Recent researches evidenced that organic matter (OM) in shale is the major control on its adsorption behavior, but in some cases, mineral components (MC) may also play a role. Herein, we focus on the alteration of porosity due to the presence of OM-MC interface and their influence on gas adsorption, these cannot be simply attributed to either OM or MC as frequently reported in the previous publications. In this context, OM from a shale sample was purified following reported methodology, while a universal procedure for extraction of MC was established. Further studies on the porosity and adsorption behavior were carried out on OM, MC, and shale, which were then compared with a hypothetic mixture (HM) from OM and MC bearing the same composition of shale. For the first time, we demonstrate experimentally the profound effect of porosity at the OM-MC interface on gas adsorption of shales particularly at temperatures more relevant to reservoir conditions. The current work deepened the understanding on gas adsorption of shale, and thus shed meaningful lights on related areas such as gas-in-place (GIP) estimation, CO2 sequestration in shales, and particularly the utilization of CO2 for enhanced shale gas recovery.