Study on Competitive Adsorption and Displacing Properties of CO[sub.2] Enhanced Shale Gas Recovery: Advances and Challenges

CO[sub.2] enhanced shale gas recovery (CO[sub.2]-ESGR) draws worldwide attentions in recent years with having significant environmental benefit of CO[sub.2] geological storage and economic benefit of shale gas production. This paper is aimed at reviewing the state of experiment and model studies on gas adsorption, competitive adsorption of CO[sub.2]/CH[sub.4], and displacement of CO[sub.2]-CH[sub.4] in shale in the process of CO[sub.2]-ESGR and pointing out the related challenges and opportunities. Gas adsorption mechanism in shale, influencing factors (organic matter content, kerogen type, thermal maturity, inorganic compositions, moisture, and micro/nano-scale pore), and adsorption models are described in this work. The competitive adsorption mechanisms are qualitatively ascertained by analysis of unique molecular and supercritical properties of CO[sub.2] and the interaction of CO[sub.2] with shale matrix. Shale matrix shows a stronger affinity with CO[sub.2], and thus, adsorption capacity of CO[sub.2] is larger than that of CH[sub.4] even with the coexistence of CO[sub.2]-CH[sub.4] mixture. Displacement experiments of CO[sub.2]-CH[sub.4] in shale proved that shale gas recovery is enhanced by the competitive adsorption of CO[sub.2] to CH[sub.4]. Although the competitive adsorption mechanism is preliminary revealed, some challenges still exist. Competitive adsorption behavior is not fully understood in the coexistence of CO[sub.2] and CH[sub.4] components, and more experiment and model studies on adsorption of CO[sub.2]-CH[sub.4] mixtures need to be conducted under field conditions. Coupling of competitive adsorption with displacing flow is key factor for CO[sub.2]-ESGR but not comprehensively studied. More displacement experiments of CO[sub.2]-CH[sub.4] in shale are required for revealing the mechanism of flow and transport of gas in CO[sub.2]-ESGR.