Abstract
CO2 enhanced shale gas recovery (CO2-ESGR) draws worldwide attentions in recent years with having significant environmental benefit of CO2 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 CO2/CH4, and displacement of CO2-CH4 in shale in the process of CO2-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 CO2 and the interaction of CO2 with shale matrix. Shale matrix shows a stronger affinity with CO2, and thus, adsorption capacity of CO2 is larger than that of CH4 even with the coexistence of CO2-CH4 mixture. Displacement experiments of CO2-CH4 in shale proved that shale gas recovery is enhanced by the competitive adsorption of CO2 to CH4. Although the competitive adsorption mechanism is preliminary revealed, some challenges still exist. Competitive adsorption behavior is not fully understood in the coexistence of CO2 and CH4 components, and more experiment and model studies on adsorption of CO2-CH4 mixtures need to be conducted under field conditions. Coupling of competitive adsorption with displacing flow is key factor for CO2-ESGR but not comprehensively studied. More displacement experiments of CO2-CH4 in shale are required for revealing the mechanism of flow and transport of gas in CO2-ESGR.
Highlights
Due to increasing combustion of fossil fuels and the ensuing large-scale CO2 emissions, the high level of CO2 content in the global atmosphere is believed as the main driving force of global climate change [1, 2], which may cause climate disasters, seriously affecting human life and the earth’s ecology [3, 4]
CH4 in shale is proportional to the organic matter content, and the larger total organic carbon (TOC) can improve the adsorption capacity of
Results of experiments and molecule grand canonical Monte Carlo (GCMC) simulations in Huang et al [70] showed that the maximum adsorption capacity of gas molecules for both CO2 and CH4 on kerogen is proportional to the effective pore volumes, which increases with kerogen maturity but decreases with moisture content
Summary
Due to increasing combustion of fossil fuels and the ensuing large-scale CO2 emissions, the high level of CO2 content in the global atmosphere is believed as the main driving force of global climate change [1, 2], which may cause climate disasters, seriously affecting human life and the earth’s ecology [3, 4]. CO2 replaces the adsorbed CH4 in shale matrix (kerogen or clay) through competitive adsorption and displaces the free CH4 in the fractured pores during the process of CO2-ESGR, thereby increasing the recovery of shale gas. In order to achieve effective development of CO2-ESGR technology, some relevant key scientific problems must be solved, including mechanism of CH4 adsorption/desorption, CO2/CH4 competitive adsorption mechanism in shale matrix, and CO2 displacement CH4 flow and mass transfer mechanism. The adsorption/desorption characteristic of singlecomponent gas (CH4 or CO2) in shale has been fundamentally studied [14,15,16] Among these key scientific problems, mechanism of CO2/CH4 competitive adsorption in shale matrix and flow and mass transfer in the displacement of CO2-CH4 has not been fully understood and has attracted widespread attention and curiosity worldwide.
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