Shale is a self-sourced reservoir rock that contains tremendous natural gas resources. Shale incorporates unique gas storage mechanisms, and both free gas and adsorbed gas contribute significantly to the total gas resource. Because a considerable portion of the gas is adsorbed at the pore surfaces of the shale matrix, the study on gas adsorption is critical to the reservoir assessment and production forecasting of shale plays. This work presents a comprehensive literature survey of gas adsorption on shales and the interrelationship with gas well recovery. This review revisits the fundamental knowledge of adsorption theory and theoretical adsorption models commonly encountered in the shale gas adsorption community. This study also summarizes the relation of pore structural characteristics and geochemical properties (i.e., total organic carbon, kerogen type, thermal maturity, and mineral content) to the sorption capacity of shale. Because shale incorporates a highly heterogeneous structure, its ability to adsorb gas varies from basin to basin and no universal relationship persists between porosity and permeability. This study also outlines the influence of moisture on gas sorption in shale reservoirs, where the imbibed water from hydraulic fracturing and internal moisture content negatively impact gas storage and transport capacity of shale. Most importantly, this work carefully articulates the contribution of adsorbed gas to overall gas recovery from shale reservoirs based on the different stages of depletion. Because the pay zone of shale reservoirs has a deep burial depth with high initial pressure, especially for shale in thermogenic origin, short-term production mainly comes from gas in the free phase. In contrast, long-term production is extensively supplied by adsorbed gas as the reservoir pressure approaches the critical desorption pressure and beyond. The relative role that free gas and adsorbed gas play in shale gas production is still obscure. It is recommended that further studies will be required to analyze the role of sorption on gas transport processes, including desorption, diffusivity, and permeability.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)
- Fuel Technology
- Energy Engineering and Power Technology