The progress of enhanced gas recovery (EGR) in shale gas reservoirs: A review of theory, experiments, and simulations

Due to the combined advantages of injecting CO2 for boosting natural gas recovery efficiency and sequestration of CO2 in depleted shale gas reservoirs, the enhanced gas recovery (EGR) approach has recently attracted the attention of researchers. To analyze the viability of the increased gas recovery technique, many published studies were reviewed based on theoretical, experimental settings, and simulation models in this manuscript. The underlying link between geological and petrophysical factors is discussed, as well as how they affect CO2 and CH4 sorption. According to numerous studies, 30-55 percent of the CO2 injected into the shales is adsorbed on the pores surface of the rock matrix, resulting in CH4 desorption and additional natural gas recovery of 8-16 percent. For the application in diverse shales reservoir conditions, the best fit adsorption models (Langmuir, Ono Kondo, and D-A) were summarized. The theoretical findings of this work are anticipated to add to current studies on CO2 adsorption and sequestration, as well as CH4 desorption characteristics and the myriad simulation studies have revealed that well spacing, fracture permeability, injection pressure and strategies are key consideration for effective field demonstration for CO2-EGR projects. Despite the availability of theoretical explanations, experimental verification, and modeling findings, field-scale trials remain limited due to the risk of CO2-CH4 mixing and the high cost of capturing, purifying, and re-injecting CO2 into depleted reservoirs. Furthermore, the unpredictable heterogeneity of the shale formation still poses challenges on the gas recovery. The setbacks and limitations highlighted in this study will encourage academia and researchers to conduct more research into appropriate EGR technologies and their economic implications.