Preliminary Study of Influence Factors and Estimation Model of the Enhanced Gas Recovery Stimulated by Carbon Dioxide Utilization in Shale

Carbon dioxide (CO2) injection into shale enables enhanced gas recovery (EGR) associated with geological CO2 sequestration. Although primary research has been conducted on the CO2-based EGR technique in shale, the factors that influence the EGR efficiency remain unclear and need to be examined. This study presents a novel nuclear magnetic resonance (NMR)-based methodology to measure the EGR efficiency caused by CO2 injection into shale samples with various properties. Accordingly, the effects of shale properties on the CO2-based EGR efficiency were revealed, and a calculation model for estimating the EGR efficiency was established. The results indicated that CO2 injection enables significant production enhancement of shale gas, with the EGR efficiency averaging 23.54% and ranging from 16.22 to 34.34%, thus indicating that the EGR efficiency varies with the shale properties. The results also indicated that a higher CO2-based EGR efficiency, while adhering to a higher CO2-sequestration capacity, usually occurs in shales with a higher content of total organic carbon, higher methane-adsorption capacity, lower permeability, and lower clay mineral content. Moreover, an estimation model is developed to forecast the CO2-based EGR efficiency according to the shale properties. In general, these far-reaching results are of significance for predicting the benefit of CO2 utilization in different shale reservoirs.