CO2, N2, and CO2/N2 Mixed Gas Injection for Enhanced Shale Gas Recovery and CO2 Geological Storage

In this work, using fractured shale cores, isothermal adsorption experiments and core flooding tests were conducted to investigate the performance of injecting different gases to enhance shale gas recovery and CO2 geological storage efficiency under real reservoir conditions. The adsorption process of shale to different gases was in agreement with the extended-Langmuir model, and the adsorption capacity of CO2 was the largest, followed by CH4, and that of N2 was the smallest of the three pure gases. In addition, when the CO2 concentration in the mixed gas exceeded 50%, the adsorption capacity of the mixed gas was greater than that of CH4, and had a strong competitive adsorption effect. For the core flooding tests, pure gas injection showed that the breakthrough time of CO2 was longer than that of N2, and the CH4 recovery factor at the breakthrough time (RCH4) was also higher than that of N2. The RCH4 of CO2 gas injection was approximately 44.09%, while the RCH4 of N2 was only 31.63%. For CO2/N2 mixed gas injection, with the increase of CO2 concentration, the RCH4 increased, and the RCH4 for mixed gas CO2/N2 = 8:2 was close to that of pure CO2, about 40.24%. Moreover, the breakthrough time of N2 in mixed gas was not much different from that when pure N2 was injected, while the breakthrough time of CO2 was prolonged, which indicated that with the increase of N2 concentration in the mixed gas, the breakthrough time of CO2 could be extended. Furthermore, an abnormal surge of N2 concentration in the produced gas was observed after N2 breakthrough. In regards to CO2 storage efficiency (Sstorage-CO2), as the CO2 concentration increased, Sstorage-CO2 also increased. The Sstorage-CO2 of the pure CO2 gas injection was about 35.96%, while for mixed gas CO2/N2 = 8:2, Sstorage-CO2 was about 32.28%.