Adsorption/Desorption Behavior of CH4 on Shale during the CO2 Huff-and-Puff Process

The CO 2 huff-and-puff method has been widely adopted for enhancing CH 4 recovery in shale reservoirs. Revealing the behavior of CH 4 adsorption/desorption in shale during the CO 2 huff-and-puff process clarifies the recovery mechanisms of CH 4 from shale reservoirs. In our work, the question of how CO 2 plays a role in affecting the adsorption/desorption of CH 4 is investigated using the low-field nuclear magnetic resonance technique. In addition, phase transition of CH 4 is also analyzed to investigate how the existing states of CH 4 transform in shale during this process. Specifically, the states in which CH 4 resides in shale are first recognized by analyzing the measured T 2 spectrum of shale after injecting CH 4 . CO 2 huff-and-puff tests are then conducted to investigate how CO 2 impacts the adsorption/desorption behavior of CH 4 on shale samples. Furthermore, the T 2 signals of shale during depressurization are measured to investigate the state transformation of CH 4 in shale during the CO 2 huff-and-puff process. Test results show that three states are observed for CH 4 storage in shale samples, that is, bulk CH 4, free CH 4 at the pore center, and adsorbed CH 4 on the pore surface. After injecting CO 2, the adsorbed CH 4 will be desorbed from the shale surface, which thus increases the free CH 4 at the pore center. During depressurization, the free CH 4 is more readily produced from the shale samples, whereas the adsorbed CH 4 is hard to be recovered; more advanced technology should thereby be proposed for enhancing the adsorbed CH 4 from shale reservoirs. This work is expected to inspire new understanding of the mechanisms of CH 4 recovery using CO 2 huff-and-puff methods.