CO2/N2 injection into CH4 + C3H8 hydrates for gas recovery and CO2 sequestration

Abstract

CO2 injection into CH4 hydrate (sI) is a promising method for simultaneous energy production and global warming mitigation. In fact, the natural abundance of sII hydrates have been also confirmed. The gas exchange kinetics in sII hydrates were significantly different from those in CH4 hydrate. In this work, the injection of CO2 with or without N2 into CH4-C3H8 hydrate-bearing sediments was experimentally investigated to explore gas recovery and CO2 sequestration during the replacement. The results indicated that the CH4-CO2 exchange in large cages was always controlled by gas diffusion through newly-formed mixed hydrate layer. Differently, the CH4-N2 exchange in small cages changed from gas exchange-controlled to gas diffusion-controlled with the increase of N2 partial pressure. It was found that the recovery of CH4 was much more continuous than that of C3H8 in gaseous CO2/N2 replacement while CH4 and C3H8 were released synchronously in liquid CO2 replacement. CH4-C3H8 hydrate particles soaked in liquid CO2 may undergo almost complete structural transitions from sII to sI hydrate rich in CO2. The analysis of heterogeneous compositions of the replaced hydrates indicated that the presence of N2 may have inhibition effects on the structural transition of CH4-C3H8 hydrates. With the injection of gaseous CO2/N2, there was only a surface structure-transition followed by an iso-structural conversion in the deeper layers of hydrate particles. The structural transition played an important role in the replacement of sII hydrate, which made sII gas hydrate reservoirs to be promising target regions for efficient gas production and CO2 sequestration.