Experimental and numerical study on supercritical CO_2/brine transport in a fractured rock: Implications of mass transfer, capillary pressure and storage capacity

摘要

This study presents the impact of fractures on CO_2 transport, capillary pressure and storage capacity by conducting both experimental and numerical studies. A series of laboratory experiment tests was designed with both a homogeneous and a fractured core under CO_2 storage conditions. The experimental results reveal a piston-like brine displacement with gravity override effects in the homogeneous core regardless of CO_2 injection rates. In the fractured core, however, two distinctive types of brine displacements were observed; one showing brine displacement only in the fracture whereas the other shows brine displacement both in the fracture and matrix with different rates, which were dependent on the magnitude of the pressure build-up in the matrix. The injectivity in the fractured core was twice of the homogeneous core, while the amount of calculated CO_2 in the homogeneous core was over 1.5 times greater than the fractured core. Salt precipitation, which is likely to occur near injection wells, was observed in the experiments; X-ray images enabled the observation of salt-precipitation during CO_2-flooding tests. Finally, numerical simulations predict free-phase CO_2 transfer between fracture and matrix in a fracture-matrix system. Pressure gradients between the fracture and matrix enforced CO_2 to transfer from the fracture into matrix at the front of the CO_2 plume, whereas, the reversal of pressure gradients at the rear zone of the CO_2 plume reversed the transfer process. The variation of CO_2 saturation within the fracture was caused by fracture aperture variations, and local variations of fracture permeability control the free-phase CO_2 transfer between the fracture and matrix.