LAUSR

Abstract Water-saturated CO2 (wsCO2) injection into oil reservoirs has recently been proposed to provide better oil recovery and CO2 storage than can be obtained by pure CO2 injection. However, the interaction of wsCO2 with oil under different miscibility conditions and injection rates has not been investigated. We inject wsCO2 and pure CO2 at a "low rate" of 0.5 cc/min (to simulate gravity dominance) into Bentheimer sandstone cores at 70°C and at three pressures:14.5 MPa (2100 psia), 11.7 MPa (1700 psia), and 9.0 MPa (1300 psia) to represent miscibility, near-miscibility, and immiscibility, respectively. At near-miscibility, we also inject these fluids at a "high rate" of 2.5 cc/min in order to simulate gravity-viscous transition. The oil phase consists of a mixture of 0.65 hexane and 0.35 decane by molar fraction. For the low injection rate, at miscibility, the mobility ratio of wsCO2 injection is found to be 99% less than that of CO2 injection; at near-miscibility, 85% less; and at immiscibility, nearly the same. The greatly reduced mobility ratio at miscibility and near-miscibility yields higher oil recovery (4–11%), and higher net CO2 stored (4–9%) than that obtained by CO2 injection. However, at immiscibility, wsCO2 injection is found to yield 3% less oil recovery than does CO2 injection and to yield the same net CO2 storage as does CO2 injection. The high injection rate at near-miscibility also finds wsCO2 to be superior, although by a smaller margin. In summary, wsCO2 injection is found superior to CO2 injection only under miscible and near-miscible conditions.