Sweep Efficiency of Miscible Floods in a High-Pressure Quarter-Five-Spot Model

Abstract

Summary Evaluation and improvement of sweep efficiency are important for miscible displacement of medium-viscosity oils. A high-pressure quarter-five-spot cell was used to conduct multicontact miscible (MCM) water-alternating-gas (WAG) displacements at reservoir conditions. A dead reservoir oil (78 cp) was displaced by ethane. The minimum miscibility pressure (MMP) for ethane with the reservoir oil is approximately 4.14 MPa (600 psi). Gasflood followed by waterflood improves the oil recovery over waterflood alone in the quarter five-spot. As the pressure decreases, the gasflood oil recovery increases slightly in the pressure range of 4.550–9.514 MPa (660–1,380 psi) for this undersaturated viscous oil. WAG improves the sweep efficiency and oil recovery in the quarter five-spot over the continuous gas injection. WAG injection slows down gas breakthrough. A decrease in the solvent amount lowers the oil recovery in WAG floods, but significantly more oil can be recovered with just 0.1 pore volume (PV) solvent (and water) injection than with waterflood alone. Use of a horizontal production well lowers the sweep efficiency over the vertical production well during WAG injection. Sweep efficiency is higher for the nine-spot pattern than for the five-spot pattern during gas injection. Sweep efficiency during WAG injection increases with the WAG ratio in the five-spot model.