Recovery efficiency of a 28°API crude-oil system as a function of voidage replacement ratio

Crude-oil recovery processes with a voidage replacement ratio (VRR = injected volume/produced volume) less than 1 combine solution gas drive mechanisms and viscous crude-oil displacement. Previous laboratory and field studies have shown that a VRR less than 1 for a period of time is optimal for recovery of some viscous and heavy crude oils that form emulsions and/or bubbly oil in situ. This study extends understanding of the effect of VRR to recovery of live, less-viscous crude oils from sands. The crude oil used is roughly 28 °API, has a dead-oil viscosity of 26 cP, a live-oil viscosity of 0.41 cP, and an asphalt content of 2.1 wt%. It is found in the laboratory that the final crude-oil recovery (70.5%) using a 0.01 PV/h depletion rate and VRR equal to 0.7 is more than 8% greater than recovery with VRR equal to 1 (65.1%). Recovery using a 0.001 PV/h depletion rate with VRR equal to 0.7 (79.1%) was nearly the same as the sum of oil recoveries for solution gas drive (VRR = 0, 24.5%) plus waterflooding (VRR = 1, 53.4%). For these two depletion rates, the oil recovery obtained using VRR equal to 0.7 is more favorable than VRR = 1 (waterflooding). Relative permeability was inferred from experimental results. Substantial critical gas saturations (8.1–16.1%) also contributed to oil production. A concave shape of the oil/water relative permeability curve indicated an emulsion-like effect that also promoted recovery when the VRR was less than 1.