Mobility and conformance control for carbon dioxide EOR—Part 2 ///
The density of carbon dioxide at CO2-EOR reservoir conditions is 0.4–0.8 g/cm3, which is typically less than oil and brine, resulting in CO2 gravity override. The viscosity of CO2 at typical EOR conditions is 0.04–0.08 cP, which is very low relative to brine and oil, and promotes viscous fingering, exacerbates gravity override, and leads to early breakthrough and high ratios of CO2 to produced oil. The CO2 density cannot be significantly increased at CO2-EOR operating conditions. However, several strategies greatly reduce CO2 mobility—most commonly, the water-alternating-gas (WAG) process. The WAG process increases water saturation and decreases CO2 saturation, thereby reducing the relative CO2 permeability. Field studies where 80% hydrocarbon pore volume CO2 had been injected indicated that WAG floods leave behind about one-third to two-thirds of the oil remaining after waterflooding.1
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