Effect of Oil Viscosity, Permeability and Injection Rate on Performance of Waterflooding, CO₂ Flooding and WAG Process in Recovery of Heavy Oils

The economic life of a typical heavy oil reservoir under primary or secondary recovery schemes can be short lived or near their limit, with recovery factors in the range of 5–15%. Waterflooding alone has been successfully practiced in the Lloydminster area of the Western Canada for decades. Heavy oils located in thick reservoirs have benefited from the application of thermal, gravity drainage processes; however, thin, unconsolidated heavy oil reservoirs are unsuitable due to lack of drainage height. These reservoirs may benefit from immiscible CO₂ or CO₂-WAG processes.

This paper examines the effect of oil viscosity, permeability and injection rate on the performance of heavy oil waterflooding, immiscible CO₂ flooding and immiscible CO₂ water-alternating-gas (WAG) processes. A series of 11 sand packfloods were conducted using 440 and 1,500 mPa·s heavy oils and sand packs with absolute permeabilities of 12 and 40 μm². Water injection volumes for the waterfloods were 1.5 pore volumes (PVs) at rates of 0.112, 1.124, and 5.62 cc/min. For the CO₂ flooding process, 4.5 PVs of gas were injected at a rate of 1 cc/min. For the WAG process, the water:CO₂ slug ratios were varied from 1:1, 1:2 and 2:1. A 99.9% purity CO₂ gas stream was used for all gas floods. All experiments were performed at a controlled temperature of 25°C and 345 kPa.

Among the 11 sand packfloods conducted, the waterfloods consistently yielded the highest recovery factor for both heavy oils and sand packs, with 48-52% OOIP recovered and most of this recovery occurring during 0.112 cc/min. During CO₂ flooding of the 440 mPa·s oil, 48.5% OOIP was produced from the 40 μm² sand pack. From the same fluid-sand system, a 1:1 slug ratio, CO₂-WAG process produced 42% OOIP. For the 1,500 mPa·s heavy oil, 1:2 and 2:1 slug ratios of CO₂-WAG both produced ~25% OOIP from the 40 μm² sand pack and a 1:1 slug ratio produced 35% OOIP from the 12 μm² sand pack. All WAG injection schemes were compared on a 2.5 PV injected basis. These results suggest that the role of displaced fluid viscosity plays the most prominent role in the recovery of heavy oil.