Heavy oil recovery by liquid CO2/water injection

Alaska's North Slope reservoirs contain a massive heavy oil resource. There has been some success producing the less viscous crudes in the West Sak formation by waterflooding and water-alternating-gas injection. CO2 injection could also have potential applications as an enhanced oil recovery (EOR) process with the added benefit of providing a solution to the problem of produced CO2 present in the associated gas. CO2 is generally in supercritical state in normal reservoir conditions. However, permafrost causes an unusually low reservoir temperature and as a result CO2 will be in liquid state for these heavy oil fields.

In this study we consider West Sak reservoir and investigate different injection strategies in which available water and CO2 can be utilized individually or combined for EOR and CO2 storage purposes. A three-phase, three-dimensional, black oil simulator was constructed accounting for the oil swelling and viscosity reduction due to dissolution of liquid CO2.

The results indicate that a combination of water and CO2 injection could be an attractive recovery method from the West Sak with higher oil recovery obtained when CO2 and water injection carried out early in the production life of the reservoir. If depletion precedes CO2 injection, CO2 storage is at its maximum. The recovery under liquid CO2 injection was almost the same as that of water flood due to its reduced mobility compared to CO2 gas. Water-Alternating-CO2 injection and Simultaneous-Water-CO2 injection showed significant improvement compared to water or CO2 injection alone. At Water-Alternating-CO2 (volumetric) ratio less than one, lower CO2 is required and higher CO2 storage per standard barrel of produced oil is obtained if CO2 is injected at higher rate for shorter period. Whist at Water-Alternating-CO2 ratio greater than one, reducing duration of CO2 injection favoured oil recovery but lowered CO2 retention factor compared to lowering CO2 injection rate.