Effects of CO₂ Addition to Steam on Recovery of West Sak Crude Oil

A high pressure one-dimensional laboratory displacement study was undertaken to evaluate the effects of the addition of carbon dioxide to steam on the recovery of West Sak crude oil. In addition a run was made below bubble point pressure to assess the effect of a free gas phase on recovery. Experiments were conducted in an unconsolidated sand pack two inches in diameter and four feet in length. The sand pack was saturated with "live" West Sak crude oil (19.2°API) with a bubble point pressure of 1690 psig. Volumetric flow rate was held constant to focus on the effect of CO₂ addition to steam. The temperature profile and pressure drop along the length of the sand pack were recorded along with ultimate recovery and effluent properties.

Carbon dioxide affects oil recovery by several mechanism. (Gas displacement, swelling, viscosity reduction.) Measurements of: CO₂ swelling and CO₂/steam distillation of West Sak crude are presented. The results indicate that the addition of CO₂ steam increases ultimate recovery, increases rate of recovery and decreases required heat input. Furthermore the presence of a free gas phase greatly reduces the ultimate recovery. The results also indicate the existence of an optimum steam/CO₂ concentration. A 1:1 molar ratio increases recovery by 5.5% and a 3:1 molar ratio increases recovery by 12 .8%. Holding the steam/CO₂ molar ratio constant at 3:1 and decreasing the injection temperature resulted in no significant change in recovery.

The current study indicates that the simultaneous injections of CO₂ and steam increases recovery over steam alone and allows the use of lower injectant temperature and hence reduces the heat input required. If a suitable source of CO₂ is available, this process may enhance the economics of thermal recovery of West Sak oil.