SCALED MODEL STUDIES OF HEAVY OIL-RECOVERY FROM AN ALASKAN RESERVOIR USING GRAVITY-ASSISTED GAS INJECTION

Gas injection is one of the oldest enhanced oil recovery techniques used for recovering light oils. Recently, there has been renewed interest of gas injection in both light and heavy oil reservoirs. This recovery technique is particularly of interest for the heavy oil reservoirs of Alaska where the application of thermal techniques is limited due to reservoir depth and the existence of permafrost. It has been pointed out that, in order to apply gas injection, gravity stabilization is required. In absence of a natural dip, such gravity stabilization may be achieved through horizontal wells placed on top of the reservoir while producing through another horizontal well placed at the bottom of the reservoir. The possibility of such recovery technique has been investigated in this study. A series of scaled model studies has been conducted using nitrogen, propane and carbon dioxide as injection fluids. The West Sak reservoir of North Slope, Alaska is used as the prototype for the present modeling study. Results indicate that close to 65% of the oil in place may be recovered by using gas injection from the top of a heavy oil reservoir. Even though similar ultimate recovery is obtained with gravity drainage alone, the time required to reach such a recovery is extremely long. However, these experimental runs are sensitive to the rate of gas injection. Experimental results suggest that there is an optimal gas injection pressure for nitrogen. Recovery mechanism is found to be different with propane than with nitrogen. However, the oil recovery remains higher with propane for all the cases studied. The highest recovery is obtained with carbon dioxide. In all experiments, profuse viscous fingering takes place with different degrees of severity. This is found to be in agreement with a previously published stability theory. Viscous instability is considered to be harmless as final recovery is very high for all the cases. Final recovery with gas injection is significantly higher than the one with a waterflood.