Steam injection is a widely used oil recovery method that has been commercially successful in many types of heavy oil reservoirs, including oil sands of Alberta. Steam is very effective in delivering heat that is the key to heavy oil mobilization. In the distant past, and also recently, solvents are being used as additives to steam for additional viscosity reduction. This was done previously in California heavy oil reservoirs also. The current applications are in SAGD (Steam-Assisted Gravity Drainage) and CSS (Cyclic Steam Stimulation) field projects.
The past and present projects using solvents are reviewed, and evaluated viz ES-SAGD (Enhanced solvent SAGD) and LASER (Liquid Addition to Steam for Enhancing Recovery). The theories behind the use of solvents with steam are outlined. These postulate (1) additional heavy oil mobilization; (2) oil mobilization ahead of the steam front, and (3) oil mobilization by solvent dispersion due to frontal instability. The plausibility of the different approaches is discussed.
Recent theoretical work is described that compares thermal and solvent diffusion, showing that the time scales of the two processes are quite different casting doubt on the effectiveness of the use of solvents with steam. The numerical and analytical solutions have been compared for effect of cold solvent, hot solvent, steam only, and co-injection of solvent and steam on bitumen mobilization.
The outcome of this study can be used for better understanding of mechanisms and theories behind co-injection of solvent with steam.