Viscous Fingering Effects in Solvent Displacement of Heavy Oil

Abstract A number of solvent-based processes for the recovery of heavy oil have been proposed in recent years. One of the phenomena that characterizes all such processes, to varying degrees, is viscous fingering. This paper describes the results of a combined experimental/simulation study aimed at characterizing viscous fingering under conditions typical of heavy oil recovery (very high ratios of oil to solvent viscosity). The study also sheds light on other phenomena that are part of such processes. We describe a set of four experiments carried out in heavy oil saturated sand packs contained within a 30 cm x?60 cm x?1.4 cm visual cell. Three of the experiments involved injection of a miscible, liquid solvent at the bottom of the sand pack, with subsequent upward displacement of the heavy oil; the fourth involved top-down injection of a gaseous solvent. The miscible liquid displacements were dominated by a single solvent finger, which broke through quickly to a producing well at the other end of the sand pack. Observed breakthrough times were consistent with a correlation that describes reported results at lower viscosity contrast. The gaseous solvent experiment exhibited fingering but also had features of a gravity-driven VAPEX process in its later stages. Numerical simulations using a commercial reservoir simulator have been successful in reproducing key features of the experiments. Realistic fingering patterns are produced in the simulations by assuming small, random spatial variations of permeability. The correct modelling of dispersion is crucial in matching the observed phenomena. For gaseous fingering and VAPEX processes, capillary effects are significant and should be included in simulations. Introduction Solvent-based processes for the recovery of heavy oil have attracted increasing attention in recent years. Much of this attention has focused on the Vapour Extraction or "VAPEX" process(1), a solvent analogue of steam assisted gravity drainage (SAGD). However, it has been suggested that for thin reservoirs, and particularly primary-depleted reservoirs, a cyclic solvent process might be preferred. Whereas VAPEX is analogous to SAGD, a cyclic solvent process would be analogous to the cyclic steam stimulation process. A concept for a cyclic process is shown in Figure 1. In this concept, solvent would be injected for a period of time, then oil produced from the same well; and this process would be repeated. A number of questions may be asked about the basic mechanisms of a cyclic solvent process and the resulting efficiency of oil recovery. The work reported here was aimed particularly at understanding the phenomenon of viscous fingering, which characterizes any such process in which a low viscosity solvent is injected into a high viscosity oil. Viscous fingering is an instability phenomenon which occurs when one fluid is displaced by another fluid of lower viscosity. The displacing fluid is said to "finger" into the resident fluid. The two fluids may be either miscible or immiscible, and the displacement may take place in a porous medium or even a Hele- Shaw cell(2, 3).