Mathematical modeling of dynamic mass transfer in cyclic solvent injection

In a cyclic solvent injection (CSI) process, a vaporized solvent is cyclically injected, soaked, and released to produce heavy oil. Through this process the operating pressure is increased and decreased periodically. This causes a dynamic fluid flow across a heavy oil-solvent mixing zone, leading to a convective mass transfer in addition to molecular diffusion. This study aims at modeling the effect of convection on the heavy oil-solvent mass transfer in the transition zone of the CSI process. A convection-diffusion mathematical model is developed and semi-analytically solved. It consists of two sub-models, a pressure model and a solvent concentration model, and considers realistic viscosity and convection velocity profiles. Three practical injection and production schemes are modeled in order to provide some meaningful suggestions for the CSI field applications from the viewpoint of mass transport. Results show that convection can significantly enhance solvent dissolution during the injection period and accelerate solvent exsolution during the production period. In addition, it is also found that a smaller solvent injection rate does not make a big difference in the solvent dissolution for a long injection period. Moreover, during the production period, solvent exsolution induces a sharp pressure gradient that is believed to be a key factor for foamy-oil flow. It causes a viscosified front and solvent "inverse" diffusion that results in several rounds of foam-oil flow. Finally, a considerable amount of solvent remains dissolved in heavy oil at the end of the production period when the reservoir pressure is depleted, and is difficult to be retrieved in a short time.