Solvent SAGD hybrid processes have attracted considerable attention in recent years. The perceived benefits of solvent addition to steam in SAGD are higher oil rate, lower energy and water consumption, higher recovery by lowering residual oil saturation (Sor) and higher return on investment. Despite numerous investigations that have been published regarding different aspects of solvent SAGD processes, this hybrid process is poorly understood and the solvent effects are difficult to predict. In fact, there is no available theory to model to the transport phenomena and the role of solvent within the steam chamber. Numerical simulation studies typically model the viscosity reduction of bitumen by solvent dissolution but do not capture other plausible mechanisms that yield higher oil rate and recovery, for example, lowering of Sor or partial in-situ upgrading. Laboratory experiments at realistic reservoir conditions are needed to gain more insight into these hybrid processes.
This paper presents the results of a series of laboratory experiments for evaluation of solvent addition to SAGD. These experiments were conducted at different representative reservoir pressure in a 3-D scaled physical model. Hexane, which has shown the best performance in many studies, was co-injected as solvent with steam in these experiments. Oil rate, recovery, and steam oil ratio were compared and the hybrid solvent/SAGD process performance was evaluated at different operating conditions. Additionally post-test sand samples were extracted from the model to examine residual oil saturation in different parts of the model after each experiment.
Experimental results showed improved performance of SAGD with addition of hexane, both at high and low operating pressure. However, the impact of hexane on the shape of the steam chamber and distribution of residual oil was significantly affected by operating pressure. This behavior of hexane, which appears to be related to its phase behavior, shows that solvent SAGD processes are considerably more complex than first thought.