Dynamic Modelling of Solution-Gas Drive in Heavy Oils

Abstract

Abstract Some heavy oil reservoirs in Canada show atypically high production rates and high primary oil recoveries under solution-gas drive. Much attention has been given to the anomalous behaviour observed in such heavy oil reservoirs, and several models have been suggested to explain these anomalies. There are two classes of effects responsible for the unusual behaviour of solution-gas drive in heavy oil reservoirs, including fluid effects and rock/geomechanical effects. This study focuses exclusively on fluid effects. In at least two ways, solution-gas drive in heavy oils differs from that in light oils. In heavy oils, the concentration of gas in the oil can be significantly greater than the equilibrium value; the oil could be significantly supersaturated. Additionally in heavy oils, recovery and gas mobility show rate dependent behaviour. Both of these effects are taken into consideration in this study. In this paper, we develop a dynamic model that captures many important processes that affect heavy oil recovery. The non-equilibrium early time behaviour is modelled by introducing a kinetic equation describing the rate of evolution of solution gas into free gas. The equation is derived based on a phenomenological analysis which takes into account bubble nucleation and growth. A second component of this model captures the low gas mobility in heavy oil reservoirs and its dependency on viscous forces. To account for the effect of viscous forces on gas mobility, relative permeability functions are introduced that not only depend on gas saturation but also on local oil phase velocity and viscosity. While many of the previous models apply several kinetic equations associated with a large number of parameters, we have shown that the modifications suggested in this study enable predicting many of the unusual behaviours observed in solution-gas drive in heavy oil reservoirs, using only one kinetic equation with a smaller number of fitting parameters.