Effect of Gas Dissolution on Heavy Oil Well Performance: Rheology Considerations

Abstract Crude oils may exhibit shear rate dependent viscosity under reservoir temperature and pressure conditions due to many factors such as wax content, asphaltenes, solids, and water droplets. In a previous experimental study we investigated the shear rate-shear stress relationship for heavy crude oil using capillary viscometer. The study showed mildly non-Newtonian behavior of heavy oil and the rheological power-law parameters (flow behavior index and flow consistency) depend on the extent of gas dissolution and at higher gas dissolution the fluid flow behavior index approaches unity. Impact of non-Newtonian flow behavior on oil inflow performance can be significant. In this study, a two dimensional unsteady state model is developed to represent the multiphase flow behavior of heavy oil, accounting for the non-Newtonian behavior and the model is solved using a fully implicit finite difference scheme. In essence, the approach is that of a traditional black oil type of reservoir modeling, but with the viscosity of the heavy oil represented by a power law model. Power law parameters for saturated oil at high temperature and different pressures, obtained from experiments in a companion study are used in modeling the rheology of heavy oil. Modified Darcy’s law is used to model non-Newtonian flow of heavy oil through porous media. Results of the numerical solution are validated with analytical solution of single phase non-Newtonian flow through porous media available in the literature. Non-Newtonian behavior under reservoir conditions can increase the pressure drawdown required for production by several times and hence affect the production. The well inflow performance calculated using the simulator shows that, as the exponent of the power law model decreases, the production flow rate decreases. Thus, in comparison to a Newtonian behavior, a power law behavior results in a poorer inflow performance. The results presented here may be used to explain higher recoveries in heavy oils which display non-Newtonian behavior. Additionally, strategies for production optimization could be achieved by controlling the fluid rheology and hence the power law parameters by maintaining solution gas in the oil phase.