The Effect of Pressure Decline Rate and Pressure Gradient on the Behaviour of Solution Gas Drive in Heavy oil

Abstract

Abstract Heavy oil recovery under solution-gas drive is affected by several interacting factors. Among such factors are pressure decline rate and pressure gradients. It has been suggested that a high pressure decline rate (dp/dt) generates larger super- saturation and faster nucleation that leads to more dispersed gas bubbles, while a high pressure gradient (∇p) increases the viscous forces acting on the gas phase enhancing bubble break up and gas dispersion. Both effects lead to lower gas mobility affecting oil recovery; however the relative importance of each is not known. Finding this is important for development of mathematical models and to allow extrapolation of experimental results to field conditions, where the relative importance of these factors changes with time and space. Previous experimental studies were affected by a combination of the two effects. In this paper we distinguish between the effect of pressure decline rate and pressure gradient on gas mobility and oil recovery, by varying these independently. In the experimental work reported in this paper, change in confining pressure is used to create a change in pressure decline rate, and a change in production rate is used to change the pressure gradient. Several depletion experiments at varying pressure decline rate and production rate are reported here. At a constant pressure decline rate, the recovery factor tripled when the flow rate was increased by one order of magnitude. Similar experiments were conducted when the pressure decline rate was increased by one order of magnitude but flow rate was kept constant. In this case, the recovery factor did not change significantly. The results of this study clearly indicate that the effect of pressure gradient on gas mobility and oil recovery is much more than the effect of pressure decline rate. This paper presents the experimental results and their analysis, and the implications of these findings on modelling of solution-gas drive in heavy oils.