The Effect of Pressure-Decline Rate and Pressure Gradient on the Behavior of Solution-Gas Drive in Heavy Oil

Abstract

Summary Heavy-oil recovery under solution-gas drive is affected by several interacting factors including pressure-decline rate and pressure gradients. It has been suggested that a high pressure decline rate (dp/df) generates larger supersaturation 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 to develop 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 the 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 rates and production rates 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 the flow rate was kept constant. In this case, the recovery factor did not change significantly. The results of this study clearly indicate that the pressure gradient has a much greater effect on gas mobility and oil recovery than pressure-decline rate has. This paper presents the experimental results and their analysis, along with the implications of these findings on modeling of solution-gas drive in heavy oils.