Experimental study on pressure control strategies for improving waterflooding potentials in a heavy oil-methane system

Abstract

Due to the adverse mobility ratio conditions in heavy oil waterflooding processes, the viscous force is not as significant as that in conventional waterflooding in terms of enhancing oil recovery. Instead, pressure related mechanisms, such as foamy oil flow and solution gas drive, might have significant effects on waterflooding performances when an operating strategy is beneficial (e.g. voidage replacement ratio less than one).In this study, a series of waterflooding tests are performed to study the beneficial operating strategies in a heavy oil-methane system from the pressure control aspect. The pressure control strategies include primary-ended waterflooding (Strategy 1), primary-plus waterflooding (Strategy 2), pseudo-continuous injection with cyclic production (Strategy 3), and cyclic injection with cyclic production (Strategy 4). The depletion rates include 0.45kPa/min, 1kPa/min, 2kPa/min, and 3kPa/min.Experimental results show that the optimal pressure control strategy for the heavy oil-methane system is Strategy 2. The ultimate oil recovery and overall average production rate are inversely changed with the depletion rate increasing in Strategy 2. To get a balance between the ultimate oil recovery and overall average production rate, the optimal depletion rate, 2kPa/min, is obtained. By comparing the performances of Strategy 2 under different depletion rates, the relationship of voidage displacement ratio (VRR) with the depletion rate as well as the optimal VRR for waterflooding are obtained. Moreover, it is found that, with depletion rate increasing, positive effects of depletion rate decrease at foamy oil flow stage and increase at solution gas drive stage.