The role of injection fluid elasticity in microscopic displacement efficiency of residual non-wetting phase: An in-situ experimental investigation

Abstract

Polymer flooding has been employed extensively over the years as a mature enhance oil recovery (EOR) technique to improve volumetric displacement (sweep) efficiency in oil reservoirs. In the last few years, groundbreaking laboratory and pilot tests have demonstrated that the use of polymers with viscoelastic properties may potentially enhance the efficacy of polymer flooding and result in improvements in both volumetric (macroscopic) and microscopic (pore-scale) displacement efficiencies. The transport and pore-level displacement mechanisms responsible for such microscopic improvements, however, have been poorly understood. This study was thus designed to improve the current understanding of the displacement of waterflood residual oil with viscoelastic polymers in natural porous media. We performed a series of micro-scale two-phase core-flooding experiments on miniature water-wet Berea sandstone samples at elevated pressure. During the flow tests, heavy oil was displaced with two Newtonian fluids (brine and Glycerin) and a viscoelastic polymer to investigate the impact of injection fluid elasticity on mobilization of capillary-trapped oil. We purposely selected a viscoelastic polymer that had a similar viscosity, but significantly different elastic characteristics compared to those of Glycerin. The rheological behavior of the fluids was characterized using an oscillation rheometer. The flow experiments were conducted using a miniature core-flooding setup coupled with a high-resolution micro-computed tomography (µ -CT) scanner enabling us to directly visualize morphologies of oil globules in the pore space. The core sample initially saturated with brine was subjected to primary oil drainage to establish initial water saturation (Swi) and then a waterflood to establish remaining oil saturation. This process was then followed by (i) a Glycerin flood and (ii) a viscoelastic polymer flood, all under low pressure gradients (i.e., capillary numbers of approximately 1E-5). The injection of Glycerin, after the waterflood, reduced the remaining oil saturation from 45.8 to 38% due to a favorable mobility ratio, which was believed to be the reason for the improvement in volumetric sweep efficiency. Furthermore, in-situ saturation data revealed that the use of the viscoelastic polymer resulted in an additional 12% oil recovery. Since both the viscoelastic polymer and Glycerin had similar mobility ratios, their volumetric sweep efficiencies were believed to be comparable, and hence the observed incremental oil recovery was attributed to additional pore-scale oil mobilization caused by the injection of the viscoelastic polymer. This mobilization mechanism is triggered by elastic forces and is not due to dynamic effects generated usually under high capillary numbers. Moreover, investigation of pore-scale fluid occupancy maps provided clear evidence of fragmentation and mobilization of trapped oil clusters as the mechanisms responsible for the observed enhancement in recovery. The fragmentation and mobilization of the oil globules were related to high elastic properties of the viscoelastic polymer used. Finally, we present a new correlation linking the reduction in residual oil saturation to the elasticity of the injected fluid.