Polymer Flooding in Oil-Wet, 2D Heterogeneous Porous Media

Abstract

The displacement of viscous oils typically involves unstable immiscible flow. The microscopic and volumetric efficiency is further exacerbated if the reservoir is oil-wet and heterogeneous, respectively. The goal of this work is to systematically compare the performance of secondary vs. tertiary polymer flooding for viscous oil recovery in an oil-wet 2D, layered, heterogeneous system. It focuses on performing flow visualization to capture the effect of cross-flow and viscous fingering in both modes. First, contact angle experiments were performed to ensure that the reservoir crude oil results in oil-wet sand. Second, rheological analysis of HPAM polymer solution was performed to find the optimal injection concentration. Third, oil displacement experiments were performed in an in-house, custom-built 2D sandpack with the front face made of a transparent acrylic sheet for flow visualization. It was packed with two communicating layers of silica sand — bottom layer with 20-30 mesh and a top layer with 100-120 mesh, which resulted in a permeability contrast of 8:1. The system was vacuum-saturated with a viscous crude oil with a viscosity of 157 cp. Polymer floods were conducted in secondary and tertiary modes and the oil displacement profiles were continuously monitored using a camera. At the end of the experiments, the sandpacks were cut in 16-equal zones and were analyzed for the amount of crude oil using UV-Spectroscopy to quantify the residual oil saturation achieved in each zone. Finally, the results were compared with analogous floods in 1D sandpacks to understand the effect of heterogeneity. The contact angle experiments revealed that the reservoir crude oil used in the present work resulted in highly oil-wet sand after aging. In the oil displacement experiments in the layered sandpack, the secondary waterflood recovery after 1 PV was low (∼25% OOIP) due to channeling in the bottom high-permeability region, leaving the top low-permeability region completely unswept. Tertiary polymer flooding leads to improvement in sweep efficiency in both regions. It resulted in an incremental oil recovery of 53% OOIP with an ultimate recovery of 78% OOIP. Conversely, polymer flooding in secondary mode resulted in 46% OOIP in 1 PV injection. But the overall recovery was 69 % OOIP which was less than the tertiary mode. Different flow phenomena, such as, cross-flow, gravity segregation, and viscous fingering, were observed in these visualization experiments.