Post-primary recovery from some mobile heavy-oil reservoirs in Western Canada cannot be improved using thermal methods due to environmental concerns and technical difficulties. Moreover, miscible gas injection suffers from low initial production rates, premature breakthrough, and possible, formation damage. Low-tension polymer flooding (LTPF) can be an alternative in these reservoirs. However, a major technical challenge in LTPF is that a fingered displacement front may occur. This instability reduces displacement efficiency and may invalidate normal method of simulating LTPF performance based on relative permeability and capillary pressure concepts. Also, it introduces an additional scaling requirement for using results of experimental tests in larger scales. Therefore, it is important to predict the nature of instability, to avoid viscous fingering, or, where it is inevitable, to be capable to include it as an additional factor in modeling displacement.

Previous experiments of viscous fingering in immiscible displacements have been conducted in presence of high single-phase permeabilities and linear displacement schemes. The question is whether previous findings are valid in displacement schemes similar to oil-field patterns (e.g., five-spot) in which one should deal with varying velocity profiles from injector(s) to producer(s). Hence, the effect of dispersion caused by varying velocity profiles has not been tested completely on viscous fingering.

To help understand viscous fingering in LTPF in heavy oil reservoirs and to overcome the aforementioned limitations, we conducted experiments in low-permeability, one-quarter five-spot patterns. Foremost parameters including oil recoveries at different times to breakthrough, ultimate oil recovery, pressure drops, cumulative saturation profiles, mean local saturations, fingers length and width, dynamic level of bypassing, dynamic population of fingers, rate of growth of population of fingers and number frequency of the fingers were measured. We have correlated some of these parameters with displacement time and front position.

Analysis of experimentally-observed fingering patterns of LTPF in this study is the most detailed interpretation performed to date, which provides new insights into the onset of fingering and fingers development. Results would help numerical simulation and stability theory to satisfactorily reproduce qualitative and quantitative features of finger growth.

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