Abstract
The applications of carbon dioxide sequestration in brine-saturated reservoirs, as well as recovery of oil from oil-wet reservoirs, involve the injection of a less- viscous, non-wetting fluid into a porous medium occupied by a more-viscous, wetting fluid: i.e., drainage with an unfavorable viscosity ratio. In these cases, there is a competition between capillary fingering and viscous fingering. In standard treatments of two-phase flow in porous media, the flow is assumed to be compact, with a uniform residual saturation behind a front, which advances linearly with time. This view of two phase flow is inconsistent with the cases of fractal capillary fingering at zero capillary numbers, and fractal viscous fingering resulting from injection of an inviscid fluid. Earlier work has shown that when the flow characteristics are not precisely at their fractal limit, the fractal fingering behavior crosses over to standard behavior at a characteristic time which is inversely related to the distance of the flow characteristics from their fractal limit. This earlier work was limited to crossover from one type of fractal fingering to standard flow. We present results from pore-level modeling for a range of capillary numbers and unfavorable viscosity ratios. The results are analyzed to determine how the competing capillary and viscous fingerings cross over to compact flow. These results are compared with predictions of a scaling hypothesis based upon experience with the aforementioned, simpler fractal-to-compact crossovers from one type of fractal fingering to standard/compact flow.
Published Version
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