Microchannel formation driven by instability in heavy oil chemical cold production

Rapid water-cut increases following chemical injection remains a major challenge in heavy-oil cold production. Although viscosity reducers can effectively mobilize heavy oil, they frequently induce premature water breakthrough and severe sweep inefficiency. In this study, the mechanism of spontaneous microchannel formation during viscosity-reducer flooding was investigated through a combination of linear stability analysis and two-dimensional (2D) physical simulation experiments. A modified instability model was developed by incorporating the effects of viscosity reduction, interfacial-tension attenuation, and fractional-flow variances by chemical agents. The results indicated that the heavy-oil displacement system was unstable to perturbations longer than a centimeter-scale cutoff wavelength, whereas the light-oil system was destabilized only above a meter-scale cutoff. Consequently, even centimeter-scale heterogeneities could evolve into dominant preferential-flow pathways during heavy-oil displacement. To validate the theoretical prediction, a macroscopically homogeneous 2D sand-packed model with a five-spot well pattern was constructed to eliminate the influence of large-scale reservoir heterogeneity. Despite the macroscopic homogeneous porous medium, viscosity-reducer flooding resulted in pronounced production divergence among the four producers, characterized by early water breakthrough and reduced recovery along the dominant flow direction. Additional experiments demonstrated that the introduction of a profile-control agent effectively suppressed channel development, redistributed injected fluids toward unswept regions, and significantly improved production uniformity. The combined viscosity-reduction and profile-control strategy transformed the displacement behavior from channeling-dominated flow to volumetric sweep. The results established a mechanistic understanding of instability-driven microchannels in heavy-oil cold production and provided a theoretical basis for designing chemical flooding schemes that simultaneously improve oil mobility and control flow distribution.