Historically a voidage replacement ratio (VRR) of 1 is assumed to be optimal for oil recovery regardless of whether recovery occurs from an unconventional heavy oil reservoir or a conventional oil reservoir. That is, it is assumed that of all scenarios, the most oil recovery occurs when the amount of fluids injected into the subsurface equals the amount produced. Recent work publications have analyzed both field and core-scale data to conclude that a VRR of 1 is suboptimal for certain viscous and heavy-oil reservoirs. In this work, we use numerical simulation to seek an understanding of the conditions under which a VRR of 1 is suboptimal. Models are core-size to enable better interpretation of mechanisms. We tested the sensitivity of the optimal VRR to the curvature of our relative permeability relationships (i.e., Corey exponent), the critical gas saturation, the three-phase flow model, potential chemistry of the oil, the statistics of permeability values, the connectivity of low and high permeability regions, and the reference scale at which results are compared. Realistic relative permeability curves based off rock and fluid interactions observed in the literature were developed and used in the majority of our simulations. We find that gas mobility is an influential parameter in determining the optimal VRR at core scale. As the gas mobility decreases relative to oil and water and/or the reservoir heterogeneity increases, a VRR below 1 becomes more favorable. The heterogeneity and its connectivity also influenced the optimal VRR to a lesser extent.

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