Optimal Heavy Oil Waterflood Management May Differ From that of Light Oils

Abstract

Abstract Current paradigms and regulatory mandates implicitly assume that waterflood reservoir management practices successful in light oils can be extended unmodified to heavier oils. In particular, complete voidage replacement is considered optimal irrespective of oil chemistry; furthermore, it is assumed that the Buckley-Leverett multi-phase flow formulation, successful in light oils, is equally applicable with heavier crudes. Surprisingly, despite the paramount importance of these concepts to successful reservoir management, there is little public domain documentation on any empirical tests of these assumptions using field data. We here report that our ongoing empirical examination has accumulated observations that suggest that optimal heavy oil waterflood management may differ from that of light oils. The literature has anecdotal accounts of the difficulty of maximizing oil recovery for heavy oil reservoirs while attempting to achieve complete voidage replacement. In the North Slope of Alaska, efforts to maximize oil production early in the waterflooding of isolated hydraulic blocks have led to a VRR < 1. For heavy oils, we have previously identified a flow regime with WOR ∼ 1 for extended periods of time prevalent for reservoirs worldwide. In Alaska, where we possess detailed fluid, well and reservoir information, we have correlated this regime with hydraulic units with incomplete voidage replacement. The WOR ∼ 1 flow regime can be interpreted as a water-in-oil emulsion flow which is intrinsic to the water/oil system chemistry and not to the details of the reservoir stratification, explaining its widespread prevalence. Laboratory heavy oil waterfloods with a VRR = 0.7 recover more oil than those with VRR = 1, and provide evidence of in-situ water-in-oil emulsion formation. Furthermore, the laboratory floods suggest that the recovery prize for optimal voidage strategy may be estimated by a simple heuristic equation: optimal recovery process (VRRopt) ∼ recovery pure waterflood (VRR = 1) + recovery pure solution gas drive (VRR = 0).