Heavy Oil Waterflooding Enabled by Optimal Waterflood Voidage Management

Increasingly the remaining conventional oil resources to be developed are offshore and heavy. The North Sea leads this transition, with several projects planned for development with an API of less than 15. Implicitly the industry assumes that waterflood practices & paradigms developed for onshore light oils can be applied largely unmodified for offshore heavy oils. This paper presents experimental data that question one of the key assumptions: the optimal voidage replacement ratio (VRR) equals one. The experiments were motivated by the accumulation of field empirical observations suggesting that water injected to displace heavy oils forms in the reservoir channel-like communication paths from the injectors to the producers. Once formed – typically early in the waterflood - the channels can degrade further economic recovery of the heavy oil as the water to oil ratio increases significantly. The large offshore spacings will likely exacerbate this effect for most depositional environments. This study reports on the results from a laboratory program focused on designing optimal waterflood management after water breakthrough for an Alaskan 12 API heavy oil representative of a very large but remote & expensive to produce resource base. To physically simulate reservoir waterflood behavior under the existence of a communication path, a large scale ‘big can’, five feet long with a ten inch by ten inch cross section, was designed and constructed that allowed for the creation of a highly reproducible communication path from the injection to production end of the can. This was a mandatory requirement for accurate comparison between alternative reservoir management strategies whose differences would otherwise be hidden by variations in random communication path formation. The design has proven to be highly successful. Our first objective was to test whether the industry paradigm and the regulatory mandated practice in some jurisdictions of maintaining a voidage replacement ratio (VRR) of one throughout the entire waterflood is optimal. Live 12 API Alaska North slope oil was used to saturate four Darcy sand that filled the big can. Upon creation of the communication path, four VRRs were tested: 1.0 (conventional waterflood), 0.7 & 0.5 (hybrid waterflood/solution gas drive), and 0.0 (conventional solution gas drive). The VRR=0.7& 0.5 runs outperformed the conventional VRR=1.0, suggesting that whilst the optimum depletion plan may approach a VRR of 1.0 over the life of a field, periods of under injection may improve heavy oil waterflood response upon formation of injector-producer communication.