Fluid characterization and phase behavior studies of oil from the frozen reservoir of Umiat Oil Field, Alaska

Abstract

Umiat oil field is the largest oil accumulation in National Petroleum Reserve (NPRA) No. 4 of Alaska. Shallow reservoir depths, low reservoir pressures, and low temperatures with most of the oil-producing zone in a continuous layer of permafrost are unique characteristics that make Umiat reservoir unconventional and difficult to produce. However, unavailability of fluid characterization and phase behavior data needed for reservoir simulation studies pose challenges in developing an effective production strategy. Given the conspicuous lack of complete fluid data on Umiat oils and the unavailability of live oil samples from Umiat, an experimental study was undertaken to characterize and quantify phase behavior of an available small volume of dead Umiat oil. The oil composition characterized experimentally was found to be severely weatherized and not representative of original Umiat oil. Comparison of components in the dead oil sample origina one characterized by Pedersen method enabled determination of the mass of each component that would be need to be added to the weathered sample in order to compensate for the evaporated light ends. The re-created sample was subsequently used for constant composition expansion (CCE) laboratory PVT test. The bubble point pressure at reservoir temperatures, and densities and viscosities of single-phase reservoir fluid at various pressures were measured. The phase behavior of the pseudo live oil was also simulated using Peng-Robinson equations of state (PR-EOS). The EOS model was tuned with measured experimental data to simulate differential liberation tests in order to obtain the PVT data needed for reservoir simulation studies.