Integrated geologic modeling and reservoir simulation of Umiat: A frozen shallow oil accumulation in national petroleum reserve of Alaska

Abstract

Current high oil price and availability of new technologies allow re-evaluation of oil resources previously considered uneconomic. The Umiat oil field is one such resource: a unique, shallow (275−1055ft), low-pressure (200−400psia) reservoir within the permafrost zone with no initial gas cap, located north of the Arctic Circle, 80miles west of the Trans Alaska Pipeline System (TAPS) with an estimated 1.5 billion barrels of oil in place.A static model was built based on reinterpretation of original log and core data and seismic information. A permeability anisotropy ratio of 0.45 was incorporated into the geologic model. A Monte Carlo simulation was conducted to estimate the different degrees of uncertainty in the original oil in place (OOIP) estimates. To cover the wide permeability range (0–500md), three sand groups (rock types) were defined and assigned appropriate capillary pressure and relative permeability curves. These were included in the dynamic model along with measured PVT data and gas–oil relative permeabilities in the presence of ice to evaluate the performance of immiscible gas injection using a multilateral wagon wheel well pattern with horizontal well length of 1500ft.The new simulation results show that with 50years of gas injection, recovery factors for the base case (400psia injection pressure) and two cases with 600 and 800psis injection pressures are 10.81%, 13.77%, and 16.66% respectively, keeping other parameters constant. These recovery numbers will reduce by 14% and 9% when producing GOR is restricted to 5000scf/STB and 10,000scf/STB, respectively. Due to unknown characteristics of natural fractures, several permeability anisotropy (Kv/Kh) ratios were considered. Simulation results indicate that lower anisotropy ratio will reduce oil recovery, probably due to inhibition in the downward movement of gas.The result obtained by this study contributes to the understanding of uncertainties in resource estimates and evaluating ranges of oil recovery in reservoir modeling. Despite limited data and lack of production history to tune the model, the results demonstrate that the economics of the proposed development plan bear a high degree of uncertainty and risk. These findings strongly encourage the operator to include development plan strategies to reduce the risk and enhance the quantity and quality of input simulation data.