Assessment of resource and recovery potential of Ugnu tar sands, north slope, Alaska

Extensive shallow sand deposits on the North Slope of Alaska contain up to 35 billion barrels of oil in place. These Cretaceous and Tertiary sands are divided into two distinct units: the upper unit is termed as Ugnu Sands and the lower unit is known as West Sak Sands. The Ugnu Sands, which occur at 2200 feet below the surface in the southwest, dip towards northeast to a depth of 4300 feet. These sands are divided into upper and lower sand members. The Upper Ugnu Sands are discontinuous with lower reservoir quality, whereas the Lower Ugnu Sands are on the average 275 feet thick with good reservoir properties. The Ugnu Sands contain approximately 15 billion barrels of 8-14{degrees}API gravity crude, characterized as tar sand bitumen. The Ugnu reservoir sands are described using well logs, core description, and petrographic analysis of core material. Based on the well log data and petrographic analysis, various reservoir parameters were calculated to (i) characterize each pay zone, (ii) correlate lithofacies distributions, (iii) estimate hydrocarbons-in-place, and (iv) evaluate the environment of deposition. The oil in these sands is biodegraded and highly viscous (50,000 to 10 million cP) due to low reservoir temperatures (45 to 65{degrees}F) andmore » proximity to overlying permafrost. According to Byramjee, the Ugnu crude oil can be classified as extra heavy oil to bitumen. Due to high bitumen viscosity, only thermal recovery processes are considered to be appropriate for recovery of this resource. A detailed simulation study is presented to determine the performance of five novel thermal recovery processes in the Ugnu reservoir. Effects of various reservoir and process parameters such as well spacing, well pattern, preheating period, heterogeneity, anisotropy, and shale barrier configurations on the recovery performance parameters, namely, oil production rate, cumulative oil recovery, and oil-steam ratio (OSR) are determined and are compared in this study.« less