Evaluation of hybrid enhanced oil recovery strategies for Ugnu heavy oil reservoirs in the Alaska North Slope

Abstract

The Alaska North Slope (ANS) holds a vast reserve of heavy oil, primarily in the Ugnu formation, estimated at 12-18 billion barrels. However, the recovery of these resources presents challenges due to high oil viscosity and proximity to continuous permafrost, which precludes thermal recovery methods that could cause disastrous environmental damage. Recently, low-salinity water flooding (LSWF) and low-salinity water with polymer (LSP) have been considered for enhancing oil recovery from moderately viscous oil reservoirs on the slope. This study explores non-thermal hybrid enhanced oil recovery (cEOR) techniques, focusing on solvent pre-treatment (e.g., CO2) and low-salinity water (LSW) within a Water Alternating Gas (WAG) injection method. Commercial silica sand packed in cylindrical sandpacks was used throughout the study to evaluate the recovery performance of the proposed methods on both dead and live Ugnu heavy oil. Various injection modes were also examined to optimize recovery performance. Cumulative oil production and pressure drops were measured and recorded, while oil recovery factors and residual oil saturation after each flooding were determined based on material balance. The displacement test results reveal that combining liquid CO2 with LSW in a WAG process significantly enhances recovery, achieving up to 83.5% of original oil in place (OOIP), more than double that of continuous LSW flooding. This improvement is attributed to CO2-induced viscosity reduction and swelling, with additional benefits of CO2 storage in the reservoir. Hydrocarbon lean gas (HLG) was also evaluated as an alternative solvent, but the performance was lower (68.4% of OOIP) compared to liquid CO2 due to differences in mass transfer and live oil interactions. Simulation studies optimized key parameters for the CO2-WAG process, such as soaking time and CO2 slug volume, highlighting the potential for maximizing recovery while sequestering greenhouse gases. Another promising approach, CO2-saturated low-salinity water (CWI), demonstrated substantial recovery benefits, achieving an additional 36% OOIP beyond secondary LSW injection and 40% in tertiary stages. The effectiveness of CWI lies in viscosity reduction and solubility trapping of CO2 in residual oil, with 28-41% of injected CO2 stored during the process. However, integrating low-salinity water polymer (LSWP) in tertiary recovery was less effective due to polymer degradation. Overall, these studies underscore the significant potential of CO2-based cEOR methods, particularly liquid CO2-WAG and CWI, for Ugnu heavy oil recovery. These techniques not only improve oil recovery but also contribute to greenhouse gas mitigation through efficient CO2 utilization and storage.