Extensive Miscible Water Alternating Gas WAG Simulation Study for a Giant Offshore Oil Field

Abstract

Abstract The subject field is a giant offshore reservoir with light oil and a planned long production life extending beyond 100 years. In order to sustain oil production, EOR will be implemented at the appropriate time. As a result of EOR screening, miscible water-alternating-gas (WAG) injection was identified as one of the suitable methods. In order to evaluate miscible WAG potential, extensive WAG simulation studies were conducted in each step including 1D, 2D and 3D simulation. First, 1D simulations were conducted for tuning the equation of state (EOS). Second, 2D conceptual model simulations were carried out for the preliminary evaluation of WAG. Then, 3D conceptual models for two typical geological areas of the subject reservoir, the "Homogeneous" and "Heterogeneous" geologic areas were generated and findings from the 2D simulation study were validated. Finally, 3D sector model simulations were conducted using the history-matched simulation model to evaluate the miscible WAG potential for full field implementation and its economics. In all simulation studies, CO2 and hydrocarbon (HC) gas were evaluated as miscible injectants. 1D simulations replicated a slimtube test and EOS parameters were tuned to match the minimum miscible pressure (MMP) of CO2 and HC gas. As a result of 2D simulations, "Tapered" WAG appears the most attractive WAG injection scheme in terms of gas utilization and oil recovery. In the Tapered WAG concept, the durations of gas injection varies where longer gas injection cycles are completed initially progressing towards shorter gas injection cycles with progressive WAG sequences. The Tapered WAG concept was also tested using the 3D conceptual model and similar findings were obtained. To evaluate miscible EOR WAG for the full field, due to the size of our field, the high resolution gridding required to capture fluid transport and the 9 component tuned EOS, the required computing resources exceed typical computing capacity; hence, two sector models which represent the Homogeneous and Heterogeneous areas were generated. Based on simulation results, normalized type curves for the hydrocarbon pore volume injected (HCPVI) with incremental recovery factor were generated for each geological area. A production and injection profile for full field WAG implementation was generated by applying these type curves to each pattern and incremental oil production was estimated. The potential incremental oil production for WAG application for a giant offshore oil field was successfully assessed utilizing extensive simulation scenarios from a preliminary concept using simplified models to a detailed full field analysis using complex models. Through this step-by-step analysis, we were able to efficiently identify the key criteria impacting full field recovery including Tapered WAG, impact of injectant and impact of multi-scale heterogeneity (e.g. 2D-3D).