Development of CO2 EOR Techniques for Unlocking Resources in Tight Oil Formations

Abstract

Tremendous resources of tight oil are located in North America and widely distributed in 24 oil reservoirs, which are characterized by both low permeability and low porosity. Although long horizontal wells together with massively fracturing techniques have been utilized to promote fluid injectivity and productivity, primary recovery factor of a tight oil reservoir still remains as low as only 5-10 % of original oil in place (OOIP). Due to low permeability, waterflooding results in low injectivity. Although CO2 injection may be the most suitable enhanced oil recovery (EOR) option under immiscible or miscible conditions, its flooding mechanisms and performance have not been well understood. The traditional injection strategy including continuous CO2 injection, water-alternating-CO2 injection (CO2-WAG), simultaneous water and CO2 injection, and CO2 huff-n-puff injection may perform rather differently in tight formations compared to the conventional reservoirs. Therefore, it is of practical and fundamental importance to evaluate the suitability of CO2 EOR techniques for unlocking resources from tight oil formations. In this study, techniques have been developed to experimentally and numerically evaluate performance of CO2 injection in tight oil reservoirs for the purpose of enhancing oil recovery. Experimentally, coreflooding experiments have been performed to evaluate performance of waterflooding, continuous CO2 flooding, CO2-WAG flooding, and CO2 huff-n-puff processes in tight oil formations, respectively, while effects of various operational parameters on recovery performance are examined. Subsequently, numerical simulation is performed to match the experimental measurements and optimize the operational parameters. Finally, field-scale simulation is further conducted to evaluate the performance of various CO2 EOR schemes in a tight oil formation. As for experimental measurements and simulation, the CO2-WAG flooding is found to result in higher sweep efficiency, leading to better recovery performance compared to waterflooding and continuous CO2 flooding. As for CO2 huff-n-puff process, oil recovery is experimentally found to be lower than that of CO2 flooding. There exists a good agreement between the experimentally measured and numerically simulated oil recovery and pressure, indicating that the overall mechanisms of CO2 injection in tight oil reservoirs have been captured by numerical simulation, while the operational parameters can be optimized to maximize oil recovery form tight oil formations. As for field-scale simulation, miscible CO2-WAG flooding is found to result in higher oil recovery efficiency compared to other CO2 flooding schemes, while CO2 huff-n-puff processes show favourable recovery performance, provided that enough wells in the field are put into production. Oil recovery is found to be improved by optimizing the operational parameters during CO2 injection processes, while production and injection pressures play dominant roles on oil recovery in tight formations.