(535b) A Novel Fluid System to Integrate Stimulation, EOR, and CO2 Storage in Shale Oil Reservoirs

Due to the ultra-low permeability nature of shale oil reservoirs, the oil production rate is much lower than from conventional hydrocarbon resources. CO2 has been preliminarily proven an effective way to extract the tremendous unconventional resources. Also, CO2-EOR is an effective way for CO2 utilization and storage, thus, helping reduce carbon emission and realize the “carbon neutral” target. A new fluid system was proposed to integrate stimulation, EOR, and CO2 storage in shale oil reservoirs.

The new working fluid system was established by adding cosolvent and surfactant into supercritical carbon dioxide (SC-CO2). The fluid was used for stimulation to create hydraulic fracture networks and for soaking to displace oil present in the matrices. Bottle tests were carried out to investigate screen proper cosolvent and surfactant candidates and the stability of the fluid system at supercritical conditions. Phase behavior experiments were conducted to study the minimum miscible pressure (MMP) and the miscibility behaviors with and without cosolvent. Imbibition tests were performed to look into the oil production rate, equilibrium oil recovery and the wettability index.

Results show that the new fluid system was stable under supercritical condition without precipitation. Phase behavior experiment results demonstrate that the addition of cosolvent reduced the MMP and help to achieve miscibility with shale oil. The influence of cosolvent type and concentration was studied. Imbibition tests results suggest that the new fluid system promoted the spontaneous imbibition process. The imbibition reached the equilibrium status in a shorter time. That is, the oil present in the shale matrices was displaced out more rapidly. In addition, a higher equilibrium oil recovery efficiency was obtained with the proposed fluid system. Wettability alteration was observed after exposure to the working fluid. The wettability index was estimated and the results indicate that the shale medium became less oil-wet. The effects of cosolvent type and concentration, and surfactant type and concentration were tested. The system was optimized according to the tests. As a result, the cosolvent facilitated miscibility of CO2 and shale oil, and the surfactant promoted imbibition of the working fluid into the matrices. The enhanced miscibility and imbibition help CO2 penetrate and be trapped in the shale matrices (i.e., enhanced CO2 sequestration).

Systematic experimental and theoretical studies were carried out to test the feasibility of a new working fluid of SC-CO2/cosolvent/surfactant combination for shale oil development. The work demonstrates the potential of the proposed fluid system in improving the performance of stimulation, economic recovery factor, as well as CO2 sequestration in shale oil plays.

Key words: Shale oil; Well stimulation; Supercritical CO2; Fracturing fluid; CO2 storage