Further Investigation of Effects of Injection Pressure and Imbibition Water on CO2 Huff-n-Puff Performance in Liquid-Rich Shale Reservoirs

Shale oil production has increased rapidly in the past decades, especially in the United States, and results in a revolution in the energy landscape. However, one main problem existing in the shale reservoir development is the sharp decline of liquids production in all the hydraulically fractured wells. In recent years, CO2 huff-n-puff injection has been proved to be a potential method to enhance the oil recovery. In this study, the effects of injection pressure and imbibition water on CO2 huff-n-puff performance were further investigated. Eagle Ford core samples and Wolfcamp dead oil were used in this experimental study. The microscopic pore characteristics of Eagle Ford shale core samples were analyzed, and the results show that 98.08% of the pore sizes are distributed between 3 nm and 50 nm. The experimental results demonstrate the great potential of CO2 huff-n-puff EOR. The cumulative oil recovery can reach 68% after seven huff-n-puff cycles. The oil recovered in each cycle deceases as injection cycle number increases due to the permeability damage caused by asphaltene precipitation, oil saturation reduction, and low injected CO2 sweep efficiency. The effect of injection pressure was studied by injecting CO2 at both immiscible and miscible conditions. CO2 huff-n-puff has better performance (more than 9.1% EOR) under miscible conditions than immiscible conditions. After that, a novel experiment was designed to saturate the core samples with both water (15 wt % KCl) and Wolfcamp dead oil to investigate the influence of imbibition water on CO2 huff-n-puff EOR performance. The existence of imbibition water impedes oil production in shale core samples. The oil recovery decreased about 45.3% after seven huff-n-puff cycles compared to the condition without water. A simulation study provides a better understanding of CO2 huff-n-puff application in liquid-rich shale reservoirs, which is fundamentally important for applying and optimizing CO2 huff-n-puff in field production.