Enhanced oil recovery mechanisms of pre-fracturing CO2 injection in high-maturity shale oil reservoirs: Integrated phase behavior experiments and field-scale numerical simulations

CO₂-EOR in shale oil reservoirs serves as an effective method for enhancing oil recovery while facilitating geological carbon sequestration. This study proposes a novel pre-fracturing CO₂ injection (PFCI) technique, where controlled-volume CO₂ injetion is systematically implemented prior to hydraulic fracturing in high-maturity shale oil reservoirs, aiming to modulate fluid phase behavior and replenish reservoir elastic energy. Through experimental and numerical investigations of PFCI in high-maturity shale oil reservoirs and field-scale simulations, it was revealed that the PFCI method demonstrates substantial advantages over CO₂ huff and puff (HnP) in enhancing oil recovery and carbon sequestration. PFCI significantly promotes interphase mass transfer, reinforces elastic energy, and enhances crude oil mobility. Furthermore, PFCI improves CO₂ sweep efficiency and suppresses fracturing fluid flowback. Phase behavior analysis indicates that CO₂ injection into high-maturity shale oil reservoirs induces dramatic phase transformations. The reservoir fluid undergoes sequential phase state transitions during CO₂ injection: the transformation progresses from light volatile oil reservoirs to near-critical light volatile oil reservoirs, then to near-critical gas reservoirs, and ultimately to gas reservoirs. PFCI exhibits exceptional energy replenishment capability, reducing reservoir oil bubble-point pressure during this process and facilitating the dissolution of free gas into formation crude oil. During production stage, the oil drainage radius expands by 1.55 times with more balanced recovery of hydrocarbon components. Cumulative oil production from PFCI can be further increased by 18–25 % compared to CO₂ HnP, and achieving 47.67 % CO₂ sequestration efficiency within one-year production. The integration of enhanced oil recovery mechanisms and carbon sequestration effects establishes PFCI as an efficient development strategy for high-maturity shale oil reservoirs.