Parametric study of oil recovery during CO₂ injections in fractured chalk: Influence of fracture permeability, diffusion length and water saturation

Experimental secondary and tertiary CO₂ injection tests in fractured, strongly water-wet, chalk core samples were performed to investigate the influence of fracture permeability, diffusion length and initial water saturation on the oil recovery potential. Liquid CO₂ was injected in partially oil and brine saturated core plugs and larger blocks at miscible condition with the oil phase (n-Decane) at 20 °C and 90 bar. High final oil recoveries, up to 100%OOIP, were observed during secondary miscible CO₂ injections in whole, unfractured core samples within 2 pore volumes (PV) injected. High recoveries were also observed in fractured systems (79–93%OOIP), but recovery was less efficient in terms of PV injected (4–12) as a result of increased system permeability and increased diffusion length. Tertiary CO₂ injections in core samples after waterfloods were less efficient in terms of final oil recoveries (62–69%OOIP) compared to secondary CO₂ injections. Additional oil recovery during tertiary CO₂ injection ranged from 0% to 15%OOIP. An adverse effect of water was observed both during secondary and tertiary CO₂ injections, where higher water saturation decreased the oil recovery efficiency by diffusion, indicative of a water shielding effect.