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

• Laboratory CO2 injection in fractured chalk produced on average 83% oil in place.
• Recovery was dominated by CO2 diffusion from fracture to oil-saturated rock matrix.
• Oil recovery decreased with increased diffusion length and/or system permeability.
• An adverse effect of water was observed during secondary and tertiary injections.

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.