Mechanisms of Miscible and Immiscible scCO‌2 Displacement Efficiency: Analytical Evaluation of Experimental conditions

Abstract

CO2 injection has proven to be one of the most successful EOR (Enhanced Oil Recovery) methods, as compared with other injection gases CO2 miscibility with oil is easier to achieve. During gas injection into reservoirs, oil might be bypassed on either a micro- or macroscopic scale because of different types of heterogeneities. In this work, the performance of first-contact-miscible (FCM) and immiscible (IM) CO2 injections were investigated experimentally using outcrop sandstone core samples. Decane was also used as the hydrocarbon phase as it has a relatively low minimum miscibility pressure (MMP) with CO2 (12.4 MPa). Core flooding experiments were conducted at two pressures of 17.2 MPa and 9.6 MPa and the common temperature of 343 K. Furthermore, analytical calculations of dimensionless numbers are used to study the dominant forces and mechanisms which are correlated with the results of the core flooding experiments. The impacts of gravity, swelling and vaporization on the end results were inferred from the oil recoveries, variations in the pore pressure and dimensional analysis. For CO2 injection in homogeneous core samples, a maximum recovery of 93.5% and 76% was achieved for the FCM and IM displacements, respectively. The higher recovery results of FCM is attributed to the vanishing capillary pressure between displacing and displaced phases. Dimensional analysis showed that the flow is at the capillary-gravity equilibrium at immiscible conditions, while there is dominance of gravity-viscous forces at miscible conditions.