This paper presents a modeling study of CO2 injection in a chalk core based on experimental data reported by Karimaie (2007). The experiment consisted of a vertically-oriented 19.6 cm long chalk outcrop core initially saturated with reservoir synthetic oil consisting of C1 and n-C7 at a temperature of 85°C and pressure of 220 bar. After saturating the core with the oil mixture by displacement a small "fracture" volume surrounding the core was created by heating the solid Wood's metal which originally filled the volume between the core and core holder.
Gas injection was first conducted using an equilibrium C1-n-C7 gas at 220 bar, resulting in no recovery by (thermodynamic) mass transfer but only from immiscible displacement with Darcy flow driven by induced pressure gradients and a minor impact of gravity-capillary equilibrium. Once oil production ceased in this first displacement, a second period with pure CO2 gas injection followed.
Our modeling was conducted with a compositional reservoir simulator. The 2-dimensional r-z model used fine grids for the core matrix and surrounding fracture. Automated history matching is used to match experimental data. The match to reported production data gave a high degree of confidence in the model. Oil recovery improved significantly by CO2 injection.
Our model study indicates that the recovery mechanism in the Karimaie experiment was dominated by Darcy displacement because of a low conductivity in the surrounding fracture. Another observation made in our study was the strong influence of surface separator temperature on surface oil production. Finally, gas injection rate changes had a significant impact on recovery performance for CO2 injection. Gravity-capillary recovery mechanism was of minor importance in the Karimaie experiments.