Numerical modeling CO2 injection in a fractured chalk experiment

This paper presents a numerical modeling study of CO2 injection in a chalk core based on experimental data, as 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 that originally filled the volume between the core and core holder.Gas injection was conducted initially using an equilibrium C1–n-C7 gas at 220 bar. This gas should have had no recovery by thermodynamic mass transfer, only from immiscible Darcy-controlled displacement driven by pressure gradients and gravity-capillary forces. Once oil production ceased in this first displacement, a second period with pure CO2 gas injection followed.Our numerical modeling was conducted with a compositional reservoir simulator. The 2-dimensional r-z model used fine grids for the core matrix and the surrounding fracture. Automated history matching was used to match the experimental data (surface volumetric oil production profile). The match to reported production data gave a high degree of confidence in the model. Oil recovery improved significantly with CO2 injection.Our numerical 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, with little impact of capillary–gravity displacement. Another observation made in our study was the strong influence of surface separator temperature on surface oil volumetric 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.

Research Highlights
► Numerical modeling study of CO2 injection in a chalk core based on experimental data.
► Gas injection was conducted initially using an equilibrium gas and followed by pure CO2 gas injection.
► Our numerical modeling was conducted with a compositional reservoir simulator.
► Automated history matching was used to match experimental data (oil production profile).
► Our study indicates that the recovery mechanism in the experiment was dominated by Darcy displacement.