CO₂ IOR on North Sea Chalk: Laboratory Experiments at Reservoir Conditions

The performance of CO₂ IOR on low-permeable chalk was tested in two reservoir condition experiments modelling the Syd Arne oil field of the Danish North Sea. Conditions were P_fluid=380 bar, NOB=120 bar, and T=115 °C. A core sample with porosity=29.4 %, and k_g=1.6 mD was used. Fluids were recombined live crude oil, simulated formation water, and pure supercritical CO₂. The oil-CO₂ system was above the minimum miscibility pressure.

Starting at an S_wi of 0.147 PV, a water-flooding was conducted that resulted in the production of 0.597 PV of oil and an S_orw of 0.256 PV. This represents the end-point for the water-flooding experiment, and was the starting point for the CO₂-flooding experiment. CO₂-flooding produced only water until injection of 0.476 PV of CO₂ where breakthrough of a miscible oil+CO₂ phase occurred, followed by a sharp rise in production of oil+CO₂ without detectable fingering. At breakthrough the miscible oil+CO₂ phase had high contents of hydrocarbon, which gradually decreased. After injection of 1.51 PV of CO₂, the cumulate oil production during CO₂-flooding was 0.197 PV and the sample had reached an S_orCO2 of 0.059 PV. After this no further oil production occurred. The efficient sweep of the CO₂-flooding is attributed to increased viscosity of the miscible oil+CO₂ phase and to transverse layering within the sample. Corrosion and compaction of the rock sample were insignificant. Permeability change was absent within the precision of the experiment.

The experiment indicates a potential for CO₂ IOR to produce significant amounts of oil from low-permeable chalk, even after a water-flooding operation. It further indicates adverse effects from corrosion and compaction to be limited.