CO2 Injections for Enhanced Oil Recovery Visualized with an Industrial CT-scanner

The effect of micro-scale heterogeneities on front instabilities during secondary, liquid CO2 injections for enhanced oil recovery in standard-sized chalk core plugs was investigated. The rock structure and displacement process was imaged in an industrial CT-scanner to probe the effect of micro-scale heterogeneities on the flow patterns and development of plume and CO2 fingers during injections. Heterogeneities in the chalk samples include fractures, healed shear bands and remnants of burrows. A one-component mineral oil was placed in contact with CO2 at the experimental conditions to promote reproducibility between repeated tests. The chalk is considered homogeneous on a standard-sized plug level, and varies only slightly in porosity and permeability within a large number of cores. The high spatial resolution CT scanning revealed sub-mm healed shear bands running through the length of the core which potentially can cause a permeability decrease or diversion of the injected fluid. Total oil recovery from CO2 injection was around 90% regardless of heterogeneities, and there was no visible difference in CO2 arrival at the outlet. With no permeability contrast through the length of the core, the production of oil took place with less than one pore volume (PV) of CO2 injected. With a permeability contrast through the length of the core, more than one PV of CO2 was required to reach end-point oil saturation. Imaging the dynamic properties of a CO2 flood in the industrial CT showed how micro scale heterogeneities impact the flooding characteristics of a small core sample, as the healed shear bands diverted flow to a certain degree. It is also demonstrated how a larger permeability contrast will make the recovery more dependent on diffusion, which is a slower process than viscous displacement. The results demonstrate the need for characterization of micro-scale heterogeneity, because high permeability streaks and fractures will dominate flow during CO2 injection for EOR.