Experimental Investigation of the Interaction of Phase Behavior With Microscopic Heterogeneity in a CO2 Flood

Summary This paper reports results of an experimental investigation of the effects of microscopic heterogeneity on local displacement efficiency in a CO2 flood. Flow-visualization experiments for first-contact miscible displacements are described and compared with effluent composition measurement for the same models. High-pressure flow-visualization experiments for multicontact miscible CO2 floods are also described. The displacements were performed in two-dimensional (2D) etched glass models made from thin-sections of San Andres carbonate core from the Maljamar field. Techniques used in preparation of the models are described briefly. Observations of first-contact miscible displacements in heterogeneous models indicate that early breakthrough and a long transition zone occur when preferential flow paths exist in the pore structure. This behavior corresponds to a flowing fraction less than 1 in the Coats-Smith model. Because the 2D models contained no dead-end PV, the results presented indicate that a low flowing fraction can occur if flow velocities in different portions of the pore space differ significantly. Coats-Smith parameters obtained for models were comparable with values obtained in floods performed in reservoir samples. Visual observations of the CO2/crude-oil displacements indicate that there is an interaction between phase behavior and microscopic heterogeneity. Mixing of nearly pure CO2 in the preferential flow path with oil in adjacent regions leads to a residual oil saturation (ROS) that forms in the preferential path after oil initially present has been displaced.