This paper reports results of an experimental investigation of the effects of microscopic heterogeneity on local displacement efficiency in a CO 2 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 CO 2 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 CO 2 /crude-oil displacements indicate that there is an interaction between phase behavior and microscopic heterogeneity. Mixing of nearly pure CO 2 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.

Results of secondary and tertiary displacements of Maljamar crude oil by N2 and CO 2 are compared to examine the effects of solubility and extraction on local displacement efficiency, The flow visualization experiments were performed in pore networks etched in glass plates, In those experiments, the much higher solubility of CO 2 in the oil caused only marginal improvement in displacement efficiency over that observed for N 2 , which was much less soluble, At pressures high enough for CO 2 to extract hydrocarbons efficiently, however, displacements were much more efficient.' Secondary and tertiary displacements in a more heterogeneous glass model are also compared, In a secondary displacement with high solubility but low extraction, adverse capillary and viscous effects limited the area swept to preferential flow paths, High CO 2 solubility did not appear to have significant effects, When water was present in tertiary displacements with efficient extraction and high solubility, combined effects of viscous instability, capillary forces, and heterogeneity sharply reduced sweep efficiency below that observed in secondary displacements in the same model.