PORE-LEVEL MECHANISMS OF RESIDUAL OIL FORMATION DURING MISCIBLE DISPLACEMENT

Miscible flooding of oil from porous sedimentary formations by the injection of high-pressure solvents incorporates a complex interaction of heterogeneity, fingering, multiphase flow, and phase behavior. The objective of this work is to identify pore-scale mechanisms that lead to formation of a residual oil saturation in so-called miscible floods in the absence of water shielding. Oil displacements are carried out in two high-pressure, two-dimensional, transparent micromodels, which are simplified analogs of porous rocks. Displacement mechanisms are visualized for different solvents. Results show that the interaction of bypassing and phase behavior can lead to an immiscible residual with so-called first-contact miscible solvents. An equimolar mixture of ethane, propane, and n-butane (C2C3C4) recovers all the oil in one-dimensional flow, but can leave a nonzero residual in dead-end pores of micromodels. Ethane develops multicontact miscibility with this oil in one-dimensional flow. Three hydrocarbon phases appear during the oil displacement by the methane-diluted equimolar mixture of ethane, propane, and n-butane. Capillary-driven flow recovers oil out of dead-end pores in ethane- and methane-diluted equimolar C2C3C4 displacements. The interaction of viscous fingering and phase behavior can also contribute to miscible flood residuals in multiple contact displacements. © 1994 by Academic Press, Inc.