Low IFT drainage and imbibition

Abstract

Experiments were performed in four cores to investigate the effects of interfacial tension (IFT) changes and phase density difference on drainage and imbibition of oil/water/alcohol mixtures. In imbibition experiments, in which the cores were initially saturated with oil, both total recovery and the rates of recovery increased when IFT and density differences were reduced, despite reduced capillary and gravity driving forces. Time scale analysis shows that a transition from capillary-dominated flow to gravity-dominated flow as IFT is reduced, accounts for the observations. At high ratios of capillary to gravity forces (hight inverse Bond numbers, N B −1) the flow is capillary dominated and counercurrent. At low N B −1, gravity segregation dominates the flow. When the initial wetting phase saturation is zero, snap-off residual oil droplets are partially suppressed for N B −1 < 1.0. When an initial oil saturation is present, however, entrapment of residual oil is not suppressed for N B −1 as low as 0.04. In free drainage experiments, in which a core filled with an equilibrated wetting phase was surrounded by an oil phase, the wetting phase was completely retained in the core for N B −1 > 1.0. For N B −1 < 1.0, however, gravity forces were sufficient to overcome capillary entry pressures and free damage occurred. Significant reductions in oil saturation (60%) were observed for N B −1 < 0.1. The potential for recovery of oil from fractured reservoirs based on free drainage during gas injection is discussed. Critical scaling shows that a multicontact miscible gas injection process can be used to create the low value of N B −1 required. Estimates of N B −1 for fractured reservoirs indicate that use of gravity drainage is possible even for low-permeability reservoirs as long as fractures exist with vertical connections of sufficient height.