Mobilization of Residual Oil

During the early stages of a waterflood in a water-wet reservoir system, the brine exists as a film around the sand grains, and the oil fills the remaining pore space. At an intermediate time during the flood, the oil saturation has been decreased and exists partly as a continuous phase in some pore channels but as discontinuous droplets in other channels. At the end of the flood, when the oil has been reduced to residual oil saturation, S_or, the oil exists primarily as a discontinuous phase of droplets or globules that have been isolated and trapped by the displacing brine.

The waterflooding of oil in an oil-wet system yields a different fluid distribution at S_or. Early in the waterflood, the brine forms continuous flow paths through the center portions of some of the pore channels. The brine enters more and more of the pore channels as the waterflood progresses. At residual oil saturation, the brine has entered a sufficient number of pore channels to shut off the oil flow. The residual oil exists as a film around the sand grains. In the smaller flow channels, this film may occupy the entire void space.

The mobilization of the residual oil saturation in a water-wet system requires that the discontinuous globules be connected to form a continuous flow channel that leads to a producing well. In an oil-wet porous medium, the film of oil around the sand grains must be displaced to large pore channels and be connected in a continuous phase before it can be mobilized. The mobilization of oil is governed by the viscous forces (pressure gradients) and the interfacial tension forces that exist in the sand grain–oil–water system.

There have been several investigations of the effect of viscous forces and interfacial tension forces on the trapping and mobilization of residual oil.^{4–7,17,21,22} From these studies, correlations between a dimensionless parameter called the capillary number, N_vc, and fraction of oil recovered have been developed. The capillary number is the ratio of the viscous force to the interfacial tension force and is defined by Eq. (11.2).

where υ is velocity, μ_w is the viscosity of the displacing fluid, σ_ow is the interfacial tension between the displaced and displacing fluids, k_o is the effective permeability to the displaced phase, φ is the porosity, and Δp/L is the pressure drop associated with the velocity.

Figure 11.4 is a schematic representation of the capillary number correlation in which the capillary number is plotted on the abscissa, and the ratio of residual oil saturation (value after conducting a tertiary recovery process to the value before the tertiary recovery process) is plotted as the vertical coordinate. The capillary number increases as the viscous force increases or as the interfacial tension force decreases.

Figure 11.4 Capillary number correlation.

The correlation suggests that a capillary number greater than 10^–5 for the mobilization of unconnected oil droplets is necessary. The capillary number increases as the viscous force increases or as the interfacial tension force decreases. The tertiary methods that have been developed and applied to reservoir situations are designed either to increase the viscous force associated with the injected fluid or to decrease the interfacial tension force between the injected fluid and the reservoir oil. The next sections discuss the four general types of tertiary processes: miscible flooding, chemical flooding, thermal flooding, and microbial flooding.