Effects of capillary pressure and use of polymer solutions on dense, non-aqueous-phase liquid retention and mobilization in a rough-walled fracture

In this laboratory study, perchloroethylene (PCE) was permitted to migrate through a horizontal rough-walled limestone fracture under controlled condi tions to assess fracture retention capacity. Retention of immiscible-phase PCE in the absence of an applied wetting-phase hydraulic gradient varied be tween 11% and 26% of the fracture volume. A portion of this residual could be removed through water flooding; however, even at the maximum applied hyd raulic gradient of 1.0, residual PCE remained in the fracture. The observed correlation of reduced residual saturation with capillary number (N-C) dem onstrated that this rough-walled fracture exhibited behavior similar to tha t of a porous medium under water-flooding conditions. For a given hydraulic gradient, polymer-enhanced floods (using xanthan gum) were not as successf ul as conventional water flooding at removing residual from the fracture. T he traditional form of the capillary number became an increasingly poor pre dictor of mobilization behavior as the viscosity of the displacing phase wa s increased. Incorporation of (mu(W)/mu(NW))(-0.5) into the traditional cap illary number provided a more appropriate dimensionless group with which to correlate residual PCE saturation in the fracture as mu(W) increased.