THE INFLUENCE OF WATERFLOOD DESIGN ON THE RECOVERY OF MOBILE DNAPLS

This study examines the effectiveness of various waterflooding strateg ies to recover pooled dense nonaqueous phase liquid (DNAPL) from the s ubsurface at an industrial facility. The relative influence of horizon tal injection/recovery well configuration, established hydraulic gradi ent, and fluid properties is investigated for a site characterized by a homogeneous silty sand underlain by an impermeable clay layer. The t op of the clay layer is located 5 m below the water table and supports a laterally extensive 2 m deep DNAPL pool. The sensitivity study empl oys a two-phase flow numerical model that simulates both DNAPL infiltr ation and redistribution, including the formation of immobilized DNAPL residual. This is accomplished with constitutive relations featuring hysteretic capillary pressure-saturation pathways in which the local a mount of residual formed is a function of the maximum nonwetting satur ation attained during infiltration. Sixteen simulations, performed in two-dimensional vertical cross-section, demonstrate that strategies ef fecting increased wetting phase gradients, namely increasing drawdown at the recovery drain, adding injection wells, and reducing their dist ance to the recovery drain, result in an increased DNAPL volume recove red with time at the expense of increased volumes of ground water remo ved per unit volume of DNAPL recovered. Strategies which do not increa se wetting phase gradients result in DNAPL recovery with a minimum vol ume of produced contaminated ground water. Three pulsed pumping simula tions indicate that increasing the length of pump shut-down time decre ases the recovery of DNAPL with time but increases efficiency with res pect to ground water pumped. Decreased nonwetting density and increase d interfacial tension result in poorer DNAPL recovery with respect to both time and volume of ground water removed, while reduced nonwetting viscosity corresponds to dramatically increased efficiency in both re spects.