Experimental observations and modeling of the effect of wettability on immiscible viscous fingers at the Darcy scale

Abstract

In this work, we present a Darcy-scale study of immiscible unstable displacement in large (15 × 15 × 1 cm3) slabs of Bentheimer sandstone. Oil is displaced by water under two wetting conditions, a weakly water-wet system and a weakly oil-wet system (at two initial water saturations), with a viscosity ratio of μo/μw = 100. The system is directly visualized using x rays and the fluid distribution monitored throughout the displacement. Following a period of water flooding, the injection water is viscosified with glycerol to act as a pseudo tertiary polymer flood. The experiments are then modeled directly using numerical simulation applying a recently developed fractional flow approach, which has been shown to reproduce experimental immiscible fingering, before upscaling the results to the reservoir scale. From the observed results, we can conclude that typical wettability modification using crude oil aging may generate a wettability similar to that found in oil reservoirs, but this may still result in an exaggerated capillary dampening of the viscous fingers. However, as the system is scaled up in size to the field dimensions, viscous forces will dominate, and fingering will re-emerge due to the increase in the viscous/capillary force balance. But if the input relative permeabilities are constructed from an excessively capillary stabilized experiment, the shock front mobility of the fractional flow will still overestimate the stability of the system. Indeed, we derive the same water saturation above Swi for the three floods, but with different shock front mobilities.