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Summary This paper examines a conformance control technique in which gel is placed into a heterogeneous porous medium by first injecting a high-viscosity uncrosslinked polymer solution and second, a low-viscosity crosslinker. The latter step causes an unstable displacement to occur and allows the crosslinker to flow selectively into high-permeability strata. A visual flow cell was used to test the procedure, and complete fluid diversion was achieved in a dual-zone medium with a 4:1 permeability ratio. In addition, experiments and numerical simulations were used to study fundamental changes in viscous fingering due to the crosslinker-polymer reaction. When the crosslinking reaction is fast enough so that it occurs during injection, the viscous instability becomes somewhat damped, resulting in less distinct fingers and a nearly plug-flow displacement in some cases. The differences are because gelation causes a large pressure gradient to form just behind the displacement front, whereas unreactive fingering is caused by a small pressure gradient within the fingering zone.

The pore-level mechanisms by which a crosslinked gel changes a porous medium's physical morphology and multiphase permeability are examined using relative permeabilities, pulse tracers, and micromodel visualization. Coreflood experiments indicate that to retain oil permeability while reducing water-permeability, the maintenance of oil-phase pathways after a gel treatment is crucial. However, because the gel changes the medium's morphology, the original pathways for oil are disrupted if the posttreatment fractional flow of water is nonzero. This redistribution of phases generally causes a loss of oil-phase permeability that cannot be regained under normal flow conditions. Relative permeabilities during drainage are not affected significantly by the gel, but during secondary imbibition, the hysteresis in relative permeability is much more pronounced. Tracer experiments indicate the reason for this behavior is that the gel reduces the connectivity of the medium and the fluids distribute themselves more inefficiently, causing a large fraction of dead-end or isolated nonwetting-phase fluid to exist. These conclusions are particularly important for water-control applications where water saturation increases with time. The mechanism suggests that even if a gel exhibits preferential permeability reduction, the loss of absolute permeability and the shift in saturation can be highly damaging to the oil-phase permeability of a treated zone.