Injection of a pH-sensitive polymer into a heterogeneous reservoir as a novel deep-penetrating mobility control method has been proposed earlier (Al-Anazi and Sharma 2002b). A polyelectrolyte that forms molecular-network microgels in solution is injected into high-permeability zones under acidic conditions. Upon contact with reservoir rock, the injected fluid experiences an increase in pH due to geochemical reactions between the injected fluid and carbonate and other mineral components in the rock. The pH increase swells the polymer, which drastically increases the apparent viscosity of the polymer "solution??, significantly lowering the mobility of water in the high-permeability zone. For the controlled application of this novel process, the mechanisms for each of its three sub-processes need to be understood: (1) dependence of polymer viscosity on ionic (pH) conditions in the reservoir; (2) geochemical characterization of pH change in the rock; and (3) polymer microgel transport in porous media. Viscosity characterization has been reported earlier (Huh et al. 2005). The geochemical characterization and the microgel transport are reported in this paper.

For geochemical characterization, brine at pH=2 was injected into a Berea sandstone core. The pH and cation concentrations in the effluent were matched, employing our geochemical simulator, KGEOFLOW. An approximate representation of sandstone with three mineral components adequately matched the pH change during the acid-injection coreflood. For a preliminary characterization of soft microgel transport, a pure-silica sandpack at a constant pH was employed to remove any geochemical effects (i.e., no microgel swelling). From the pressure gradient data available from internal pressure taps, the permeability reduction due to polymer retention was obtained as a function of polymer structure (represented approximately by rheological parameters) and polymer concentration, and with addition of surfactant. The retention can be explained by adsorption on the silica surfaces, with a small contribution from straining by size exclusion.