Water and gas shutoff in naturally fractured reservoirs is traditionally achieved with cross-linked gels. Low sweep efficiency is also an important problem in waterflooding such reservoirs which can be treated with gel. In-depth placement of gels is the key to success, and this requires careful control of the cross-linking chemistry. This work examines the feasibility of an alternative gelation mechanism in fractured rock: pH-triggered polymer microgels. The microgel particles are small enough to pass through fractures but not pore throats in a matrix. The pH change occurs naturally and inevitably; it thus offers a simpler means of obtaining deep placement.

When an acidic polymer solution is injected into the formation, several factors affect the pH: rock mineralogy, reactive surface area of the minerals, temperature and dilution due to mixing with residual water. The viscosity of the solution depends strongly on pH and upon polymer concentration and salinity of the polymer solution. The rate of pH increase relative to the rate of fluid advance determines the depth of placement.

We used commercially available polymers which exhibit low viscosity at a pH below 3 but transform to gels at pH > 4. Polymer solutions were injected through artificial fractures in outcrop cores. Both sandstone and carbonate rocks raise the polymer solution pH. The presence of acid-soluble minerals containing cations such as calcium can independently trigger viscosification by precipitating the polymer. After polymer injection, a shut-in time allows further reaction to increase the pH and thus affects the Permeability Reduction Factor (PRF), the ratio of original fractured core permeability to treated core permeability. The PRF was measured to be in the range of 200 to 5 during the various experiments. The gelation is faster and PRF is higher in carbonates than in sandstones. Because the neutralization capacity of a core is large, it is possible to approximate reactive transport in a reservoir by continuously re-circulating the polymer effluent. Experiments showed gel-like characteristics after 30 pore volumes of recirculation in Berea sandstone.