Cross-linked gels can be made over a wide pH range. Carboxylated polymers perform best from pH 3 to 4. However, derived guars, such as CMG(Carboxy Methyl Guar) and HPG (HydroxyPropyl Guar), may be cross-linked up to a pH of 9 to 10. A change in pH may occur through use of the preferred operational technique for massive hydraulic fracturing treatment offshore, which is to use a liquid gel concentrate mixed with filtered seawater. A major incompatibility arises with use of seawater, because it contains multivalent metal ions. These ions may precipitate as hydroxides, lowering the pH of the fluid with a drastic effect on the fluid properties. The metal ions may also poison enzymatic breakers (which breakdown cross-linked guar) and clog the fracture, preventing the release of oil or gas.
