Polymer crosslinked solutions support many beneficial applications in the oil industry, such as water shutoff and hydraulic fracturing. These techniques pose challenges, and, in order to reach a favorable result, high quality gels are required.

This study investigates the effectiveness of four zirconium-based, one aluminum-based, and one aluminum-zirconium-based crosslinkers that can be functional over a wide pH range (3.8 to 10) in crosslinking CMHPG (Carboxymethylhydroxypropylguar). Zirconium chemistry in aqueous solutions is quite complicated. Zirconium is prone to hydrolysis and readily polymerizes. It is highly sensitive to pH, temperature, and ionic strength. A high viscosity gel is a function of and a combination of the correct selection of crosslinkers, buffer systems, pH, polymer loadings, crosslinker concentrations, and temperatures. Not only do the type of ligands attached to the zirconium impact gel performance, but also the order of addition and reaction temperatures. Thus, two very similar crosslinkers can display very different performances.

This research further investigates the effects on gel formation of specific crosslinker concentrations (0.7 to 1.3 gpt) in a pH environment of 3.8 to 10.8. After forming a gel, the impacts of temperatures at 60 to 121°C (140 to 250°F) and the varying shear rates on the viscosity of formed gels were tested. The viscosity of crosslinked CMHPG using the crosslinker at a low pH (3.8) remained above 1,000 cp after 60 minutes at 60°C (140°F) and 10 s−1. Increasing the shear rate to 170 s−1 caused the viscosity to decrease to 300 cp. However, the viscosity stabilized at 300 cp for almost 40 minutes. At a low pH (3.8), delay curves were developed for the gelling process. The fast crosslinking reaction has always been a limitation of crosslinkers at high pH; however, the new aluminum-zirconium dual crosslinker in this study overcame this shortcoming. The optimum crosslinked gel was tested for proppant-carrying purposes along with the static leakoff tests. The results also revealed gel proppant-suspending capabilities and acceptable leakoff rates.

Extensive lab research undergirds a successful field treatment, and these results indicate that a high viscosity at high shear rates cannot be used to interpret the particle settling properties of a crosslinked gel. In addition, the chemistry of crosslinkers is very important as this study reveals that a viscosity buildup delay at a high pH, which has always been a shortcoming, is possible through the right choice of crosslinkers.

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