Crosslinked polymer gels have been extensively used in upstream petroleum industry for water shut-off, profile modification and hydraulic fracturing. The successful history of those applications has led to opening of a new frontier for solving one of the most severe and costly drilling problems known as lost circulation. Xanthan chains can be crosslinked by borate ions released from borax in appropriate pH conditions and consequently convert to a three-dimensional gel structure that can block thief zones.
Steady shear viscometry method was applied in which the gel system was under constant shear rate while apparent viscosity had been recording. Apparent viscosity is almost constant up to initial gelation time in which viscosity starts growing up. Initial and final gelation times, initial and final viscosities are parameters that can describe the behavior of a gel system and are reported in this paper. The effects of concentrations of xanthan blended with borax, a pH-controller, and magnesium chloride as well as mixing time, shear history and temperature were investigated. Moreover empirical models are suggested for prediction of initial and final gelation times/viscosities as a function of the above-mentioned variables.
Although the initial gelation time decreases when mixing periods increases, the summation of those is constant. In other words, the reaction causing gelation cannot be postponed by mixing. This is also true when the gel is at rest. On the other hand, it was observed that gels with high-shear rate history demonstrate similar initial gelation time as gels under 3 rpm steady shear. Moreover this low shear rate was found to not interfere in initial gelation time. Temperature and pH-controller reduce the time required for initiation of gelation. Magnesium chloride reduces the viscosity. Consequently the final gel is weaker and occurs in a longer time. Xanthan blended with borax contributes to the system mainly by increasing the viscosity. Finally the empirical models can be used to manage lost circulation treatment job. They help to have enough time to pump the pill with a known viscosity and wait enough on a gel with a desired quality.