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Laboratory investigation on gelation behavior of Xanthan crosslinked with borate intended to combat lost circulation

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2010
Mokhtari, Mehdi
This thesis addresses the application of xanthan/borate gel for lost circulation treatment. Steady shear viscometry method was applied in which the gel system was under constant shear rate while apparent viscosity was being recorded. The apparent viscosity was constant up to initial gelation time in which viscosity started to build up. Four parameters: initial and final gelation times as well as initial and final viscosities are defined and a correlation is derived between those parameters and four variables: polymer blended with crosslinker, pH-controller, and magnesium chloride concentration as well as temperature. These correlations can help the drilling industry to manage the lost circulation treatment job in a way to have enough time and pressure to pump the fluid and to optimize the time and quality required for final gel. The effects of those variables besides mixing time and shear history on gelation were also investigated. Temperature and pH-controller shortens initiation of gelation. Poly-cross shifts viscosity upward. Retarder postpones the final gelation time. Shear history does not affect initial gelation time and increase of mixing time reduces initial gelation time. This thesis also investigates the rheological model behavior of this gel system before initial gelation time which is the time allowed for pumping the fluid. Shear stress was measured at 0.1,1,50, and 450 rpm besides the conventional readings. Then residual mean squares for six common rheological models were obtained. Sisko was found to be the best fitting model based on this statistical approach. Moreover a modified Bingham-plastic and low shear yield point model are suggested.