The high pressure and high temperature (HPHT) encountered in geothermal wells make the drilling operation challenging. Drilling in such environments requires special drilling fluid formulations with high thermal stability and suitable rheological properties to function as drilling fluid. Rheological properties impact many drilling parameters such as hole cleaning, fluid and wellbore stability, wellbore hydraulics, torque and drag, and other performance indicators. Therefore, great efforts should be put into selecting suitable additives to maintain the performance of drilling fluid throughout the operation. In this study, the effectiveness of different additives was evaluated in maintaining drilling fluid rheology at HPHT conditions. The additives considered in this investigation are bentonite, xanthan gum (XC), low-viscosity and regular polyanionic cellulose (PAC-L and PAC-R), hydroxyethyl cellulose (HEC), other synthetic polymers and clay such as THERMA-VIS. Fluid samples were prepared in various concentrations and left to hydrate for 20-24 hrs. The rheological analysis was performed under HPHT conditions using a rheometer. Different parameters were considered in the screening such as temperature, concentration, shear rate, and aging time. The results of this study showed that temperature has a significant impact on the stability and rheological properties of the fluids. The rheological behaviors of tested additive-containing fluids vary substantially with temperature, and THERMA-VIS was superior to the other additives in terms of viscosity and thermal stability. The viscosity of THERMA-VIS-based fluid was low at room temperature and increased with temperature up to 375℉ (maximum testing temperature). During the test, the fluid became viscous enough to suspend rock cuttings and other heavy solids. The optimum concentration of THERMA-VIS was found to be 3.0 lb/bbl. Exceeding this concentration resulted in a highly viscous fluid and mud gelation at high temperatures.

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