Starch is one of the most widely used biopolymers in water based drilling fluids to control fluid loss. The thermal stability of starch in common drilling fluids is low (93 °C). In this study, the thermal stability of starch has been evaluated in sodium/potassium formate and potassium chloride fluids. Samples of mud were prepared by formate salts (sodium and potassium) and potassium chloride with the same formulations. The rheological and filter loss properties and shale recovery were studied at different temperatures. The results showed that despite the relatively low concentration of starch (4 gr⁄350cm3 ) and low density of mud, the sodium and potassium formate salts increased the thermal stability of starch up to 150 °C for 16 hours. The rheological and fluid loss properties of formate fluid after 16 hours hot rolling slightly changed compared with potassium chloride fluid. Also, the results showed that the shale recovery of formate fluid is better than that of potassium chloride fluid. Thus, formate salts improved the thermal stability of starch in water based drilling fluids
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