The increasing global demand for energy, coupled with the depletion of conventional hydrocarbon reserves, has prompted the petroleum industry to drill deep wells to access unconventional reserves with high pressure-high temperature (HP-HT) conditions. To minimize drilling-fluid-related challenges encountered at such conditions, thermally stable drilling fluids (DF) with appropriate filtration and rheology are required. However, commonly used biopolymers and their modified versions have low thermal stability, limiting their operational effectiveness. More thermally stable synthetic polymers (SPs) are being developed to improve DF properties under HP-HT conditions. This study synthesizes a thermally stable SP, acrylamide/styrene/maleic anhydride terpolymer (ASMTP), and evaluates its technical performance in a water-based DF in terms of its rheology and filtration properties at high temperature. Using an inverse emulsion polymerization technique, the ASMTP was synthesized, and its chemical structure was characterized using three common chemical characterization methods. The ASMTP was then incorporated into a bentonite fluid system at four concentrations ranging from 0.05 to 0.2 wt%, and the rheological and filtration properties were assessed at both the low pressure-low temperature (LP-LT) and HP-HT conditions (3.45 MPa; 140 °C). To evaluate the 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thermal stability, rheological and filtration analyses were performed before and after 16 h of thermal aging of the DF systems at 140 °C. The rheology of these fluids was theoretically modeled by coupling the extra tensor concept with generalized bracket formalisms. The results confirm that this novel terpolymer significantly enhances the rheological and filtration properties of the bentonite fluid. The presence of 0.2 wt% ASMTP in the DF reduced the volume of HP-HT filtration by approximately 67 % and 69.9 %, and improved plastic viscosity by 82.9 % and 202.9 % before and after thermal aging, respectively. Observations indicate that ASMTP is able to share/link hydrophobic sites more extensively under high-temperature conditions. Furthermore, the evaluation of the technical performance and financial impact of the ASMTP compared to three commonly used commercial SPs confirms its potential applicability in deep drilling. ASMTP outperformed the commercial SPs in enhancing rheological and filtration properties at a lower cost than commonly used commercial SPs.

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