Bentonite, a primary constituent of water-based drilling fluid, is traditionally used as a rheology and filtration loss control additive. However, the bentonite undergoes temperature-induced flocculation and gelation under high pressure and high temperature (HPHT) conditions, resulting in poor drilling fluid deliverability. Further, bentonite becomes inert and acts like a drilled solid under higher chloride-bearing salt contamination. Functional polymers with appropriate chain length, molecular weight, and molecular weight distribution may replace traditional water-based additives, e.g., bentonite and chemical thinners, to typically drill an ultra-HPHT reservoir. Though various synthetic polymer-based water-based drilling fluids have been developed recently with improved thermal stability (>= 200 degrees C). However, these drilling fluids require a particular concentration of bentonite to achieve filtration and rheological characteristics at higher temperatures. Therefore, it is necessary to design a functional polymer-based clay-free drilling mud with the appropriate molecular weight and molecular weight distribution to achieve the required thermal stability without affecting the desired performance of the drilling fluid under HPHT conditions. In the present study, polyanionic cellulose (PAC: an exceptional filtration loss control agent below 150 degrees C) and xanthan gum (XG: an excellent viscosifying agent below 150 degrees C) are modified using multiple functional monomers with strong viscosifying, deflocculation, and lubrication characteristics at elevated temperature to enhance the thermal stability of the formulated drilling mud. A graft radical polymerization is adopted to synthesize functional copolymer of PAC and XG. Thermally stable PAC- and XGgrafted copolymers with desired molecular weight and molecular weight distribution were synthesized through minimum experiments using the central composite design (CCD) assisted response surface methodology (RSM). Optimally synthesized grafted copolymers were used to formulate better-performing clay-free drilling muds satisfying filtration loss, viscosifying, and lubrication criteria under HPHT and salt contamination conditions. The newly developed clay-free mud performs satisfactorily at temperatures <= 200 degrees C with NaCl and CaCl2 contamination <= 7.5 wt% and 2.5 wt%, respectively.
