Preparation and Evaluation of Viscosity Reducer with Strong Adsorption Characteristics

High temperature high pressure (HTHP) wells require high-density drilling fluids to control well pressure, which complicates the development of water-based drilling fluid systems (WBDFs). Unfortunately, high temperature degradation of drilling fluid additives and high fraction of weighting materials cause thickening and higher filtration loss of drilling fluid, further lead to serious well control incidents and lost circulation. It is a great challenge to balance the water-based drilling fluid’s properties including rheology, filtration loss and sedimentation stability. To prevent the rheological deterioration by high concentration of weighted solid particles in ultra-high-density drilling fluid, a mixture of low molecular weight carboxylic acid polymer and phosphoric acid polymer (M-CAPAP) was prepared in this study. The effects of M-CAPAP on reducing viscosity in high-density barite suspensions was explored. Additionally, the viscosity reduction mechanism was analyzed through nuclear magnetic resonance (NMR) analysis, Zeta potential measurement, and ultraviolet visible spectrum analysis (UV-vis). Performance test confirmed that, following thermal aging at 180 °C for 16 h, the apparent viscosity of both barite suspension and drilling fluid (2.4 g/cm3) was reduced by at least 45% with 2 w/v% M-CAPAP, while the HTHP filtration loss was also decreased by 60%. The presence of a strong adsorption between M-CAPAP and all-lattice plane of barite cell, which enables M-CAPAP molecules to be dispersed across the surface of barite particles, resulting in the release of trapped free water molecules between aggregated weighting particles. Meanwhile, the adsorption of M-CAPAP formed strong repulsive interactions between weighting particles, which effectively reduced the collision friction between particles and improved the rheological properties of ultra-high-density drilling fluid. This novel viscosity reducer realizes a balance between rheology, filtration loss and sedimentation stability of a high-density water-based drilling fluid under high/ultra-high temperature conditions. It can achieve maximum weights of water-based drilling fluid and shows promise in application in ultra-deep well drilling.