Experimental investigation and computational insights of enhanced rheological stability of water-based drilling fluids by microspherical polymers

Three-dimensional (3D) bulk polymer, as an alternative to linear polymer, has exhibited large potential in formulating high performance water-based drilling fluids. Understanding mechanism behind the enhanced rheological stability of drilling fluids by microspherical polymers is critical for designing and developing new high performance drilling fluids. In the present work, we conducted a pioneering investigation that integrated experimental techniques with computational modeling, to explore the enhancement mechanism involved in the targeted drilling fluids. Inverse emulsion polymerization experiments were firstly carried out to fabricate the microspherical polymer P(AA-AM-AMPS), and then physicochemical properties of microspherical polymer were characterized. Subsequently, the performance of drilling fluids with microspherical polymer as an additive was systematically evaluated. Finally, molecular simulations were employed to investigate the characteristics of chemical active sites, molecular conformation and the structural variation at various temperature. The results showed that the final microspherical polymer has a core-shell structure, with an average size of 198.3 nm and a molecular weight of 6.2 × 106 g/mol. The 3D structure exhibits good thermal stability, and thermal decomposition occurs above 220°C. The drilling fluids formulated with the microspherical polymer showed better rheological stability in the medium-low (4 - 65°C) and medium-ultra high (40 - 240°C) temperature ranges, compared to the relevant drilling fluids with the parallel linear polymer. Analyses on electrostatic potentials (ESP) and frontier molecular orbital (FMO) revealed that active groups within the confined sphere domain mainly include carbonyl C=O, and amide -CO(NH2). Additionally, these active groups exhibit a hierarchical distribution in the outer molecular region. Analyses on the radius of gyration (Rg) and the radial distribution function g(r) further validated the core-shell structure of microspherical polymer and its temperature-resistant stability. Moreover, a new self-consistent structural compensation model was proposed to rationalize the structure-activity relationship of microspherical polymer in drilling fluids. The computational results align well with the experimental findings. This pioneering work will provide valuable information for both synthesis of new functional additives and formulation of tailored performance drilling fluids.