A thermo-associating copolymer integrated with biogenic nanosilica as a novel viscosifier in low solid drilling fluids

Smart low-solid drilling fluids (SLSDFs) with thermo-controllable rheological properties and attractive thickening characteristics have recently captivated profound attention due to their low formation damage and enhanced cuttings lifting capacity. However, their applications to deep hole drilling at high temperatures have remained limited because of the thermal instability and environmental constraints of the thermo-associating polymers as additives. This work explored the synergistic benefits of thermo-associating polymer and biogenic nano-silica (B-SiNP) extracted from rice husk to improve the thermo-stability of SLSDF. This study shows that the nano-hybrid, TAP-S based on vinyl-terminated B-SiNP could potentially mitigate the limiting performance of conventional LSDF (F-2) caused by the failure of thermo-associating copolymers under elevated temperatures. TAP-S bearing drilling fluid (F-3) could preserve more than 5.6-fold of its initial properties (ca. apparent viscosity, plastic viscosity, yield point, and gel strength) with a nearly flat-gel profile in the temperature range of 25–230 °C, which was higher than those of the counterpart F-2 and base fluid according to the results of rheological tests analysis. In addition, TAP-S exhibited an abrupt thermo-thickening characteristic with a magnitude declining by only 1.05-fold and the activation Gibbs free energy of 1339 kJ/mol above the plateau (ca. 130 °C), reflecting its less sensitivity compared to F-2 under a continuous heating process. As a result, a lower temperature was required to drive the dehydration of the residual fraction of lower critical solution temperature (LCST) in nano-hybrid structures than TAP according to the results of DSC analysis. Thus, lower energy was expected to disintegrate the residual hydrogen bonds formed between the LCST chains and surrounding water molecules at elevated temperatures. Moreover, TAP-S formed a solid-micro-crosslinking structure network which exhibited a more stable hydrodynamic diameter as revealed by DLS analysis. Compared with TAP, TAP-S consisted of a larger composite B-SiNP-TAP integrated spatial network structure based on the results of environmental scanning electron microscope, which conferred a degree of thermal conductivity characteristic for improved temperature resistance. This contributed to the effective binding onto bentonite particles for protection and maintained a relatively stable bentonite particle dispersion according to the results of EPM and particle size distribution analyses. Consequently, TAP-S fortified drilling fluid demonstrates improved rheological and filtration performance under severe downhole conditions. Therefore, TAP-S, the thermo-associating copolymer integrated with B-SiNP could find potential application as an eco-friendly viscosifier in LSDFs for deep-well drilling operations.