Hydrothermal synthesis of nitrogen and sulfur co-doped reduced graphene oxide anchored cuprous oxide nanocomposite for enhancing rheological and filtration performance of water based drilling fluids

Borehole instability driven by reactive shale hydration and excessive filtrate invasion remains a critical challenge in drilling operations. To address this, we synthesized a novel nitrogen and sulfur co-doped reduced graphene oxide/cuprous oxide (N,S-rGO/Cu 2 O) nanocomposite to reinforce the rheological and filtration properties of environmentally friendly water-based muds. The nanocomposite, prepared via a facile one-pot hydrothermal method, was characterized structurally and compositionally, then evaluated in both polymeric and high-solid base fluids. Performance was assessed through standard and high-pressure/high-temperature (HPHT) filtration, rheological modeling, and hot-rolling shale recovery tests. Results demonstrated that an optimal nanocomposite concentration of 1000 ppm maximized fluid efficiency, with rheological behavior conforming closely to the Herschel-Bulkley model. Crucially, the additive significantly mitigated fluid loss, achieving a 58.8% and 61.3%reduction in API filtrate volume for polymeric and high-solid fluids, respectively, alongside HPHT fluid loss reductions of 57.6% and 64.1%. Furthermore, hot-rolling experiments confirmed enhanced shale stability, yielding a 20% increase in shale recovery. These targeted improvements are attributed to the formation of a dense, hydrophobic nanoplug that physically seals pore throats and chemically inhibits clay swelling, though concentrations exceeding 1000 ppm caused particle agglomeration and diminished returns. Ultimately, the N,S-rGO/Cu 2 O nanocomposite operates through a synergistic mechanism of robust physical plugging and rheological networking, providing a highly effective solution for stabilizing reactive shales in complex subterranean environments.