Preparation of Oleic Acid‐Modified Graphite/Biodiesel Composite Lubricants and Its Synergistic Lubrication Mechanism in Water Based Drilling Fluids

ABSTRACT With the gradual shift of global oil and gas exploration and development toward offshore, deep, ultra‐deep, and high‐displacement unconventional wells, the drilling process is increasingly challenged by excessive torque and high frictional resistance, which severely restrict drilling efficiency and operational safety. Meanwhile, under the background of increasingly stringent global environmental protection regulations, traditional oil‐based drilling fluid lubricants are gradually becoming inapplicable due to poor biodegradability and severe environmental pollution. Thus, developing high‐performance, environmentally friendly water‐based drilling fluid lubricant has become an urgent demand in the oil and gas drilling industry. To address this critical issue, this study proposes a two‐step strategy for modified graphite/biodiesel (MG/BD) composite lubricants: graphite was first surface‐modified with oleic acid to enhance its dispersibility and compatibility with organic phases, followed by compounding with biodiesel (a renewable biomass derivative) to obtain the composite lubricant. Systematic tribological tests were conducted to assess the lubricating performance of the composite lubricant. The results demonstrate that the MG/BD composite lubricant exhibits excellent friction‐reducing and anti‐wear properties at room temperature. When 1.0 wt% of the MG/BD composite lubricant with 60% MG content was added to the base slurry, the friction coefficient was reduced by 89% compared with the base slurry without lubricant; even after thermal aging at 150°C for 16 h, the friction coefficient still maintained a 73% reduction efficiency, indicating good thermal stability. Comparative experiments with commercial water‐based drilling fluid lubricants further confirm that the MG/BD composite lubricant has superior lubricating performance. The mechanism analysis reveals that the excellent tribological properties of the composite lubricant stem from a synergistic effect of bentonite dispersion regulation and solid–liquid composite lubrication. On one hand, BD components adsorb on bentonite surfaces to form a steric hindrance layer, while nano‐MG particles intercalate into bentonite interlayers, jointly inhibiting bentonite agglomeration and enhancing drilling fluid stability. On the other hand, a stable solid–liquid composite lubricating film is formed on the metal friction pair surface: a composite solid film consisting of a continuous graphite solid lubricating film and an inorganic reaction film formed by iron oxides was formed to reduce direct contact between friction pairs. Simultaneously, BD forms a complementary liquid lubricating film to fill micro‐pits and reduce shear resistance between the solid film and metal substrate. Even after 150°C aging, partial decomposition of BD and increased defects in the graphite film slightly degrade anti‐wear performance, but the residual solid–liquid composite film still maintains effective lubrication. This study provides a novel approach and technical support for the development of high‐performance, environmentally friendly water‐based drilling fluid lubricants, and has broad application prospects in unconventional well drilling operations.