ROP optimization is an important technical approach to reducing drilling costs. It has been found that the water-based drilling fluid composition have a significant impact on ROP in field applications for many years, but the underlying mechanisms are not yet clear. In this study, targeting the difficult-to-drill formations of the Shaximiao Formation in the central Sichuan region, the interaction mechanisms between drilling fluids and rocks, as well as their effects on rock mechanics and drillability, were experimentally explored by altering the composition of the drilling fluids with the aim of enhancing the infiltration capability of fluid. The research indicates that among water-based drilling fluids formulated with bentonite, sepiolite, palygorskite, and nanoclay, the nanoclay-based fluid exhibits the highest rapid infiltration capability into the tight sandstone of the Shaximiao Formation, followed by sepiolite, palygorskite, and bentonite. Nanoclay, due to its low clay platelet content, has a slow filter cake formation rate and a higher instantaneous filtration loss, resulting in a stronger rapid infiltration capability for the configured drilling fluid. The stronger the rapid infiltration capability of the drilling fluid, the more deteriorated the mechanical properties and drillability of rock, and the lower drillability index value, leading to higher rock fragmentation efficiency. The underlying mechanism lies in the promotion of hydration reactions in the Y/ M layer by rapid infiltration, which disrupts the original cementation state within the rock and reduces its resistance to fracturing under external forces. Field tests have shown that after optimizing the drilling fluid, the mechanical drilling speed increased from an average of 1.25 m/h in the upper wellbore section to 2.29 m/h, indicating that improving the rapid infiltration capability of the drilling fluid has good adaptability for ROP optimization in the tight sandstone formations of the Shaximiao Formation and is worthy of further promotion and application.
2023, 46 (4):
7-12.