Water-Based Drilling Fluids Rheological Properties and Clay-Silty Sediments Mechanical Behavior Response

Natural gas hydrates are characterized by shallow burial and a narrow pressure window. During actual drilling operations, drilling fluids can cause a series of impacts such as cyclic disturbances and invasion into the formation, altering the properties of formation fluids and leading to engineering issues such as formation instability. The rheological properties of drilling fluids directly affect their flow behavior in the wellbore, cuttings transport capacity, and the extent of formation invasion. Therefore, optimizing the rheological performance of drilling fluids is a prerequisite and guarantee for ensuring drilling safety and the safe extraction of gas hydrates. In this study, focusing on the rheological properties of marine water-based drilling fluids, traditional water-based drilling fluids were prepared with different additives (Bentonite, NaCl, Sulfomethylated Phenolic Resin, and Polyethylene glycol). The rheological characteristics of the drilling fluids were systematically investigated through rotational shear tests, stress oscillation tests, and frequency oscillation tests. Additionally, the impact of drilling fluid invasion on the mechanical properties of clay-silty sediments was examined by conducting triaxial shear tests on soil samples containing different drilling fluids. The results indicate that (1) bentonite and sulfomethylated phenolic resin significantly increase viscosity and enhance viscoelasticity, thereby reinforcing the gel structure and improving the rheological performance of the drilling fluid. NaCl reduces drilling fluid viscosity; at low concentrations, it enhances viscoelasticity, whereas at high concentrations, it destabilizes the drilling fluid structure and decreases viscoelasticity. Polyethylene glycol exerts a minimal effect on drilling fluid viscosity and viscoelasticity, leading to only a slight reduction. (2) The rheological curves of all drilling fluids exhibit distinct yield-pseudoplastic flow behavior, which is better described by the Herschel-Bulkley model. The adjusted R2 values for the fitted models were mostly above 0.99. (3) All soil specimens exposed to drilling fluids exhibited strain-hardening response. Compared to water-saturated soil specimens, those exposed to drilling fluids exhibited more pronounced strain hardening and a significant increase in failure strength. (4) The incorporation of sulfomethylated phenolic resin and polyethylene glycol significantly enhanced the failure strength of soil specimens exposed to drilling fluids, whereas NaCl addition led to a reduction in failure strength.