An Amide-Carboxylic Acid Compound as Gel Structure Breaker to Improve the Rheology of Oil-Based Drilling Fluids.

High-density oil-based drilling fluids (OBDFs) are widely used in drilling operations, but during their application, the viscosity of the fluid typically increases due to the enhancement of the solid-phase gel network structure. This can lead to issues such as impaired fluid circulation, increased blowout risks, and accelerated drill bit wear. In this study, a compound (OCD), synthesized from tall oil fatty acids, diethylene triamine, and maleic anhydride, was developed to disrupt the strong gel structure in high-density OBDFs, thereby reducing the viscosity of the OBDFs. Rheological properties, including viscosity, yield point, and gel strength, were tested to evaluate the viscosity-reducing effect of OCD on both laboratory-prepared and field high-density OBDFs. Additionally, the effects of OCD on electrical stability (ES), high-temperature high-pressure (HTHP) filtration loss, and solid-phase settling stability were also tested. Finally, the mechanism of OCD was analyzed through contact angle tests, particle size analysis, and microstructural observations. The experimental results demonstrated that OCD could effectively reduce the viscosity of various high-density OBDFs. Adding 2 wt% of OCD reduced the apparent viscosity of laboratory-prepared OBDFs by 20.4%, and reduced the apparent viscosity of field OBDFs with a density of 1.7 g/cm by 29.2%. Furthermore, OCD showed good compatibility with OBDFs, having negligible effects on HTHP filtration loss and ES, and maintained good viscosity-reducing performance even at 180 °C. Mechanistic studies revealed that OCD enhanced the hydrophobicity of the solid phase, reduced the particle size of solids, and prevented the formation of excessive network structures in the oil. Therefore, this study provides significant practical value for controlling the rheological performance of the gel system in OBDFs.