Study on Key Mechanisms of Constant Rheology Synthetic Base Drilling Fluids

An amidated dimer acid derivative (a flow-pattern optimizer) and organophilic montmorillonite are two core additives used in formulating the FLAT-PRO constant rheology fluid. By studying these two additives from macroscopic and microscopic perspective and rheological analyses, the constant rheology mechanisms of the FLAT-PRO fluid was extensively investigated. Rheological experiment under controlled temperatures and long-time standing test results showed that, at low temperatures, the volume of the oil phase is reduced, the dispersibility of the organophilic clay in the oil phase becomes poor, and the viscosity and gel strengths were increased greatly. With temperature increasing, the volume of the oil phase expands and the organophilic clay becomes well dispersed in the oil phase, the viscosity and gel strength of the drilling fluid were correspondingly reduced. When the flow-pattern optimizer was added into the drilling fluid, changing of the viscosity and gel strength of the drilling fluid with temperature becomes less progressive, and the drilling fluid thus showed to some extent a constant rheology characteristics. Microscope observation, XRD analysis and SEM observation showed that the molecules of the flow-pattern optimizer can intercalate in between the micro crystal layers of the organophilic montmorillonite, thereby increasing the c-space of the clay and improving the dispersion of the clay in oil. In this way a temperature-responsive tight network structure is formed in a drilling fluid by highly dispersed clay platelets, molecules of the flow-pattern optimizer and emulsified droplets. The molecular chains of the flow-pattern optimizer become stretched at elevated temperatures; compared with the viscosity and gel strength of the drilling fluid at 2 ℃, the viscosity and gel strength of the drilling fluid at 65 ℃ were increased with much higher magnitude, making the rheology of the drilling fluid change more gently between 2 ℃ and 65 ℃, showing a characteristics of constant rheology.