Investigations of rheology and a link to microstructure of oil-based drilling fluids

Abstract

Abstract Towards establishing a first-principles physical link between the field-behavior and microstructure of oil-based drilling fluid (OBDF), we study rheology of an OBDF and its variation with different contents of water, clay viscosifier, or barite weighting agent during steady and dynamic rheological phenomena, and develop a “cryo-preserved” experimental methodology towards in-situ microscopy. Rheological data was collected for shear rate ramps between 0.0001 s−1 and 1000 s−1, under temperatures ranging from 10o to 60o C, with three water/oil ratios, and multiple OBDF compositions. Oscillatory and constant shear rate measurements showed elastic and loss modulus co-dominate when stress is below ∼1 Pa, but above a ∼1 Pa “apparent yield-stress” the fluid microstructure reorganizes, elasticity decreases, and the flow follows a viscosity-dominated shear-thinning Herschel–Bulkley behavior. Signs of thixotropy are seen in increasing vs. decreasing flow curves when in the elastic regime, and elasticity shows time-dependency. Clay particles created elasticity more so than the other ingredients. Elimination of water, clay, or barite caused an approximately equal reduction of viscosity. Elimination of water also led to elastic modulus dominating over viscous modulus for a longer range of strain in the linear-elastic pre-yield state, likely due to water droplets having a shorter length scale over which yield-stress is broken than does a water-free mixture. Elimination of barite had a similar but lesser effect on elasticity. Cryological scanning electron microscopy (Cryo-SEM) and cryological x-ray spectroscopy (Cryo-EDS) methods show promise for future work to identify the spatial microstructure of the OBDF at nanometer to micrometer scales in a “cryo-preserved” state that approximates an in-situ like state. Chemical identification of individual phases within the mixture components is achieved via a quantitative elemental method. Water droplets, base oil, and barite are identifiable and future methods for in-situ imaging of sheared OBDFs is proposed.