Probing the state of water in oil-based drilling fluids

Abstract

Oil-based drilling fluids, also referred to as “invert emulsion drilling fluids”, have been widely accepted as mixtures of water-in-oil (W/O) emulsions and inorganic particles, in which the water droplet–particle interaction has not been considered. Probing the state of water plays an important role in optimizing the performance of oil-based drilling fluids. In this work, hydrophilic BaSO4 and hydrophobic polytetrafluoroethylene (PTFE) particles were added to W/O emulsions. The microstructures of the water droplets and particles in the oil phase were studied using an optical microscope. The state of water was characterized by T2 nuclear magnetic resonance (NMR) and low-temperature differential scanning calorimetry (LT-DSC). Finally, the state of water in actual invert emulsion drilling fluids was verified by two-dimensional T1-T2 NMR. The results showed that the water droplets bound to the hydrophilic BaSO4 particles formed hydrated particle aggregates in the oil phase, while the water droplets and hydrophobic PTFE particles were suspended individually in the oil phase. Thus, the so-called invert emulsion drilling fluids are in fact colloidal suspensions of hydrated particle aggregates in continuous oil phase, rather than W/O emulsions. The findings of this work provide new insights into the formulation and utilization of oil-based drilling fluids.