Probing the state of water in oil-based drilling fluids

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 BaSO 4 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 T 2 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 T 1 -T 2 NMR. The results showed that the water droplets bound to the hydrophilic BaSO 4 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. • Water droplets bound to BaSO 4 particles formed aggregates of hydrated particles in the oil phase. • Bound water showed a shorter T 2 relaxation time than water droplets. • Bound water exhibited a higher temperature of crystallization T c, peak than water droplets. • Invert emulsion drilling fluids are in suspensions of particle aggregates in the oil phase, rather than W/O emulsions.