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Magnetic resonance diffusion measurements of droplet size in drilling fluid emulsions on a benchtop instrument

Mitchell, Jonathan
Colloids and surfaces 2019 v.564 pp. 69-77
cleaning, colloids, diffusivity, drilling, droplet size, droplets, emulsions, industry, liquids, magnetism, nuclear magnetic resonance spectroscopy, petroleum, rheological properties, rheology
Oil-based drilling fluids are complex emulsio-dispersions used in the construction of subsurface wellbores. Design and control of the rheological properties are crucial for successful drilling operations because the fluid performs several critical functions, such as hole cleaning (particle transport) and ensuring wellbore stability. One of the physical characteristics of the fluid that relates to rheology is the size of the water-in-oil emulsion droplets. Here we demonstrate the use of nuclear magnetic resonance (NMR) pulsed field gradient (PFG) diffusion measurements to determine droplet size distributions in oil-based drilling fluid analogues and a commercial formulation. NMR PFG allows these optically opaque samples to be measured without dilution, and the measurement is sensitive only to the liquid components. Although the NMR technique is established, the application to drilling fluids is novel. Through simulation and experiment, the suitability of a low-field benchtop NMR instrument (commonly used for rock core analysis in the petroleum industry) for measuring droplet size distributions in these fluids is explored. Despite limitations on the available gradient strength and the bulk diffusion coefficient of the brine, droplet radii in the range a = 1 μm to 50 μm are measured. The low-field NMR instrument is shown to be suitable for characterizing drilling fluids and similar complex emulsions.