Measurement of the water droplet size in water-in-oil emulsions using low field nuclear magnetic resonance for gas hydrate slurry applications

摘要

Turbulent flow in oil and gas pipelines often results in the formation of a water-in-oil (W/O) emulsion. Small water droplets in the pipeline provide a large total surface area for hydrate formation at the water/gas saturated oil interface, which can lead to full conversion of water to gas hydrate. As a result, this may prevent the formation of large hydrate aggregates that can cause hydrate particle settling and eventually plugging. It is thus of particular interest to determine the water droplet size of an emulsion. Since water droplet size of the emulsion provides information about the hydrate particle size in the slurry, it is crucial to determine the water droplet size in a W/O emulsion. In this work, the water droplet size of model W/O emulsion systems was measured using two techniques, diffusion-transverse relaxation (T-2) experiments using low-field nuclear magnetic resonance (NMR) and optical microscopy image analysis techniques. The T-2 distribution of the emulsion was also measured. The water volume fraction was varied from 10-70 vol%. The NMR and microscopy image analysis results show the droplet size ranging from 3.5 to 4.5 mu m and 2 to 3 mu m, respectively. Both techniques show a minimum size of 2 and 4 mu m at 50 vol% water cut. There are two main reasons for the small difference in droplet size distribution (DSD) measured using these techniques: NMR provides DSD of the entire emulsion sample as opposed to the optical microscopy technique that only captures a small sample of the emulsion. In addition, since the NMR method does not require sample preparation, the characteristics and properties of the emulsion are maintained. Using microscopy images, the sample is compressed between two glass slides. This will disturb the properties of the emulsion. By combining the diffusion-T-2 and T-2 distributions, the surface relaxivity was determined to be 0.801 mu m/s for the W/O emulsion. The DSD obtained from the NMR method in this work was compared with microscopy analysis, and results show there is reasonable agreement between the two methods. This paper provides a comparison of the two methods that can be used to determine the water droplet size of W/O emulsions. This study indicates that a relatively simple quantitative NMR method can be utilized to determine the water droplet size of W/O emulsions before gas hydrate formation and hence can be used to assess the gas hydrate slurry properties and plugging risk of W/O systems.