A key processing step in the recovery of crude oil is the break-up of the stable water-in-oil (w/o) emulsion in large settling tanks. Typically these continuous flow settlers operate with a residence time between 10 and 15 minutes, reducing the water cut to below 5%. To aid separation and maintain process throughput, droplet growth is promoted in the pipeline leading up to the settler. It is generally believed that droplets smaller than 100µm at the settler inlet are carried over in the oil phase. If a significant quantity of the inlet water is carried over, the efficiency of the secondary stage cleaning process (commonly electrocoalescence) is reduced; hence the growth of sub 100µm droplets prior to separation and their removal in the settler is crucial for maintaining throughput.
To increase the droplet size from tens to hundreds of microns, chemical additives are pumped into the pipeline prior to the gravity settler. These additives (demulsifiers) act to penetrate a stable water-oil interface and allow for the coalescence of water droplets when in intimate contact. Without the addition of demulsifiers, surface active components such as, asphaltenes and resins form a stabilizing film around the water droplet preventing coalescence.
Demulsifier action and their effect under different flow conditions is still not a well understood science. In the current study 3 demulsifiers (IGEPAL CO – 520, 720 and 890) have been chosen to investigate the effect of (i) molecular weight and (ii) concentration on droplet coalescence. In addition, the fluid shear rate and the time of shear are also investigated.
‘Model' emulsions have been prepared with a 30% water cut dispersed in a light mineral oil. The water droplets are prepared in the size range of 10-30µm and stabilised by an asphaltene film. The asphaltenes are extracted from a crude sample recovered from the Grane oil field. A flow wheel (volume 2 liters) is used to shear the droplets, with 30ml samples removed and drained through the Dispersion Technology (DT-1200) acoustic particle size analyzer to measure the droplet size distribution. Droplet size distributions are compared before and after shearing to investigate the effect of both chemical addition and fluid shear on the coalescence rate.
Preliminary studies in the absence of demulsifiers have shown that for stable emulsions there is a critical flow range where droplet coalescence is observed. This study highlights the importance of chemical dosing and fluid shear when breaking stable w/o emulsions.
See more of this Group/Topical: Topical D: Chemical Engineering in Oil and Gas Production and Other Complex Subsurface Processes