A representative clean heavy oil sample (<1% BS&W) is crucial to obtain accurate fluid property data, which are required for heavy oil production, processing, and transportation designs. However, it is always a challenge to remove water from a heavy oil sample (<20° API) due to its near-water density (10° API) and high viscosity (slow separation). The objectives of this study include qualifying three heavy oil dewatering methodologies, determining their impact on heavy oil properties, and recommending suitable method(s) for preparing heavy oil samples for laboratory fluid phase behavior and property measurements.
The three demulsification methods – centrifugation, the addition of demulsifiers, and distillation – were systematically evaluated and compared. Three dead heavy oil samples ranging from 20 to 11 API gravity degrees and with viscosities ranging from 100 to 5,000 mPa·s at 310.9 K (38°C) were selected. A stable emulsion with a 30 vol% synthetic brine were prepared using each heavy oil sample, which was used to evaluate each of the demulsification methods. Viscosity, density, and composition of the dewatered samples were measured and compared with the corresponding original oil data.
This study demonstrated that centrifugation had the minimum impact on sample properties, although it was difficult to reduce the residual water content below the dewatering target of less than 1 wt%, especially for sample densities nearest the water density. The addition of chemical demulsifiers followed by mild centrifugation resulted in samples that met the dewatering target. The change in fluid properties caused by the demulsifier method was similar to the minor changes observed by the centrifugation method. Distillation could remove water from all three emulsions to below the dewatering target, if appropriate experimental procedures were followed. However, distillation had the strongest impact on dewatered oil properties due to the loss of light/intermediate components, despite precautions to minimize the mass loss.
In current open literature, the effect of the heavy oil sample preparation procedure on the measured heavy oil properties and PVT data is rarely reported in detail, which makes it a challenge to properly perform the data interpretation and use them in relative modelling and design work. This simple, systematic study provides insight into the effect of three common dewatering procedures and guidance for developing reliable laboratory sample preparation protocols for heavy oils.