This paper discusses the application of selective slug sampling using the wireline formation tester – RCX* (Reservoir Characterization eXplorer), in a clastic heavy oil reservoir and its significance to fluid characterization. The low mobility ratio between the reservoir fluid and water-based mud filtrate results in a long cleanup time to obtain samples with an acceptable level of contamination. Tool sticking and wellbore stability issues add to the challenges in open-hole fluid characterization and testing. Increasing the flow rate to reduce cleanup time does not help mostly due to formation of an emulsion between the viscous oil and the mud filtrate. The emulsion is hard to disperse, leading to a contaminated sample. A higher flow rate results in sanding and loss of seal.

In selective slug sampling, phase segregation is achieved by lowering and optimizing the flow rate, reducing the pressure drawdown that curtails emulsion formation and movement of grains thus increasing the sealing efficiency. The segregated oil slugs can be selectively collected in sample tanks multiple times downhole. Even a water cut of 60 to 70% is sufficient to collect high-quality samples, which is a significant improvement in sampling time compared to conventional fluid sampling, where 10% or less water cut is required.

The technique was applied in an unconsolidated clastic heavy oil reservoir in Vietnam. Four 14- to 22-°API single-phase oil samples were collected in two hours each. Laboratory results indicate less than 10% contamination; water cut being 50 to 75% at the time of the sampling. It was the first time that successful heavy oil samples were collected from these layers.

Fluid sampling is critical to identifying compositional variation of reservoir fluids along the depth, resulting in appropriate sampling strategy and eventually correct fluid characterization. Sampling in unconsolidated heavy oil reservoirs is improved using the selective slug sampling technique, making it possible to secure fast, yet high-quality samples, aiding fluid property characterization.

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