Heavy Oil Sampling Using Wireline Formation Tester - Case Studies of Highly Viscous Oils from Bohai Bay

Abstract

Abstract Collecting high quality representative heavy oil samples is an essential requirement for detailed PVT and flow assurance analysis. Heavy oil is often discovered in shallow and highly unconsolidated reservoirs, which results in significant formation fluid sampling challenges. To overcome the potential problems of sand influx, emulsion formation and packer seal loss, an operationally optimized and efficient operating procedure has to be applied to sample the highly viscous heavy oil. Using the optimized sampling methodology, high quality representative fluid samples (priority samples > 95% formation oil) were collected even with a water-cut of 40-70% at the time of sampling by adopting the selective slug sampling technique, thus saving time and money. Low permeability (< 10 md) oil bearing formations were also addressed by applying this fit-for-purpose methodology, thus assisting the oil discovery. The API gravity of the collected samples ranged between 8 ° API and 15 ° API and the viscosity at surface conditions is several centipoises (cp) in many cases. Based on the successful heavy oil sampling operations, this paper discusses important observations, which can be useful for future planning of similar jobs. These include early oil breakthrough, emulsion effects on sampling and changing mobility during clean-up. Unlike the water-base mud (WBM) invasion characteristics of regular oil bearing formation, which is mainly dominated by overbalance pressure and is a function of time, the invasion of heavy oil bearing formation is highly influenced by viscosity dynamics. The resultant of such dynamics is delayed oil breakthrough and the formation of emulsion during the sampling process, resulting in an undesirably long pumping time before collecting a representative sample with acceptable contamination level. Lastly the changing mobility aspect is discussed, which is caused by drag-force effects and viscosity changes during the clean-up.