PERFLUOROCARBON TRACER STUDIES AT THE GULLFAKS FIELD IN THE NORTH-SEA

The Gullfaks field in the North Sea is a complex reservoir with oil production from five different formations. The field is laterally divided into nearly 40 fault blocks with a varying degree of communication. The main production strategy is pressure maintenance above bubble point by water injection. A WAG pilot was started in spring 1991. To improve the evaluation of the WAG pilot, it was decided to inject tracers in the gas phase at the beginning of the first two gas injection periods. The WAG pilot forms the basis for the decision on full-field WAG. The tracers applied are two nonradioactive chemical tracers, perfluoromethylcyclopentane (PMCP) and perfluoromethylcyclohexane (PMCH). These compounds were never before used in field reservoir studies. Extensive laboratory testing, studying these compounds in dynamic flow experiments under reservoir conditions, are, however, performed. The compounds are measured in samples of produced ps and oil by gas chromatography (GC) in connection with an electron capture detector (EC). The compounds have a higher partitioning to the oil phase than the ideal tracer tritiated methane, causing minor retention of the tracer with respect to the average gas velocity in the reservoir. The results of this tracer study is a valuable contribution to the interpretation of the WAG pilot mechanism and communication in the area.