This paper discusses the application of non-radioactive gas tracer in the two off-shore fields Gullfaks and Sleipner at the Norwegian shelf of the North Sea. The tracers applied are perfluorodimethy lcyclobutane (PDMCF3), perflrroromethylcyc[opcntane (PM{’ P), perfluoromethylcyc lohexane (PMC’H), 1,3-pcrfluorodimethylcycloitexarrc (1,3PDMCH) and sulphur hexafluonde (SFC). The Gullfaks ticld consists of a complex reservoir with oil production from different formations. The field is laterally divided into nearly 40 fault blocks with varying degrees 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 evaluation of the pilot it was decided to inject tracers in the gas phase early in the first two gas injection periods Production from the Slcipner fscld was started in August 1993. Reinjcction of gas started in April 1994 and the first tmcer, PDMIIt, was injected in June 1994. The purpose of this iojcction wds to investigate the travel time of reinjccted gas and to monitor the reservoir performance. Samples of oil and gas were collected from the separator and analysu.i by gas chromatography (GC) coonectcd to an clcctroo captorc detector (ECD), Sampling continued throughout the pilot period to establish the trwcr product[oo protilc. The tracer compounds have a somewhat higher partitioning to the oil phase than methane, causing a minor retention of the tracer with respect to the average methane gas velocity in the reservoir, The tracer results have given valuable contributions to the interpretation of the W A(i pilot mechanism and communication in the fields. Int reduction Tracer technology has for many years been applied as a tooi to itnprove reservoir description. According to literature the most widely applied gas tracers have been tritiated methane and *$Kr (1). However, since 1991 per-fluorocarbon (PFC) Society of Petroleum Engineers tracer technology has been growing and is today appiied in several of the most important fields in the North Sea. In addition to the PFC, sulphur hexafluoride has aiso shown excellent fleid tracer properties, A gas tracer program was started in the Guilfaks field in 1991. Since then five PFC tracers and SF. have been injected in different welis. Preliminary results from these tracer studies were pubiished by Ljosland et. ai (2) in 1993. [n two of the weiis, where WAG programs were performed, different tracers were applied in two subsequent gas injection periods to monitor the differences in gas movement after water had been injected. The tracer program at Sicipncr was started in 1994. The tracers applied in this field are perfluoro dimcthylcyclobutane (PDMCB), pcrfluoromcthylcy ciopcntane (PMCP), perfluoromcthyicy ciohexanc (PMCH), and sulphur hexafluoridc (SFC). The PFC tracers are aii liquids at standard (ambient) conditions (see Table I). The tracers were injected by highprcssurc pumps directly into the main injection gas iine at a rate of approximately 300 ml/min. The amount of PFC tracers injected in each wcil were in the range of 10 kg to 100 kg, corresponding to 6-60 i.. Gas samples from production were coilected in pressure cylinders and sent to the Tracer laboratories, Institute for Energy Technology (lFE), for anaiysis. The samples were primarily taken from the test separator at a pressure of Wwroximateiy 70 bar. Due to limited capacity on the test separators. some samples were coilccted directly from the main separator or from the production flowiine. Tracer Evacuation Pcrfluorocarbons (PFC) are hydrocarbons in which ail hydrogen atoms arc substituted with fluorine atoms. The general fbnnula of the molcculcs is C. F,. The PFC tracer rcchnoiogy is now weil established as a tool in atmospheric transport studies (3), in house ventilation examinations (4) and even in groundwater (5) and marine (6) tracing and water mixing processes (7). For usc in water, an emulsion technique is needed due to very low direct volubility. The success of the PFC compounds is mainiy due to chemical inertness, high thermal stability and high detectability by gas chromatography with an eiectron capture detector (GC/ECD). Molecules with a cyclic structure and with onc or more aiiphatic branches attached, such as PDMCB, PMCP, PMCH and 1.3-PDMC’H (Fig.1), are those which are most sensitive to electron capture (EC)