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 perfluorodimethylcyclobutane (PDMCB), perfluoromethylcyclopentane (PMCP), perfluoromethylcyclohexane (PMCH), 1,3 -perfluorodimethylcyclohexane (1,3 - PDMCH) and sulphur hexafluoride (SF6). The Gullfaks field 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 Sleipner field was started in August 1993. Reinjection of gas started in April 1994 and the first tracer, PDMCB, was injected in June 1994. The purpose of this injection was to investigate the travel time of reinjected gas and to monitor the reservoir performance. Samples of oil and gas were collected from the separator and analysed by gas chromatography (GC) connected to an electron capture detector (ECD). Sampling continued throughout the pilot period to establish the tracer production profile. 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 WAG pilot mechanism and communication in the fields. Introduction Tracer technology has for many years been applied as a tool to improve reservoir description. According to literature the most widely applied gas tracers have been tritiated methane and 85Kr. However, since 1991 perfluorocarbon (PFC) tracer technology has been growing and is today applied in several of the most important fields in the North Sea. In addition to the PFC, sulphur hexafluoride has also shown excellent field tracer properties. A gas tracer program was started in the Gullfaks field in 1991. Since then five PFC tracers and SF6 have been injected in different wells. Preliminary results from these tracer studies were published by Ljosland et. al in 1993. In two of the wells, 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 Sleipner was started in 1994. The tracers applied in this field are perfluorodimethylcyclobutane (PDMCB), perfluoromethylcyclopentane (PMCP). perfluoromethylcyclohexane (PMCH), and sulphur hexafluoride (SF6). The PFC tracers are all liquids at standard (ambient) conditions (see Table 1). The tracers were injected by high-pressure pumps directly into the main injection gas line at a rate of approximately 300 ml/min. The amount of PFC tracers injected in each well were in the range of 10 kg to 100 kg. corresponding to 6-60 1. Gas samples from production were collected in pressure cylinders and sent to the Tracer laboratories, Institute for Energy Technology (IFE), for analysis. The samples were primarily taken from the test separator at a pressure of approximately 70 bar. Due to limited capacity on the test separators, some samples were collected directly from the main separator or from the production flowline. Tracer Evaluation Perfluorocarbons (PFC) are hydrocarbons in which all hydrogen atoms are substituted with fluorine atoms. The general formula of the molecules is CxFy. The PFC tracer technology is now well 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 use in water, an emulsion technique is needed due to very low direct solubility. The success of the PFC compounds is mainly due to chemical inertness, high thermal stability and high detectability by gas chromatography with an electron capture detector (GC/ECD). P. 675

The further development of the Statfjord field for the Late Life pressure blowdown phase requires the drilling and completion of many infill wells from existing slots. The overall objective of these wells is to maximize oil production in the short term whilst securing future gas delivery potential for the late life phase. Well design to meet this objective is challenging. The Statfjord and Brent formations are sand prone and robust mechanical sand control completions are required in order to secure offtake rates during late life production. Also, after 28 years of production and water injection, the drilling and completion of wells through the differentially depleted formations is challenging due to a narrow pore pressure/fracture pressure window. Furthermore, the reservoir formations are substantially interlayered with shale sections and this meant that wellbore stability issues would pose significant challenges.Initial Late-Life well designs were based on drilling long open hole sections with an oil-based drilling mud providing low Equivalent Circulating Density (ECD), displacing the well to brine, and then running sand screens and performing an open hole gravel pack (OHGP) across the reservoir section with a conventional water pack. During the last three and a half years 27 Late-Life producers have been drilled and completed. This paper summarizes the operator's experiences over this period with emphasis on well design, drilling and completion tools, well fluid technology and well productivity. It explains how ambitions and well design had to be reassesed in order to overcome several challenges with the selected drilling and completion strategy.