TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractPrecipitation from and corrosion by formation water coproduced with target hydrocarbons can cause serious problems, and an early knowledge of water chemistry is important for completion and facilities design. Owing to the high cost of interventions, this knowledge is particularly important in deepwater developments and subsea completions.Samples of formation water are obtained by producing from the well completed in the water leg or sampling after water production occurs naturally. The former method carries significant cost and the latter method may delay the knowledge of the water chemistry.Alternatively, a formation tester can be run into the wellbore to collect small samples directly from the formation.. In wells drilled with water-based mud, the results are generally mixed because successful collection of formation water requires that the tool be able to discriminate between the water and invaded filtrate. The method depends on the contrast in color or resistivity between formation water and water-based mud filtrate to determine when it is collecting water, and this contrast often doesn't exist.This paper presents a new solution. A mud tracer was used to change the optical properties of the mud filtrate to make it detectable downhole. Laboratory experiments checked the compatibility of the tracer with gypsum or glycol mud system, sensitivity to high clay content, and ability to be detected with another tracer, and individually and in combination with a dark-colored filtrate. Shop experiments were carried out to establish a relationship between different tracer concentrations and optical properties at specific wavelengths. This relationship represents the basis of tracer detection and contamination prediction in real time downhole.The first trial was carried in the Otter field in the northern North Sea. The field comprises three producers and two injectors with subsea completions. Downhole water samples were obtained during the logging phase on one of the injectors at an early stage of the field development. Sample quality and flushing time were optimized using the new technique. Sample analysis results compared well with later production tests.