Obtaining Formation Water Chemistry Using a Mud Tracer and Formation Tester in a North Sea Subsea Field Development

摘要

Precipitation from and corrosion by formation water coproducedrnwith target hydrocarbons can cause seriousrnproblems, and an early knowledge of water chemistry isrnimportant for completion and facilities design. Owing to thernhigh cost of interventions, this knowledge is particularlyrnimportant in deepwater developments and subsea completions.rnSamples of formation water are obtained by producingrnfrom the well completed in the water leg or sampling afterrnwater production occurs naturally. The former method carriesrnsignificant cost and the latter method may delay thernknowledge of the water chemistry.rnAlternatively, a formation tester can be run into thernwellbore to collect small samples directly from the formation..rnIn wells drilled with water-based mud, the results are generallyrnmixed because successful collection of formation waterrnrequires that the tool be able to discriminate between the waterrnand invaded filtrate. The method depends on the contrast inrncolor or resistivity between formation water and water-basedrnmud filtrate to determine when it is collecting water, and thisrncontrast often doesn't exist.rnThis paper presents a new solution. A mud tracer was usedrnto change the optical properties of the mud filtrate to make itrndetectable downhole. Laboratory experiments checked therncompatibility of the tracer with gypsum or glycol mud system,rnsensitivity to high clay content, and ability to be detected withrnanother tracer, and individually and in combination with arndark-colored filtrate. Shop experiments were carried out tornestablish a relationship between different tracer concentrationsrnand optical properties at specific wavelengths. Thisrnrelationship represents the basis of tracer detection andrncontamination prediction in real time downhole.rnThe first trial was carried in the Otter field in the northernrnNorth Sea. The field comprises three producers and tworninjectors with subsea completions. Downhole water samplesrnwere obtained during the logging phase on one of the injectorsrnat an early stage of the field development. Sample quality andrnflushing time were optimized using the new technique. Samplernanalysis results compared well with later production tests.