Schlumberger

Performance Report: OPTITRAK 600 Tracer Delivers Accurate, Real-Time Downhole Water Sampling

Challenge: An operator in the North Sea wanted to obtain a good quality water sample to evaluate the potential scaling problems that could develop during water injection. Because this well was drilled with a water‐base mud, it was impossible to distinguish between mud filtrate and connate water.

Solution: The operator planned to use a Modular Formation Dynamics Tester (MDT). Formation fluid is pumped through the Live Fluids Analyzer (LFA) module attached to the MDT and is discarded until the sample is uncontaminated with mud filtrate. Resistivity measurements and optical density measurements made by the LFA normally enable the logging engineer to decide when to collect the sample. The M‐I SWACO solution was to use the OPTITRAK 600 tracer, jointly developed and patented by M‐I SWACO and Schlumberger. The OPTITRAK 600 tracer is a water‐soluble dye that colors the mud filtrate, enabling it to be detected by the LFA at concentrations as low as 10 ppm. This allows the logging engineer to measure the contamination level and to judge precisely when to take the sample, which optimizes the flushing time and guarantees sample quality. The OPTITRAK 600 tracer was developed to respond to certain channels on the LFA tool. The tracer is stable for up to one week at 250°F (121°C) and shows minimal adsorption into clay minerals. The dye is not affected by pH changes or by divalent metal ions, which are commonly present in formation water. The OPTITRAK 600 dye enables real‐time decision making during formation fluid sampling operations, optimizes flushing time, potentially reducing rig time and reducing the chances of the MDT tool becoming stuck, guarantees sample quality and substantially reduces the need to rerun the MDT.

Result: On this well, tracer additions began at 7,415 ft (2,260 m). A concentration of 1,000 ppm was established in the mud filtrate and was maintained to total depth (TD) on this section. The calibrated LFA tool was run to 7,610 ft (2,319.5 m), where the formation fluid was pumped through the tool and discarded until the real‐time LFA reading indicated that the estimated filtrate contamination was 8%. Samples were retrieved on surface, and rig‐site analysis indicated the filtrate contamination level was 9.35%, representing good agreement with the downhole measurement.

Well Information
Location UK North Sea
Date March 2003
Section TD 8,071 ft (2,460 m)
Bottom Hole Temperature 173°F (82°C)
Max. Deviation 31°
Mud System Water‐base glycol/polymer
Mud Weight 10.8 lb/gal (1.30 SG)

The OPTIBRIDGE 600 tracer was run in a glycol polymer mud that also contained a dispersant that colored the filtrate dark brown. Pre-job testing ensured the dye could be detected in the filtrate and that a nitrate tracer could be run simultaneously. Both tracers are detected by a portable spectrophotometer. The LFA tool was calibrated before the job by testing water with filtrate contamination levels of 0%, 1%, 10% and 100%. The M-I SWACO engineer on the rig monitored tracer levels and adjusted concentrations accordingly.


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“Water sampling operations can be problematic if a well is drilled with a water‐base mud because there is no easy way to distinguish mud filtrate from formation water. This problem has been solved by OPTITRAK 600 tracer, which allows filtrate contamination levels to be monitored in real time during the sampling operation.”
Mike Hodder
UK Technical Services Manager
M‐I SWACO