When Weatherford introduced the GC-TRACER surface gas detector, the technology became – and remains – the only gas detection system that can measure gases directly from the mud and that has the ability to correct for changes in temperature. Prior to its introduction, conventional gas-agitator traps were considered the primary method for extracting mud gas.

“Disadvantages to this method include prolonged exposure to air, resulting in preferentially extracting lighter end alkanes and retention of a considerable amount of gas in the mud, especially in oil-based mud,” said David Tonner, Weatherford’s director of global sales. “These disadvantages can lead to costly misinterpretation of fluid typing in the reservoir. Deepwater operations accentuate the problem due to the cooling effect of water on the riser.”

The GC-TRACER’s ability to correct for changes in temperature can significantly improve the value of data gathered, and a probe device permits the tool to be installed within the mud in both the gas-in line and the gas-out line. In 2009 the technology won the formation evaluation category of Hart Energy’s Meritorious Awards for Engineering Innovation.

“The ability to measure the delta gas readings factors out the problem of gas recycling and leads to more accurate gas measurements,” Tonner said. “Finally, with more and more [managed-pressure drilling], the GC-TRACER’s unique design allows for the extraction and detection upstream of the mud gas separator. Knowing both the quantity and type of gas significantly derisks our customers’ operations.”

Since its introduction, the surface gas detector has been used in shale plays and to identify liquids-rich zones in both vertical and horizontal wells, leading to optimization of wellbore placement and productivity. “We also have successfully characterized fluids in conventional reservoirs offshore in deep water, leading to an optimal selection of wireline sampling points and, in some instances, eliminating sampling completely in doing so – saving more than US $3 million,” Tonner said.

When used on an offshore appraisal well in the UK North Sea to evaluate the Late Jurassic to Upper Magnus formation, the surface gas detector identified the full C1 to C8 range of hydrocarbons, including the aromatics benzene and toluene. The technology’s gas response mirrored gamma-ray LWD data, and the gas-ratio analysis pointed to a slightly lighter density hydrocarbon reservoir cap resting above the remaining leg of the reservoir, according to a Weatherford case study. “The GC-TRACER gas results proved the vertical extent of oil reserves in the structure, which were found to extend right to the base of the Upper Magnus sands,” the case study stated.

Several improvements have been made to the technology, including a redesign of its membrane to improve efficiency, longevity, and reliability. Weatherford also has worked to increase its extraction capabilities to include a number of C7 isomers to further expand fluid evaluation and interpretation.

Data are now viewed and delivered using the Isologica software package, which allows users to generate geochemical plots and interact with a depth-based log. This can make it easier to identify fluid contacts, compartmentalization, and connectivity and can allow data integration and in-depth reservoir evaluation for multiple wellsite geochemical datasets. In addition, the company’s Constant Volume Trap can be used with the surface gas detector to reduce the impact of fluctuating drilling parameters while extracting mud gas.

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