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High levels of hydrogen sulfide gas (H2S) have been present at Noble Energy gas drilling well pads in the Parachute Creek, CO, basin over the past three years, a company spokesman said.

The company detected the high levels of the H2S gas 46 times at 15 sites. In most of the reported incidents, gas was at between 100 parts per million (ppm) and 200 ppm, with one instance at 450 ppm, said Stephen Flaherty, community relations director for Noble Energy.

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Federal regulations list H2S as potentially toxic under lengthy exposure at 10 parts per million. At 100 to 200 ppm, inhalation causes coughing and breathing difficulties, severe eye irritation, and loss of sense of smell. The gas is fatal at levels of 500 to 700 ppm.

Flaherty said Noble has strict safety protocols for well pad workers exposed to H2S. He said the sampling results don’t mean H2S is present at high levels over long periods of time.

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“The test is a snapshot in time, and the level of H2S fluctuates,” Flaherty said. “It is not a constant flow. It’s not like the H2S is flowing at 100 parts per million consistently.”

Nor can the gas usually escape from the well equipment, he said.

“Our well pads are a closed system. There is no release to the atmosphere,” he said, of potentially toxic emissions during drilling, hydraulic fracturing or completion procedures.

That means the gas stays in the well, pipes and storage vessels “all the way to the point of sale. There is no emission or escape route.”

However, when unacceptably high levels of H2S occur in the gas stream, as is common in “sour gas” fields, the H2S separates out before the gas goes into pipelines for shipment to consumers.

Flaherty said company records indicate there have been 46 encounters of hydrogen sulfide at 15 of the company’s wells from 2009 to 2011, based on 818 samples taken.

Eleven of the 15 wells recorded only one instance each in which H2S topped 100 ppm, according to a spreadsheet documenting the H2S incidents provided by the Colorado Oil and Gas Conservation Commission. The COGCC requires operators to inform the agency any time they detect H2S at a well pad.

Three of the wells had samples topping 100 ppm on three to seven instances.

But one well tallied 21 samples of H2S over 100 ppm, from August 2009 until January 2010.

The highest level detected was 450 ppm reported March 5, 2011, at the Ramos 1-23C well south of Parachute.

In an effort to measure the prevalence of H2S launched in March, the COGCC asked Noble and other operators in the Piceance Basin to submit reports based on “selective samples” from 30 wells in the region from 2009 to 2011.

The COGCC reported measurable amounts of H2S on wells operated by Noble, Antero Resources, Williams Production RMT, Laramie Energy and EnCana Oil and Gas (USA).

The samples revealed low concentrations of H2S, from 0.1 ppm at Antero rigs up to 25 ppm at an EnCana rig, said David Neslin, executive director of the COGCC. Only Noble’s rigs had measurements at 100 ppm or higher.

In a presentation in September, Stuart Ellsworth, engineering manager for the COGCC, said in each case where they encountered H2S, the operator treated the well with chemicals to counteract the gas.

Hydrogen sulfide is in gas deposits around the U.S., though officials did not believe it was present in high concentrations in the Piceance Basin. It can also result from the introduction of chemicals in the drilling process, said Occupational Safety and Health Administration officials.

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