Methaneemissions from oil and gas production sites in northern Colorado oftenrepresent less than 1% of the gas produced, according to a recent study fromColorado State University.
The researchers looked at different stages of the extractionprocess, and found the highest methane emissionsrate occurred when companies removed liquids from a well. The flowback phase —in which the fracking fluid, produced water, oil and natural gas flow up thewell after fracking — had the second-highest methane emissions rate, the studyshowed. The production and fracking stages had median methane emissions ratesfar below the other two phases.
By taking a granular look at which stages of the extractionprocess correspond to what emissions levels, the researchers said they hoped tobe able to better predict downwind concentrations. With this data, it ispossible to feed more detailed information into models that can then accountfor different landscapes and weather conditions to determine where emissionswill spread and in what volumes, the study noted.
"While some prior studies have measured [volatileorganic compound], ethane, or methane concentrations near oil and gasoperations, ambient concentrations are strongly dependent not only on emissionrates but also on sampling location and meteorological conditions, whichgreatly affect downwind dispersion and dilution," the report said.
The researchers conducted experiments at 11 sites in theDenver-Julesburg Basin, working with EncanaCorp., AnadarkoPetroleum Corp., PDCEnergy Inc. and NobleEnergy Inc.
All told, the study found that the median volume of methaneescaping at the sites was roughly 0.23% of the gas produced.
Toward the upper end of the range of methane data recorded,the 95th percentile emissions constituted 1.03% of gas produced.
The relatively low level of emissions from the productionstage — compared to the fracking or liquids removal phases — is significant,because the production process last much longer than the drilling and completionactivities, the report added.
The effort was partially funded by the city of Fort Collins,a community with a large opposition to drilling. Fort Collins voters approved afive-year moratorium in 2013, but the Colorado Supreme Court those ordinances in May.
The data collected in Colorado State University study isexpected to be used in a state health-risk assessment, to be completed bysummer 2018, the report said.
Jeffrey Collett Jr., a professor in the university'sDepartment of Atmospheric Science, said that the data from the study can beused to generate data on expected emissions concentrations around oil and gasactivities under a variety of circumstances. "By creating scenarios ofactivity, … our emissions data can be used to estimate exposures for someoneliving, going to school, or working at a nearby location," Collett said ina Sept. 22 email.
The state's health-risk assessment project is currently outfor bid, he added.
The researchers cautioned that variability in productionrates, numbers of wells, well type, and other factors at different productionsites can shift the methane emissions rates across production sites. The studyalso underscored the importance of factoring in the drop-off in methaneemissions that comes with time as overall production at a site declines.
The research highlighted a couple issues with a key U.S. EPAmodel used to assess air pollutant dispersion. The model, known as AERMOD, "dida reasonable job predicting" concentrationscompared to the actual test results, although the AERMOD projections includedsome notable outliers, the report said.
"We were challenging it by testing against much shorter… samples," Collett said. "We were pleased to see that, even in thischallenging test, 75% of AERMOD concentrations were within a factor of 3 of ourshort-term samples."