According to a new study, hydrocarbon gas contamination in drinking water close to hydraulic fracturing activity is likely caused by breaches of well integrity from casing or cementing issues, and not from upward migration of gas from deep formations.  A study by researchers from five different universities, and published in the Proceedings of the National Academy of Sciences of the United States, found that fugitive gas contamination is most likely associated with casing or cementing issues that compromise well integrity.  “[O]ur data do not suggest that horizontal drilling or hydraulic fracturing has provided a conduit to connect deep Marcellus or Barnett Formations directly to surface aquifers.”  The study, titled “Noble gases identify the mechanisms of fugitive gas contamination in drinking-wells overlying the Marcellus and Barnett Shales,” was conducted by researchers from Duke University, The Ohio State University, Stanford University, Dartmouth College, and University of Rochester.

The researchers used noble gas and hydrocarbon tracers to distinguish between natural and anthropogenic sources of methane contamination and aimed to study two issues: (a) are elevated levels of hydrocarbon gas in dinking water aquifers near gas wells the result of natural or anthropogenic sources, and (b) if fugitive gas contamination exists, what mechanisms cause it.  Analyzing 113 samples of drinking-water wells overlying the Marcellus Shale, and 20 samples overlying the Barnett Shale, the researchers identified eight discrete clusters of fugitive gas contamination (seven in Pennsylvania and one in Texas).

The most likely cause of the fugitive gas contamination is the release of intermediate-depth gas along the well annulus due to poor cementing.

The study identified seven scenarios that may account for elevated hydrocarbon gas levels in shallow aquifers: (1) in situ microbial methane production; (2) tectonically driven migration over geological time of gas-rich brine from an underlying source formation; (3) exsolution of gas already present in shallow aquifers driven by drilling activities; (4) leakage through poorly cemented well annulus; (5) leakage through faulty well casings; (6) upward migration of gas along natural deformation features (faults, joints, or fractures) or those initiated by drilling; and (7) migration of gas through abandoned or legacy wells.

According to the study’s authors, the most likely cause of four of the cases of fugitive gas contamination is the release of intermediate-depth gas along the well annulus most probably the result of poor cementing (scenario 4 above). Three of the other cases of contamination were likely caused by poorly constructed wells, and the final case looked was found near a well with a documented underground well failure.

The researchers conclude that “where fugitive gas contamination occurs, well integrity problems are most likely associated with casing or cementing issues” and the “data do not suggest that horizontal drilling or hydraulic fracturing has provided a conduit to connect deep Marcellus or Barnett Formations directly to surface aquifers.”

You can find the study here:

http://www.pnas.org/content/111/39/14076.short