[미국] 수압파쇄 정 가스 누출…지하수 오염

오하이오 주립대학교과 로체스터대학레이저에너지 연구소, 다트머스 해군 사관학교, 스탠포드 대학교, 듀크대학교의 연구원들이 실시한 최근 연구의 결과, 수열 파쇄 부지의 케이싱 및 시멘트 우물 라이닝에 대한 향상된 건설 기준을 찾았다고 발표했다. 수열 파쇄 지역은 셰일가스 시추기술인 수열파쇄과 관련된 식수정에서 발생한 대부분의 자연 가스 오염의 원인이 된다.

연구팀은 어떻게 메탄이 지구 아래서 이동하는지 추적할 수 있는 새로운 지구화학적 방법을 고안했으며 이 연구는 펜실베니아와 텍사스 주에 위치한 8개의 오염된 식수정에서 확인됐다. 연구 결과 중 가장 중요한 것은 수평보링과 셰일 매장층의 수압파쇄 모두 자연가스 오염의 원인으로 보이지 않았다.

연구 책임자 Thomas Darrah는 " 많은 사례를 보면 의심의 여지없이 많은 자연 가스의 오염수준은 자연스럽게 발생한다. 하지만 일부의 경우, 오염에 대해 사람이 원인이 된다는 확실한 증거이기도 하다."고 설명했다. 이어 " 하지만 우리의 데이터는 오염이 발생했을 때 이 원인의 정의 열악한 케이싱과 접합으로 발생한다."고 제안하고 있다.

Gas leaks from faulty fracking wells linked to groundwater contamination, finds study
 
Results from a new study conducted by a team led by a researcher at Ohio State University and researchers at Duke, Stanford, Dartmouth, and the University of Rochester, have found that improved construction standards for cement well linings and casings at hydraulic fracturing (fracking) sites is the likely source of most natural gas contamination in drinking water wells associated with fracking.

The team recently devised a new method of geochemical forensics to trace how methane migrates under the earth. The study identified eight clusters of contaminated drinking water wells in the states of Pennsylvania and Texas. Most important among their findings is that neither horizontal drilling nor fracking of shale deposits seems to have caused any of the natural gas contamination.

"There is no question that in many instances, elevated levels of natural gas are naturally occurring, but in a subset of cases, there is also clear evidence that there were human causes for the contamination," said study leader Thomas Darrah, assistant professor of earth sciences at Ohio State. "However, our data suggests that where contamination occurs, it was caused by poor casing and cementing in the wells."

During fracking, water is pumped underground to break up shale at a depth far below the water table, he explained. The long vertical pipes that carry the resulting gas upward are encircled in cement to keep the natural gas from leaking out along the well. The study suggests that natural gas that has leaked into aquifers is the result of failures in the cement used in the well.

"Many of the leaks probably occur when natural gas travels up the outside of the borehole, potentially even thousands of feet, and is released directly into drinking water aquifers," said Robert Poreda, professor of geochemistry at the University of Rochester. Further, Avner Vengosh, professor of geochemistry and water quality at Duke, added, "These results appear to rule out the migration of methane up into drinking water aquifers from depth because of horizontal drilling or hydraulic fracturing, as some people feared."

The method that the researchers used to track the source of methane contamination relies on the basic physics of the noble gases (which happen to leak out along with the methane). Noble gases such as helium and neon are so called because they don't react much with other chemicals, although they mix with natural gas and can be transported with it.

That means that when they are released underground, they can flow long distances without getting waylaid by microbial activity or chemical reactions along the way. The only important variable is the atomic mass, which determines how the ratios of noble gases change as they tag along with migrating natural gas. These properties allow the researchers to determine the source of fugitive methane and the mechanism by which it was transported into drinking water aquifers.

The researchers were able to distinguish between the signatures of naturally-occurring methane and stray gas contamination from shale gas drill sites overlying the Marcellus shale in Pennsylvania and the Barnett shale in Texas. The researchers sampled water from the sites in 2012 and 2013. Sampling sites included wells where contamination had been debated previously; wells known to have naturally high level of methane and salts, which tend to co-occur in areas overlying shale gas deposits; and wells located both within and beyond a one-kilometer distance from drill sites.

[출처 =Water World / 2014년 9월 15일]