Article courtesy of Donald Gilliland | March 4, 2014 | Penn Live | Shared as educational material
Much of the debate over fracking for natural gas has centered around the potential to pollute drinking water. What’s not explicit in much of the discussion is that – in Pennsylvania – the only thing that has polluted people’s well water as a result of drilling is natural gas itself.
Known as “methane migration” or “stray gas,” the appearance of natural gas in people’s well water is behind the image of the flaming tap made famous in the film “Gasland.”
The residents of Dimock, in Susquehanna County, featured in that film received settlements from Cabot Oil & Gas, which the Pennsylvania Department of Environmental Protection determined improperly cemented the casings to its wells, thus allowing gas to migrate into the local aquifer and people’s wells.
A new peer-reviewed study demonstrates the appearance of gas in a water well – especially in the northeastern part of Pennsylvania around Dimock – is not necessarily the result of drilling.
Natural gas, the study shows, is often already in the water before any drilling begins.
The study, published in the February Bulletin of the American Association of Petroleum Geologists, is based on more than 2,300 gas and water samples collected from 234 gas wells and 67 private groundwater-supply wells, the largest data set of any such study thus far.
It looked specifically at what kind of gas was present and where.
There are two distinct types of gas determined by the processes that form it: “biogenic” or “microbial” gas, which is “produced at shallow depths and low temperatures by anaerobic bacterial decomposition” of organic matter; and “thermogenic” gas, which is formed deep in the earth under great pressure and heat “cracking” organic matter into oil and gas. There’s also a subset of thermogenic gas produced by further “cracking” of the oil into gas and coal.
Each has a different isotopic signature, or “fingerprint,” allowing scientists to determine the relative age of the gas and thus where it originated.
The study found that many of the shallow aquifer systems in the northeast contain microbial gas, a mix of microbial and thermogenic gasses and thermogenic gasses of different ages – and all of it predating drilling in the Marcellus.
The study found that 88 percent of the 67 water supply wells evaluated had some presence of thermogenic gas present before any Marcellus drilling occurred.
The study then tested the gas produced in the well bore of 234 wells during drilling as the drill bit sank deeper and deeper – to get a vertical stratigraphy of what types of gas are found where.
The study found that Marcellus type gas – older thermogenic gas associated with deeper geologic formations – was actually present above the Marcellus.
“The real revelation,” said Fred Baldassare, one of the study’s authors, “is that gases that look just like the Marcellus also occur in formations above the Marcellus – in the Hamilton group, Tully limestone and the Geneseo shale.”
None of that Marcellus type gas was found in any of the water wells sampled, Baldassare said: the thermogenic gas in the well water was all younger gas from shallower formations.
However, the testing at the drilling sites indicated that a component of that older Marcellus type gas – “a little bit of that gas mixed with early thermogenic gas” – is present in the stratigraphy associated with the aquifer system.
“In some areas,” the study says, “deeper thermogenic gases have migrated over geologic time and mixed with shallower thermogenic gases in the shallower strata.”
Baldassare said, “We were surprised by seeing post-mature gas up in the shallow system – I was almost in disbelief, thought we were getting noise when the first results came in – but we kept seeing it over and over again.”
That’s a key detail according to former Environmental Protection Secretary Michael Krancer, who has written an analysis of the study in his role as a legal specialist in energy issues at the Philadelphia law firm Blank Rome.
Krancer said, “Many have hastily concluded that the presence of thermogenic methane in shallow groundwater in the vicinity of drilling activities means that the methane contamination is caused by the drilling operations. This study shows that this is not necessarily so.”
The study does not dispute that drilling has the potential to cause methane migration.
“There have been real stray gas migration problems,” said Baldassare, a geoscientist who formerly worked for DEP and has been studying isotopic analysis of stray gas for more than 20 years.
Baldassare worked on the Cabot case in Dimock. “That’s a real stray gas incident,” he said, “with multiple data types and multiple lines of evidence.”
But there has been an assumption – implicitly supported by two studies out of Duke University – that the appearance of post-mature thermogenic gas in the aquifer meant hydraulic fracturing activities had connected with the aquifer system, he said.
“That’s the big bogeyman,” said Baldassare, “and our data shows that’s not the case at all.”
Baldassare said, “It’s really irresponsible for those researchers to make those gross generalizations about Marcellus gas migrating up into the aquifer system. Hopefully this paper will make people understand that a little bit better.”
The study says: “When future isotope data show a stray gas in this area to be thermogenic, that finding cannot be the sole basis for alleging that the stray gas was caused by oil or gas-well drilling.”
The study concludes: “Alleged incidents of stray gas migration require investigations at the site-specific level and evaluation and synthesis of multiple data types to determine the source of the stray gas.”
It also bolsters the argument for people in areas about to be drilled to have their well water analyzed before drilling begins.
The study – entitled “A geochemical context for stray gas investigations in the northern Appalachian Basin: Implications of analyses of natural gases from Neogene-through-Devonian-age strata” – was written by Baldassare, Mark McCaffrey, a petroleum geochemist at Weatherford Laboratories in Texas, and John Harper, recently retired from the Pennsylvania Geologic Survey.