According to a recent article in the Elmira Star-Gazette, drilling for shale gas in northeastern PA is more challenging than other areas of the state because of the geology in the area—a geology that lends itself to methane migration. The methane that migrates does not necessarily come from the Marcellus Shale layer itself, but is often trapped above the Marcellus in limestone and other rock layers.
In high-profile cases affecting 35 drinking water wells in Bradford and Susquehanna counties, state investigators have linked the stray methane to faulty drilling practices that did not account for the gas-rich and highly fractured shallow geology in northeast Pennsylvania — a hazard that has made the region one of the most difficult places in the state to drill safely into the Marcellus Shale.
As shale gas drilling has increased in Pennsylvania, so has the prevalence of methane migrating into water supplies as a result of the exploration.
The number of new Marcellus wells nearly doubled between 2009 and 2010, but the rate of methane migration more than quintupled: In 2009, there were 1.26 cases of gas migrating into groundwater for every 1,000 new Marcellus wells drilled, according to the Department of Environmental Protection.
Last year, there were more than seven cases for every 1,000 new wells.
Of the 10 confirmed Marcellus Shale stray gas cases since early 2008 — each of which may include more than one affected water well or flawed gas well — all of them have been recorded in this corner of the state.
Seven of the cases were in Bradford County and one each in Wyoming, Susquehanna and Lycoming counties.
Geologists suspect that a lack of historical drilling in the region, combined with a large amount of methane generated deep underground created a gas-charged environment in shallow sandstone layers.
The geology in Northeast Pennsylvania is also complicated and, in some rural regions, rarely studied, meaning there were few good historical maps for the drillers’ reference, said Fred Baldassare, a former stray gas inspector with DEP who now owns Echelon Applied Geoscience Consulting.
"It’s a very complex system with deep-seated fractures and deep-seated thrust faults that come to the surface," he said.
The shallow methane is not necessarily uniform in the layers above the Marcellus.
It is present in "a variety of strata — very shallow all the way down through," said Scott Perry, the director of DEP’s Bureau of Oil and Gas Management.
Natural gas wells are built with a nested series of cemented steel casings that each extend deeper underground to protect groundwater from the gas and fluids in the well.
The cement and casing is also supposed to isolate gas and fluids encountered in rock formations on the way down from migrating up the outside of the wellbore.*
*Elmira Star-Gazette (Jul 10, 2011) – Stray gas plagues NE Pa. Marcellus gas wells