A new study found that fugitive gas contamination at eight clusters of water wells in the Marcellus and Barnett shale regions might be linked to well integrity issues but not hydraulic fracturing. The results were consistent with earlier studies that found fracing is not the culprit but raised questions about whether private water wells or decades-old, undocumented oil and gas wells may have provided the conduit that facilitated methane migration.
An analysis of noble gas isotopes indicated that large pulses of methane migrated into shallow aquifers at seven well clusters in Pennsylvania and one cluster in Texas. The noble gas isotopes provided a chemical fingerprint that indicated how and where the gas formed.
"The gas had to migrate in some fashion that did not interact with water and rock," said Thomas Darrah, leader of the study and an assistant professor of geochemistry at Ohio State University. "The only way we can see that happening is if it migrated around the well annulus or as a result of a leaky well casing-or a well integrity issue."
Researchers compared water samples collected at the eight clusters with baseline water samples collected in areas more than 1 km away from drillsites in southern New York and Texas. These areas have "a natural coexistence of salt, methane, and noble gases that showed a really strongly migrated pattern, which is consistent with what you'd expect to see as gas migrates long distances in the subsurface over geological time," Darrah told UOGR.
The study, "Noble gases identify the mechanisms of fugitive gas contamination in drinking-water wells overlying the Marcellus and Barnett shales," was published in the September issue of The Proceedings of the National Academy of Sciences (PNAS).
Well construction cited
Of the eight contamination clusters, four were contaminated by gas from intermediate-depth strata. The report surmises that this likely resulted from "the release of intermediate-depth hydrocarbon gas along the well annulus, probably as the result of poor cementation."
The methane in three of the clusters was consistent with the release of Marcellus-like hydrocarbon gas, which researchers believe was likely the result of poorly constructed wells. The eighth cluster was linked to a gas well that experienced underground mechanical failure. A report to the Pennsylvania Department of Environmental Protection (DEP) showed that well experienced a casing well packer failure at depth.
Confirming a known problem
The researchers' conclusion that hydraulic fracturing was not the culprit confirmed what many other site-specific investigative studies have already revealed, said Fred Baldassare, owner and senior geoscientist at Echelon Applied Geoscience Consulting. Baldassare founded Echelon in 2010 after spending more than 20 years working for DEP. Baldassare has investigated more than 200 alleged incidents of methane migration and has been a researcher on the application of isotope geochemistry to identify gases in the subsurface since 1997.
"Fracing doesn't create gas migration … that's not really a revelation," Baldassare told UOGR.
A study financed by the US Department of Energy published in September also found that well-constructed Marcellus shale wells do not facilitate detectable migration of gas or fluids. DOE's National Energy Technology Laboratory (NETL) conducted a field study at a six-well pad in Greene County, Penn., by injecting perfluorocarbon (PFC) tracers into hydraulic fracturing fluids.
Researchers monitored the upward fracture growth out of the target zone and upward gas and fluid migration.
The NETL concluded, "For this specific location, fracture growth ceased more than 5,000 ft below drinking water aquifers and there was no detectable upward migration of gas or fluids from the hydraulically fractured Marcellus shale." Baldassare believes this study is significant because of the detail and strength of the monitoring program.
Cement jobs
During Baldassare's tenure with DEP-most of which predates the shale boom-the most common cause of methane gas migration, if well related, was an ineffective cement job that enabled gas to move along the outside of the well. The problem of methane migration, Baldassare said, is neither new nor unique to shale wells. Conventional wells, aging, abandoned wells, landfills, buried organic matter, and coal mining can all be sources of methane migration. Private water wells also provide potential methane migration conduits.
"Anytime you drill a hole in the ground you create a low pressure zone, and gas always wants to go from areas of high pressure to areas of low pressure. So, when you drill a hole in the ground, whether it's for water or it's for gas, you necessarily create a low-pressure zone. And if you don't get a good seal on the outside of your steel casing you create a conduit for that gas to move," Baldassare said.
A 2013 report by the US Geological Survey (USGS) notes research that documents the widespread natural occurrence of dissolved methane in groundwater in the Upper Delaware River Basin in Pennsylvania and New York.
Scope of the problem
An analysis of state data by the Associated Press earlier this year found that "Pennsylvania received 398 complaints in 2013 alleging that oil or natural gas drilling polluted or otherwise affected private water wells, compared with 499 in 2012." Complaints included allegations of short-term diminished water flow, as well as pollution from stray gas or other substances. In the past 5 years, AP found, more than 100 pollution allegations have been confirmed in the state.
Methane is a colorless, odorless, tasteless gas that is combustible. In confined spaces it can trigger an explosion when an ignition source, such as an open flame or an electrical spark, is present. The gas can also act as an asphyxiate in confined spaces by displacing oxygen in the blood stream.
The USGS reports that methane dissolved in groundwater acts similar to the carbon dioxide in carbonated beverages, where the gas is held within the fluid under the confining pressure of the sealed container. Reducing pressure causes some of the gas to come out of the water, which leads to bubbling and fizzing, similar to the bubbling present in a can of soda.
When groundwater enters a private water well at atmospheric pressure, "the natural gas can be released from the water, which can cause a column of gas to form above the water surface in the well or be released within a pressure tank, at faucets inside a home, or in structures enclosing the well, where it can become flammable or explosive," the USGS report said.
Every allegation of methane migration linked to gas well drilling is unique and must be investigated at a site-specific level. "Not all of these stray gas migration incidents rise to the level of a serious problem," Baldassare said.
A well can cause temporary methane migration problems when it is drilled through a shallow aquifer system. The problem is typically resolved when surface casing is set and cemented. Baldassare said since methane is a single carbon compound, it typically biodegrades easily by methanotrophs-a type of bacteria that reduces and biodegrades methane in the system-and the aquifer returns to a baseline condition without intervention. The problem can last from a few months to several years.
"I'm not trying to suggest that it doesn't matter to the homeowner. None of us want any of these impacts. But the context is some of them [these methane migration problems] are temporary and we go back to a baseline condition in fairly short order," Baldassare said.
In February 2011, Pennsylvania's DEP strengthened casing and cementing standards for new well construction. The revised standards address internal pressure rating, pressure testing of casing centralization, and certification of joint welders.
Darrah supports strengthening these standards. "I think, in reality, we've known for decades in the oil and gas industry that well integrity is one of the single biggest factors to our environmental stewardship."
A 2013 review of the revised regulations by the State Review of Oil and Natural Gas Environmental Regulations (Stronger) notes the revised regulations require "that cement used in the surface casing meet certain compressive strength and free-water specifications and isolate the wellbore from fresh groundwater; contain pressures from drilling, completion, and production; protect the casing from the geochemical effects of the subsurface; and prevent gas flow in the annulus."
Other culprits
Shale gas wells are not the only potential conduit for methane migration. Legacy wells drilled when the oil and gas industry was in its infancy exist throughout Pennsylvania and present a conduit for gas to rise to the surface.
Sir Edwin Drake drilled the world's first commercial oil and gas well in Pennsylvania in 1859. Since then, as many as 300,000 oil and gas wells have been drilled in the state. Some were drilled before the state kept detailed records and were plugged and abandoned before state oil and gas laws were standardized in 1984.
Baldassare said older wells do "pose a certain risk because a lot of them have not been abandoned properly." DEP is trying to locate and identify these wells. Baldassare said operators seek out these wells as part of their due diligence conducted before a new well is drilled.
Private water wells are another consideration. Pennsylvania is one of only two US states that lack construction standards for private water wells. Some consist only of a surface casing cemented into the ground. These open-hole water wells yield groundwater from multiple fractures in the bedrock and create more potential for the migration of naturally occurring methane.
Complicating matters is the fact the water quality has never been tested for many private water wells. "Sometimes that makes it a challenge to understand what is a baseline condition and what has been anthropogenically influenced," Baldassare said.
Ultimately, researcher Darrah said, the study had both good news and bad news.
"The bad news is, in a small subset of water wells, there is some evidence of stray gas contamination but, if you take the optimistic viewpoint, the good news is; It's not fracking causing the problems. It's well integrity. We've known about it [well integrity] for a while and future improvements in well integrity can eliminate a lot of the problems we're seeing," Darrah said.
