Groundwater contamination near North Texas shale gas wells could be caused by a faulty casing or poor cement construction surrounding the casing, according to a new study published online Monday in the Proceedings of the National Academy of Sciences of the United States of America.
Researchers from Dartmouth College, Duke University, Ohio State University, Stanford University and University of Rochester conducted the study, sampling 113 domestic groundwater wells overlying the Marcellus Shale in Pennsylvania and 20 groundwater wells overlying the Barnett Shale in North Texas.
Researchers identified eight “discrete clusters” of gas contamination, seven in Pennsylvania and one in North Texas, that showed groundwater contamination increasing over time. They linked four of the clusters to gas leakage from failures of cementing or faulty production casings in wells.
“Optimizing well integrity is a critical, feasible and cost-effective way to reduce problems with drinking water contamination to alleviate public concerns accompanying shale gas extraction,” the authors wrote, adding that future research should evaluate whether the large volumes of water used in hydraulic fracturing affect well integrity.
This study constitutes yet another chapter in the ongoing debate over the gas well drilling process called fracking. Voters in the city of Denton will go to the polls on Nov. 4 and either approve or shoot down a ballot proposition that calls for a ban on fracking within city limits.
Steve Everley of Energy in Depth, the voice of the oil and gas industry, said in an email interview that the study’s authors placed blame on well-integrity problems for the water quality issues, “but they never identified a single gas well that’s actually leaking.”
Everley said the Texas Railroad Commission, the state agency that regulates the oil and gas industry, completed a study on groundwater contamination in Parker County, one of the areas researchers sampled, which determined the gas wells were “constructed in full accordance with the law.”
Zacariah Hildenbrand, a researcher at the University of Texas at Arlington, said the study’s authors were able to determine well-integrity issues using isotopic analyses to differentiate between biochemical methane (produced by bacteria) from thermogenic methane (found in shale and intermediate layers).
In their Barnett Shale data, Hildenbrand explained, the methane discovered was coming from an intermediate layer called “The Strawn.” The source could have been natural methane migration in the Earth, or might have resulted from faulty well casings. Researchers used a process called “noble gas analysis” to determine the source.
Researchers discovered instances of “stripping” of noble gases (like argon) and other gases (nitrogen), he said, and they believe, based on these “fingerprints,” as they call them, this is evidence of methane traveling through bad casings under high pressure.
Hildenbrand also owns Inform Environmental, a company that provides unbiased scientific data for industry, municipalities, conservation groups and concerned citizens. The company enhances its clients’ scientific understanding of environmental quality by providing cutting-edge scientific methods to analyze and characterize water, air and soil quality, he said.
“It requires incredibly advanced analytical equipment to probe for these things,” Hildenbrand said, “which again, is one of the reasons why this data isn’t readily available from the Railroad Commission, Texas Commission on Environmental Quality or well drillers.”
Ohio State University’s Thomas Darrah, one of the lead researchers on the study, said his team’s gas analysis showed methane from drill sites had escaped into drinking water wells from “shallower depths through faulty or insufficient rings of cement surrounding a gas well’s shaft.”
Avner Vengosh, a Duke researcher, said based on the unique “fingerprints,” their data showed these clusters stemmed from well-integrity problems such as poor casing and cementing.
“One would hope industry would work with academics to determine what would be the best way to stop this leaking,” Vengosh added.
Cementing wells
The technology behind protecting groundwater resources after drilling a shale gas well involves a complicated process designed to isolate freshwater zones and groundwater from the inside of the well.
Surface casing is the first line of defense, according to FracFocus.org, followed by intermediate casing and then production casing, which provides a third layer of protection. Cementation of the casing adds “the most value to the process of groundwater protection.” Sealing the space between the well casing and the wall of the drilled hole with cement establishes a “hydraulic barrier” to vertical and horizontal fluid migration. The casing is cemented from the surface to 30 or 50 feet, again at 500 to 1,500 feet and finally at 5,000 to 10,000 feet.
“It’s like they’re a series of straws inside each other,” said John Tintera, a consultant for the Texas Alliance of Energy Producers and former executive director of the Railroad Commission. “The cement goes outside the straws and holds the casing.”
The cementing of wells dates back to 1919, when Erie P. Halliburton struggled with ways to set up cementing in North Texas, especially in the booming fields in Wichita Falls, according to an article in Exploration & Production magazine, an industry publication.
“Most operators were skeptical of cementing casing,” writes Bill Pike, the editor in chief. “There was the question in the back of their minds of possible well damage resulting from cementing. For Halliburton, it was to be an uphill struggle to normalize the practice of cementing a well.”
Halliburton learned the well cementing technology while working with Perkins Cementing Co. in California in 1916. Upon arriving in Wichita Falls, he used a borrowed pump, a wagon, an old tank, a clothesline for measuring depth and some homemade wooden plugs.
Today, cementing of a well involves a pump truck, a couple of water trucks and a host of workers. According to Rigzone.com, cementing begins with cement slurry deployed via pumps, displacing drilling fluids located in the well. The cement slurry flows through the casing toward the bottom of the wellbore. It fills the space between the casing and the actual wellbore and hardens, creating a seal and permanently positioning the casing.
The amount of cement needed is determined by measuring the diameter of the borehole along its depth using a caliper log, according to the website. Engineers use both mechanical and sonic means to measure the diameter of the well at numerous locations simultaneously to accommodate for irregularities in the wellbore diameter.
Tintera said surface gauges, which the Railroad Commission inspects, are used to ensure the space between the casing and surrounding earth is maintained. If a problem develops during the cementing process, operators must perform what is called a “squeeze job,” in which they identify the problem area using special diagnostic tools, go below it and punch holes in the casing, similar to the perforation process, to push cement to the surface.
If operators can’t fix the problem, he said, then the well must be plugged and a new one drilled at another location.
Well integrity
The number of Texas wells experiencing integrity issues may be small, but the problem was big enough for the Texas Railroad Commission last year to establish a revision to Statewide Rule 13, which added stricter regulations for oil and gas drillers to follow when constructing a well.
Everley, of Energy in Depth, said well-integrity issues are “an exceptionally rare occurrence,” happening in fewer than 1 percent of wells drilled across the country and, in Texas, well-integrity failure affected 0.01 percent of all wells drilled and completed in a 25-year period.
Cyrus Reed, conservation director of the Sierra Club’s Lone Star Chapter, said in response, “In the grand scheme of things, [1 percent] might not be a lot; but if you’re in that area that relies on that water, 1 percent is too much.”
Railroad Commission data on regulatory violations is important because it allows the public to get more information about oil and gas drilling.
Prior to 2012, violations were separated into categories but not available to the public. For example, in 2009 the commission issued more than 2,800 violations in a category that encompassed casing, cementing, drilling and completion problems, according to data the agency provided to the Texas Sunset Advisory Commission. In 2012, the commission started posting online for the public a summary of violation statistics that grouped categories together.
The change in how the commission decided to report violation data resulted in the public having less ability to understand the nature of the violations. In 2014, more than 45,500 violations were identified, but members of the public cannot tell what those violations specifically involved.
“Violations cited under the commission’s Statewide Rule 13 do not mean a well has faulty cementing,” said Ramona Nye, spokeswoman for the Railroad Commission. “[They] could be cited for numerous reasons, including bradenhead pressure found on a wellbore, no wellhead control, no proper wellhead equipment installed, no cashing head valves plumbed to surface, etc.”
There are numerous ways well integrity could be compromised.
Adam Peltz, an attorney for the Environmental Defense Fund, said a casing could corrode or crack. If engineers don’t know about a subsurface void, the cement could go into the void, he said.
“If you cement over an area with a flow of gas, there is a 15-minute window when the cement is liquid,” Peltz said. “During that setting time, gases and fluids can migrate up [the space] between the casings and get into places you don’t want them to go. It can happen in a very short period of time, and it’s not obvious.”
Another issue is the increase in earthquakes in North Texas affecting established shale gas wells’ integrity.
“It’s a relatively new phenomenon,” he said. “The science is still catching up.”
The Environmental Defense Fund has developed the “Model Regulatory Framework for Hydraulically Fractured and Hydrocarbon Production Wells” to provide state governments a “road map” in implementing regulations that govern subsurface aspects of the drilling, casing and cementing process.
The document offers what the environmental advocacy group thinks is “the state-of-the-art well integrity” to protect groundwater. Peltz said it was developed in conjunction with a variety of industry players and nonprofits.
“We’ve been sharing this document with regulators and [putting it] in front of policymakers,” he said, in the hope that it can be “universally applied to make sure enforcement and oversight is adequate.”
