Certain types of Pennsylvania’s scores of abandoned oil and gas wells are associated with higher emissions of the greenhouse gas methane, according to a study published Monday that could help government agencies prioritize efforts to plug the biggest leaks.
The study of 88 wells across Western Pennsylvania led by Mary Kang, a postdoctoral fellow at Stanford University, revealed that high-emitting wells tend to be natural gas wells that are either unplugged or that are plugged but vented in coal-rich areas.
Abandoned oil wells had consistently lower emissions than the abandoned gas wells in the study. Proximity to active natural gas storage fields or new shale gas wells appeared unrelated to methane flow rates from abandoned wells, according to the report in the Proceedings of the National Academy of Sciences.
The highest emitters are a particularly valuable target in efforts to curb releases of the powerful greenhouse gas, which has 86 times the heat-trapping potential of carbon dioxide in the atmosphere over a 20-year period.
Researchers found that wells with high methane emissions were leaking at steadily high rates across years of sampling, indicating that they “may have been emitting at these levels for many decades and will likely continue for decades into the future.”
Ms. Kang and her fellow researchers first reported the significant but largely uncounted contribution that abandoned wells can have on total methane emissions in a 2014 paper. That drew the attention of both state and federal regulators, who are now trying to begin to account for abandoned wells in their official inventories of emissions.
The earlier paper also showed that the bulk of measured emissions was coming from only 10 percent of the smaller sample of wells, which inspired the researchers from Stanford, Princeton and other institutions to dig deeper to define attributes of the biggest leakers.
“There are hundreds of thousands of wells in Pennsylvania. That’s a lot of wells to fix,” Ms. Kang said. “But if we only have to fix 10 percent of those, that’s a huge cost benefit.”
Officials with the state Department of Environmental Protection have been looking for just those types of distinguishing characteristics in their own ongoing study of a representative sample of about 200 of the state’s abandoned wells. That study has other goals, too, including verifying the condition of the wells and their locations.
DEP currently has limited funding — paid for by surcharges on well permits — to plug legacy wells left without owners after more than a century of oil and gas extraction. Wells that pose a safety risk are given highest priority for plugging.
Gov. Tom Wolf launched a program to cut methane leaks from active oil and gas infrastructure earlier this year, and a strategy for capping emissions from the biggest abandoned sources could complement that task.
Seth Pelepko, chief of the subsurface activities and well plugging division of DEP’s oil and gas program, said the work by Ms. Kang and her colleagues helps define the scope of the problem posed by legacy wells in Pennsylvania, which, in turn can inform efforts to develop policies and funding methods to deal with it.
“Taking steps to quantify the scope of the issue is important,” he said. “Putting some finer points on just the sheer numbers can be a real motivator — what sort of problems are we contending with and what sort of solutions might we want to propose to address them over the longer term?”
The authors of the new study also tried to refine estimates of the notoriously hard-to-determine number of abandoned wells in the commonwealth by looking to databases and historical archives.
Their new conclusion — that there are somewhere between 470,000 and 750,000 plugged and unplugged abandoned wells — is higher than some other estimates because it takes into account more than 100,000 previously uncounted enhanced recovery wells, which were used to send water into oil formations to increase the flow of oil to production wells.
Like other incompletely sealed abandoned wells, enhanced recovery wells can provide a conduit for underground methane to reach the atmosphere.
The researchers concluded that, based on the varying levels of emissions from wells with different characteristics, Pennsylvania’s abandoned wells could collectively be releasing the equivalent of 5 to 8 percent of total annual human-caused methane emissions in the state.
Plugged gas wells in coal areas are vented for safety reasons and the finding that those wells are among the highest emitters poses a challenge to regulators.
The vents can keep gas from getting into groundwater or coal seams if deeper plugs have failed, Mr. Pelepko said, but the study will help the agency explore alternatives as it prepares to update its well plugging regulations in the near future.
Laura Legere: email@example.com.