Millions of abandoned wells may be a substantial source of methane in the atmosphere, but identifying them remains a challenge -- and a big reason to study Pennsylvania, which has the longest history of U.S. oil and gas development and where scientists hope to find clues on how to fix a nationwide problem.
Initial findings, published in Proceedings of the National Academy of Sciences, is the largest study of high methane-emitting abandoned oil and gas wells in Pennsylvania, which has infrastructure dating to 1859.
"Our findings will help states prioritize where to spend their money and find the highest-emitting wells more easily," said co-author Rob Jackson of Stanford University's School of Earth, Energy & Environmental Sciences. "If you want to reduce climate change quickly, methane provides a great opportunity."
Research conducted previously at Princeton by Mary Kang, first author of the new study, determined that abandoned wells in Pennsylvania contributed a significant amount of methane to the atmosphere and should be factored into greenhouse gas emissions inventories. However, the number of abandoned wells, combined with poor record keeping, had made it difficult to determine how many wells actually existed to develop mitigation strategies.
Researchers from Stanford, Princeton, Lawrence Berkeley National Laboratory and The Ohio State University coupled 163 well measurements of methane flow rates; ethane, propane, andn-butane concentrations; isotopes of methane; and noble gas concentrations from 88 wells in Pennsylvania. They synthesized data using old books, historical documents, field investigations and state databases. Research was supported by grants from the National Oceanic and Atmospheric Administration and Vulcan Inc.
Based on their findings, researchers estimated there are 470,000-750,000 abandoned wells in Pennsylvania that contribute 5-8% of annual methane emissions in the state.
The research team improved not only the estimates about the number of abandoned wells in Pennsylvania, but also characterized key attributes that accompany "high emitters," including depth, type, plugging status and coal area designation. Researchers also estimated attribute-specific and overall methane emissions from abandoned wells.
"High emitters are best predicted as unplugged gas wells and plugged/vented gas wells in coal areas and appear to be unrelated to the presence of underground natural gas storage areas or unconventional oil/gas production," the researchers said. "Repeat measurements over two years show that flow rates of high emitters are sustained through time."
Methodology that combined new field measurements with data mining of previously unavailable well attributes and numbers of wells could be used to improve methane emission estimates and prioritize cost-effective mitigation strategies "for Pennsylvania and beyond," researchers said. "Knowing the attributes of high emitters will lead to cost-effective mitigation strategies that target high methane-emitting wells."
Most of the emissions could be reduced "just by addressing 5-10% of the wells," said Jackson, who is affiliated with the Stanford's campus-wide Natural Gas Initiative, devoted to addressing questions and funding projects related to the growth in natural gas production in the past decade.
The standard method to plug wells is to pour cement to create a combination of deep and shallow plugs, with one through every oil, gas, coal or water-bearing layer and another at the surface to isolate drinking water. It's not an inexpensive process, but if the super methane emitters can be identified, many of the problems nationwide could be addressed, Jackson said.
"The new findings highlight a need to develop more effective mitigation strategies at the wells themselves to address the most problematic sections of damaged wells," the authors said.