The development of oil and gas has a 150-year history in the US, with wells stretching across the nation from California to Texas to Pennsylvania. We continue to reap the benefits of the infrastructure we built in earlier eras. But the downside to this long history comes in the form of millions of abandoned, poorly documented wells scattered throughout the country.

Recently, a team of researchers examined some of the abandoned wells in Pennsylvania to build a better picture of how this history continues to impact us today. Measurements of methane emissions revealed that abandoned wells may still be a significant source of methane to the atmosphere.

Methane is one of the more common greenhouse gases, and its warming potential is 86 times greater than carbon dioxide over a 20-year period. So limiting methane emission is an important strategy to curb global warming. Unfortunately, little is known about the ways old wells contribute to methane emissions because they are outside of our greenhouse gas emission inventory system.

Despite the long presence of these wells in the US, there isn't much data about what happens to them after they're abandoned. Many attributes can influence leakage, including depth, plugging status, well type (oil or gas), geographic location, and abandonment method.

To tackle this problem, a group of researchers analyzed a compilation of historical documents and modern databases, and they also did some present-day field work. During the field investigations, the scientists visited numerous Pennsylvania wells to measure the flow rate of methane, the presence of different carbon isotopes in the methane, and the concentration of a variety of other gases. Their analysis focused on a few key well attributes, including depth, plugging status, well type, and proximity to subsurface energy extraction and coal mining.

The database they developed enabled the team to better estimate the number of abandoned wells in Pennsylvania. They were also able to identify key attributes that often characterize wells with high methane emission rates. Unplugged gas wells and plugged or vented gas wells in coal areas tended to be the highest emitting wells. Surprisingly, the presence of subsurface natural gas extraction appeared unrelated to the presence of wells with high methane emissions. Similarly, unconventional oil and gas production—fracking—was not related to high emitters.

To understand how these flow rates might change over time, the team measured the methane release of high emitters multiple times over two years and found that their high methane flows were sustained over the study period. This finding suggests that these wells consistently contribute to Pennsylvania's annual methane emissions.

Looking at the state as a whole, the team estimates that abandoned wells contribute 0.04 to 0.07 megatons of methane per year, accounting for roughly five to eight percent of Pennsylvania’s annual human-driven emissions. That's anything but insignificant, and it should probably be accounted for on the greenhouse gas emissions inventories. This could help policy makers design more effective strategies to mitigate the contribution of abandoned wells to climate change.

PNAS, 2016. DOI: 10.1073/pnas.1605913113 (About DOIs).