Perilous Pathways: How Drilling Near An Abandoned Well Produced a Methane Geyser by Scott Detrow, October 9, 2012, NPR State Impact
Methane is an odorless, colorless gas that exists naturally below the surface. It isn’t poisonous, but it’s dangerous. When enough methane gathers in an enclosed space — a basement or a water well, for instance — it can trigger an explosion.
Fred Baldassare worked at Pennsylvania’s Department of Environmental Protection for 25 years. He spent more than half his career investigating cases of methane migration, where gas from wells, coal mines, landfills or other sources broke loose and made its way to the surface. Baldassare investigated more than 200 different episodes. Only a handful of them, he says — perhaps five or six — involved an active drilling site communicating with an abandoned oil or gas well. But when the new and old operations did intersect, Baldassare says, the results were often “dramatic.” When energy companies drill down to the Marcellus Shale, deep below the surface, their wells pass through several smaller, shallow gas formations. Drillers go to great lengths to seal off their gas wells, and Pennsylvania regulations require companies to bond their multiple layers of steel casing with top-grade cement. Shell’s Guindon well, located a few thousand feet from the old Butters well, was lined with more than 1,200 sacks of cement, along with four layers of casing ranging from 13 3/8 inches to 4 1/2 inches in diameter. Most of the time, this casing prevents the shallow gas from moving to the surface. (That’s not always the case – just see StateImpact Pennsylvania’s reporting on a methane leak in Bradford County.)
But if an old, unplugged gas well has been drilled into the same formation already, the new activity can displace pockets of gas, through pressure changes and physical interaction. Baldasarre explains, “that gas can move to the old well, because [the well] represents a low pressure zone and a natural migration highway. “Gas always wants to go from high pressure to low pressure,” Baldassare continues. “That old well represents a low-pressure zone. Much like water wants to move downhill, gas wants to move to low-pressure zones.” The lowest pressure is near the surface, so once the gas reaches an old well, it will shoot straight up. And a new well doesn’t need to be present to trigger this migration. Gas can migrate to the surface through these pathways on its own. The state has investigated dozens of cases where unknown wells have led to gas pooling in basements, water wells, or other locations. Depending on how old the abandoned well is, the casing can be leaky, rotten, or nonexistent. Methane can easily move into natural faults and cracks, following a path toward the surface that can travel through aquifers. That’s likely how gas ended up bubbling into a creek, out of a water well and up into the 30-foot geyser in Union Township. [Emphasis added]