State regulators are considering requiring retroactive top-to-bottom cementing of about 30 natural gas wells south of Silt because of concerns about possible methane contamination of domestic groundwater.
The approach is something the Colorado Oil and Gas Conservation Commission might evaluate and possibly require in some other areas in the region as well, said Stuart Ellsworth, the agency’s engineering manager.
In the Mamm/Divide creek area, the technique is being considered for wells where shallower gas not being targeted for energy development could rise through the uncemented area between the well casing and the outer edge of the well bore. Gas related to drilling has been found in domestic groundwater in the area.
Geoffrey Thyne, a geological consultant for Garfield County, long has contended that methane levels have been rising in water wells in the geologically faulted Mamm/Divide creek area as a result of drilling, and he has recommended consideration of top-to-bottom cementing. He welcomed news that the state is evaluating the approach for some wells.
“I think overall when we work in these structurally complex areas it seems to be a good idea to cement top to bottom,” he said.
The state requires that approach in coal-bed methane development. In Piceance Basin wells, however, it requires cementing around the well casing only from the bottom up to above the gas production zone, and from the top down to below domestic groundwater levels. That leaves the intermediate area unsealed.
Chevron voluntarily cements essentially the entire length of its local wells, where it operates outside De Beque, to protect groundwater and keep the metal well casing from being corroded by water. ExxonMobil, which operates in Rio Blanco County, also normally cements from top to bottom, state officials say.
The approach costs more. But state officials are evaluating whether it might make sense in some cases where the intermediate portions of wells pass through formations containing what Ellsworth called “nuisance” gas of noncommercial volume. Water around the well casing can create enough pressure to keep the gas in place, but if not, cementing may be needed, Ellsworth said.
Unfortunately, he said, where there is enough gas pressure, it can be hard to create a seal because the gas creates channels through the cement before it hardens. When that happens, the damaged cement that is in place makes it difficult to do repairs. As a result, sometimes it’s better to try to control the gas “by other methods, so you don’t hide a problem,” he said.