An eco well is a device seen in Jak II. They were used by Baron Praxis at the strip mine to harvest eco, and then release the harvested eco to attract the Metal Heads. Jak destroyed each of these with plasmite bombs during the mission "Blow up strip mine eco wells".
Eco wells are large, gray, cylindrical containers, with a little ramp that leads upwards on its front-side. On top of the device there is a vent which will release the harvested eco as would an eco vent.
Originally the eco wells functioned as actual wells. They were to be linked into the eco grid so the eco can be used for Haven City. However, the tyrannical Baron never intended to link them into the city grid, according to Vin. This would attract more Metal Heads resulting in the strip mine being too dangerous for Vin or any other operations. Vin was paranoid about this and decided to ask Jak to close them off using plasmite bombs. Jak succeeded in destroying all six of the wells in under two minutes before they all detonated.