These include water wells, mineral exploration wells, and oil and gas production wells.
Many wells penetrate one or more aquifers.
The wells also pierce formations containing oil and gas reservoirs, mineral deposits such as uranium and lead, and water contaminated with salt, iron, selenium, sulfates, and radon. The well borehole provides a mechanism for communi cation of fluids and gasses between formations. When aquifers are involved, this poses a severe pollution threat. For example, if the borehole passes through both an aquifer and a brine-bearing formation, the brine can invade the aquifer and compromise th e quality and purity of the water. The problem escalates if the brine layer is pressurized with respect to the aquifer, causing continuous flow of brine into the fresh-water formation.