A well is a structure that is drilled into the ground for several purposes.
We will discuss two basic types of wells throughout this lesson. They are known as 1) extraction wells and 2) injection wells.
Extraction wells, also known as recovery wells, are drilled for the purpose of accessing natural resources, such as:
- Extraction of water
- Sampling of rock or soil
- Extraction of mineral resources, like oil and gas
This figure shows an artesian well for the purpose of extracting freshwater. The well pipe has been drilled into a confined aquifer, which is under pressure, to extract water at the surface. An artesian well represents the special case where the pressure in the aquifer is high enough to push the water all the way to the surface against gravity.
Injection wells are used for injecting a number of different materials for various purposes, such as:
- Injection of wastewater or other waste fluids
- Injection of water for secondary oil recovery
- Injection of CO2 for enhanced oil recovery
This figure shows an injection well for the purpose of draining stormwater runoff to the subsurface, where drainage may come in contact with the groundwater table or aquifer. Injection wells are often used for stormwater discharge to alleviate potential flooding. In this case, gravity alone (rather than pumping as can be common with some injection wells) serves as the force to inject the stormwater runoff.
As showcased in the figures above, the same material can be considered a resource we extract, or a waste product we dispose of through injection. For example, artesian wells (top figure) are typically drilled to extract water stored in pressurized, confined aquifers. In contrast, we can drill an injection well into a confined space below ground to dispose of wastewater, such as stormwater runoff (bottom figure).