Wellbore is the actual hole drilled into the earth’s subsurface to create a well, serving as the primary conduit for accessing underground resources such as oil, natural gas, water, or geothermal energy. It is formed by a drill bit that penetrates rock layers under controlled conditions, typically using a rotating drill string in rotary drilling operations. The wellbore begins at the surface (the wellhead location) and extends thousands or even tens of thousands of feet downward, and it may be drilled vertically, directionally, or horizontally depending on the target reservoir and engineering objectives. Its diameter generally ranges from a few inches to several feet and is usually lined with steel casing pipes that are cemented in place to provide structural integrity, prevent collapse, isolate formations, and protect freshwater zones from contamination. The open or cased section of the wellbore allows fluids (hydrocarbons, water, or injected materials) to flow between the reservoir and the surface, making it the critical pathway through which exploration, production, injection, or monitoring activities are conducted in the oil and gas, water, and geothermal industries.
