The hole made by drilling a well. Where casing is run in the well, the borehole is the space between the exterior of the casing and the formations. After the casing has been installed, the borehole is normally filled with various materials such as cement, drilling mud, sloughing formations, and water.
Source: API BULLETIN E3, Environmental Guidance Document: Well Abandonment and Inactive Well Practices for U.S. Exploration and Production Operations, First Edition, January 1993 (Reaffirmed June 2000). Global Standards