Annular

In well drilling, the annular space, or annulus, is the space between two concentric cylinders within the wellbore.  Specifically, it is the space between the drill string (casing) and the wellbore (another casing string).  This annular space is critical for several aspects of well control and operations, including the circulation of drilling fluids, wellbore stability, and the prevention of formation fluids from entering the wellbore.

Key Points of Annular Space in Well Drilling

Circulation of Drilling Fluids  -  Drilling fluids (also called drilling mud) are circulated down through the drill pipe and back up through the annulus.  This circulation helps to carry cuttings (rock and debris) from the drill bit to the surface.
Wellbore Stability  -  The drilling fluid in the annulus exerts hydrostatic pressure against the wellbore walls, helping to prevent the collapse of the wellbore and controlling the pressures from subsurface formations.
Cuttings Transport  -  The upward flow of drilling fluids in the annulus carries rock cuttings and other debris to the surface for removal.
Isolation of Formations  -  The annular space can be used to isolate different geological formations from each other to prevent cross-flow of formation fluids.  This is often achieved by cementing the casing in the annulus.
Preventing Formation Fluid Influx  -  By maintaining appropriate drilling fluid density and pressure, the annular space helps to prevent formation fluids (such as oil, gas, or water) from entering the wellbore in an uncontrolled manner (known as a "kick").

Components of the Annular Space

Outer Boundary   -  This can be the wall of the wellbore (in an open hole) or the inner wall of a casing string (in a cased hole).
Inner Boundary  -  This is typically the outer wall of the drill pipe or tubing.