NGWA Augering and Monitoring Well Practice Exam

The annulus is the gap between the outside of the casing and the borehole wall. This space is what is cemented or grouted to secure the casing and create a hydraulic barrier that prevents fluids from moving between formations or into the borehole. In wells with multiple casing strings, there can be annuli between casing strings as well, but the essential idea is the space surrounding the casing inside the borehole. The other described spaces—outside the borehole wall (the surrounding rock), inside the casing (the interior of the pipe), or above the surface seal—are not the annulus.