What Does Annular Filler Mean?
Annular filler is an impervious, usually bentonite, cement or a bentonite-and cement mix placed in the annulus (the space between the casing and the borehole wall) to prevent the introduction of drilling fluids or other contaminants from drilling operations into local groundwater sources.
The use of annular filler is mandated by the US Environmental Protection Agency, which also specifies the type of materials to be used, the makeup of the materials used for annular filler and the depth to which the annular filler must be placed.
Annular filler is also known as an annular seal.