Definition - 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.
Trenchlesspedia explains Annular Filler
The process of putting filler into the drilling annulus is called "grouting a well." Proper grouting not only prevents groundwater pollution, it's an important safety factor as well.
Some states, such as Wisconsin, have a requirement for the use of an annular filler; however, their requirements may not be as strict as those imposed by the US EPA. The EPA requires that annular filler (or annulus filler) consist of 90-percent bentonite, "neat" cement (cement without inclusions, such as gravel) or an unspecified mix of bentonite and cement.