Cement Additives in Drilling
Lost Circulation Prevention Additives
Lost circulation (LC) or lost returns refers to the significant and continuing loss of either whole drilling fluid or cement slurry to a formation. It should not be confused with lost filtration.
Drilling fluids or cement slurries can be lost to either natural or induced formation fractures or, less commonly, to highly permeable formations, such as carbonate formations with vugs or cavernous porosity. Cement, with its larger particle size, is less susceptible to loss in permeable formations but can still be lost to induced fractures in weak formations.
The best time to reduce the chance of such lost circulation is during drilling, when high concentrations of bridging materials and various types of plugs (pills) can be pumped into the well to fill up the fractures. During primary cementing, concentrations of LC additives must be carefully controlled to avoid bridging the casing-borehole or liner-borehole annulus, or plugging downhole cementing equipment.
Types of LC additives available for cement are blocky-granular materials (ground walnut shells, gilsonite, crushed coal, expanded perlite) and laminated materials (mica flakes, cellophane flakes) which form flake-type mats. In laboratory studies, granular material has been found to be best suited for bridging fractures.
Fibrous materials such as nylon fibers are used in drilling fluid for sealing large openings but are not normally used in cement because they tend to plug surface and downhole cementing equipment. Also, some fibrous LC materials contain organic chemicals that can seriously retard cement-thickening time.
