16.1 QUANTIFYING DRILLING WASTE

Drilling waste consists of waste drilling fluid, drilled cuttings with associated drilling fluid, and, to a lesser extent, miscellaneous fluids such as excess cement, spacers, and a variety of other fluids. The amount of drilling waste depends on a number of factors. These include hole size, solids-control efficiency, the ability of the drilling fluid to tolerate solids, the ability of the drilling fluid to inhibit degradation or dispersion of drilled cuttings, and the amount of drilling fluid retained on the drilled cuttings.

One simple expression states the amount of wet drilled solids to be discarded as:

where

• S = volume of wet drilled solids, in bbl

• ? = efficiency of solids control, expressed as a fraction

• HV = hole volume, in bbl

• F _s = fraction of solids in the discard stream.

The fraction of the solids in the discard stream varies from a maximum of about 50% to a lower value of about 25 ?30%. There is always some amount of drilling fluid associated with drilled cuttings being discarded. Solids-control systems, no matter how good, cannot totally separate the drilling fluid from the drilled cuttings. By the same token, rarely can all of the drilled cuttings be separated from the circulating system. This means that, with time, drilled solids will build up in the circulating system.