A large percentage of the solids generated in the drilling process must be removed from the drilling mud so that the mud properties can be controlled. Solids control equipment can be used to remove a portion of the solids generated. If the equipment cannot remove a sufficient percentage of the drilled solids for mud control, then discarding of whole mud is required to achieve adequate solids removal. One way to look at solids removal effectiveness for a drilling mud/solids control system is
to determine the efficiency of removal of drilled solids by the system.

Solids removal efficiency, E r, is defined as the percentage of the drilled solids generated that are removed by processing. Typical efficiencies for solids removal vary from 60 vol% to 90 vol%. Usually higher removal efficiencies are desireable. Discarding or dumping of all solids containing mud from the active system can be used to achieve low drill solids contents, but would indicate poor removal efficiency. In all but the
most simple drilling situations, this is impractical and the solids content is controlled to achieve stable mud properties regardless of the overall removal efficiency of the system. In some cases, high removal efficiencies
can be achieved, but are not economical because of high mud losses.

Control of solids control equipment to realize a desired removal efficiency can be accomplished by a variety of methods. As examples: finer screens can be installed on the shale shakers, centrifuge feed rates can be increased, and hydrocyclone discharge can be adjusted. Control of mud properties also influence solids removal efficiency. Because smaller solids are removed less efficiently, oil muds and more inhibitive water based
muds increase removal efficiency by reducing the dispersion and breakdown of solids. Solids removal efficiency can be improved by the generation of large solids at the bit and rapidly transporting those solids to the surface.

While instantaneous measurements of solids removal efficiency can be made, measurement of removal efficiency on a daily basis or by interval gives ample guidance for most drilling operations. Determination of
removal efficiency can best be made indirectly by applying the relationships developed later in this paper. For planning purposes, removal efficiency can be estimated from experience with solids control systems and analysis of offset well data. A clear understanding
of removal efficiency is one key to using these material balance concepts. Another key concept is the realization that mud is discarded with solids removal.