**1. n. [Drilling Fluids]**

Equality between the mass (density × volume) of a mixture or blend such as a drilling mud or completion fluid and the sum of the masses (density × corresponding volume) of its components. Assumptions are: (1) masses and volumes of components are additive and (2) material is neither generated nor lost from the system. As a simple example, below are the two material-balance equations for a three-component mixture of oil (o), water (w) and solids (s), where V = volume percent, D = specific density and MW = mixture weight. (This could represent a simple, weighted oil-base mud formulation.)

MW = DsVs + DoVo + DwVw

100% = Vs + Vo + Vw.

**See:**
high-gravity solids,
low-gravity solids,
make-up water,
material-balance equation,
mud density,
retort solids,
slurry