1. n. 
The process of droplet growth as small drops merge together when they come in contact. If this occurs repeatedly, a continuous liquid phase forms. Through this phenomenon, emulsions break and form two distinct liquid phases that tend to separate. In oil-base mud, the water phase is dispersed as small droplets, with oil as the continuous (external) phase. A stable oil mud will remain dispersed under normal drilling conditions because when droplets contact each other, they do not coalesce due to the strong emulsifier film around each droplet. However, when the emulsion film around each droplet becomes weakened, droplets will begin to coalesce. If not corrected, this can lead to total emulsion breakdown with solids becoming water-wetted.
Alternate Form: coalesce