(n.) Drilling Fluids, Formation Evaluation. The residue deposited on a permeable medium when a slurry, such as a drilling fluid, is forced against the medium under apressure. Filtrate is the liquid that passes through the medium, leaving the cake on the medium. Drilling muds are tested to determine filtration rate and filter-cake properties. Cake properties such as cake thickness, toughness, slickness andpermeability are important because the cake that forms on permeable zones in the wellbore can cause stuck pipe and other drilling problems. Reduced oil and gas production can result from reservoir damage when a poor filter cake allows deep filtrate invasion. A certain degree of cake buildup is desirable to isolate formations from drilling fluids. In openholecompletions in high-angle or horizontal holes, the formation of an external filter cake is preferable to a cake that forms partly inside the formation. The latter has a higher potential for formation damage. Syn: Filter Cake. See: Deflocculated Mud, Drilling Fluid, Dynamic Filtration, Filter-Cake Quality, Filter-Cake Thickness, Filtrate, Filtrate Volume, Openhole Completion, Relative Filtrate Volume, Resin, Static Filtration