The objective of Recommended Practice 13D is to provide a basic understanding of and guidance about drilling fluid rheology and hydraulics, and their application to drilling operations. The methods for the calculations used in RP 13D do not take into account the effects of temperature and compressibility on the density of the drilling fluid. Rheology is the study of the deformation and flow of matter. Drilling fluid hydraulics pertains to both laminar and turbulent flow regimes.For this RP, rheology is the study of the flow characteristics of a drilling fluid and how these characteristics affect movement of the fluid.