(1) (n.) Geology. Generally, the study of how matter deforms and flows, including its elasticity, plasticity and viscosity. in geology, rheology is particularly important in studies of moving ice, water, salt and magma, as well as in studies of deforming rocks. See: Elastic Deformation, Halite, Magma, Plastic Deformation. (2) (n.) Drilling Fluids. The science and study of the deformation and flow of matter. The term is also used to indicate the properties of a given fluid, as in mud rheology. Rheology is an extremely important property of drilling muds, drill-in fluids, workover and completion fluids, cements and specialty fluids and pills. Mud rheology is measured on a continual basis while drilling and adjusted with additives or dilution to meet the needs of the operation. in water-base fluids, water quality plays an important role in how additives perform. Temperature affects behavior and interactions of the water, clay, polymers and solids in a mud. Downhole pressure must be taken into account in evaluating the rheology of oil muds. See: Apparent Viscosity, Bingham Plastic Model, Brookfield Viscometer, Capillary Tube Viscometer, Carrying Capacity, Centipoise, Deflocculant, Direct-Indicating Viscometer, Herschel-Bulkley Fluid, High-Pressure, High-Temperature Viscometer, Kill-Weight Fluid, Mud Program, Newtonian Fluid, Non-Newtonian Fluid, Plastic Fluid, Polymer, Power-Law Fluid, PVT, Rheological Property, Shear Rate, Thixotropy, Viscosity, XC Polymer