Definition - What does Shear Stress mean?
Shear stress in drilling fluid can be defined as the force per unit area required to maintain the fluid flow at a constant rate. Drilling fluid flow regime can be plug flow, laminar flow or turbulent flow. The flow regimes change in the transition phase between these flows.
Plug flow happens at low shear rate in the gel stage, laminar flow occurs at low flow velocity, and turbulent flow is subject to fluctuations that happen in the direction of flow and velocity.
Trenchlesspedia explains Shear Stress
The drilling fluid flow properties, optimized drill operating conditions, hole cleaning and borehole stability depend on good rheological properties of the mud. Almost all drilling fluid falls in the non-Newtonian category that is the viscosity of the fluid changes under stress to more liquid or more solid.
Shear stress can be mathematically defined as T = F/A, where T - shear stress, F - shear force and A - area on which the shear force acts. A fluid placed between two plates 1 cm apart, applied with a force of 1 Dyne/ sq.cm. on the upper plate, the shear stress at any point between the two plates will be 1 Dyne/cm2.