Definition - What does Shear Rate mean?
Shear rate in the context of drilling fluid can be defined as the rate of change of velocity at which point one fluid layer passes over another layer. In order to understand hole cleaning and ability of the drilling fluid to suspend cuttings, it is important to understand the flow properties of drilling fluid at low shear rates.
Drilling fluid has three flow regimes namely plug flow, laminar flow, and turbulent flow. In between these zones are transition zones where the flow regimes change.
Trenchlesspedia explains Shear Rate
At very low shear rate when the mud is in the gel stage, the flow can be classified as plug flow, where the velocity of the mud at the center of the annulus is equal to the velocity of the mud at the sides. In laminar flow, the shear rate is a function of the shear stress of the fluid and the flow has a predictable pattern. In the turbulent flow, the flow is disorderly and difficult to describe since it is subject to random local fluctuations.
The shear rate is highest when the mud passes through the bit nozzles. The drilling fluid will resist flow initially until the shear stress exceeds a certain limit to break the interparticle bonding present in the fluid. Once the fluid begins to flow, the shear stress and shear rate have a linear relationship.
This helps the drilling fluid to suspend the solids and cuttings present in the fluid when circulation is stopped.