Hydraulics
The concept that a fluid cannot maintain a rigid shape is a basic, but important characteristic, which means that fluids cannot sustain a shearstress (a tangential force applied to the surface). Any tangential force will cause the fluid to deform and continuous deformation is known as “flow”.
Fluid flow is always considered to take place within a conductor. A conductor may be the annulus created by casing for drilling fluid or a volcano’s slope and the atmosphere, in the case of a lava flow. Generally, fluid flow can be considered the result of parallel fluid layers sliding past one another. The layers adjacent to the conductor adhere to the surface and each successive layer slides past its neighbor with increasing velocity. This orderly flow pattern is known as laminar flow. At higher velocities, these layers lose their order and crash randomly into one another with an orderly flow occurring only adjacent to the conductor. This flow pattern is known as turbulent flow.
Laminar Flow is usually found in the annulus during drilling operations. This type of flow is generally desired in the annulus since it does not lead to hole erosion and does not produce excessive pressure drops. These pressure drop calculations can be mathematically derived according to the
type of flow behavior.
Turbulent Flow is the type of flow regime found inside the drill string during drilling operations. Since high mud velocities are required to achieve turbulent flow, this results in high pressure drops. This type of flow is generally not desired in the annulus due to its tendency to cause excessive hole erosion and high “equivalent circulating densities”.
However, turbulent flow can move the mud like a plug, causing the mud to move at approximately the same rate. This provides for better hole cleaning and is sometimes required on high angle holes. Pressure drop calculations for turbulent flow are empirical rather than mathematically derived. When a force is applied to a static fluid, the layers slide past one another and the frictional drag that occurs between the layers (which offers resistance to flow) is known as “shear-stress”.
