Cuttings Slip Velocity
A cutting, traveling up the annulus, experiences a positive upward force due to the drilling fluid velocity, density and viscosity, and a negative downward force due to gravity. The rate at which a cutting falls is known as its “slip velocity”.
Several studies have enabled the following generalizations to be made:
1. The most important factors controlling adequate cuttings transport are annular velocity and rheological properties
2. Annular velocities of 50 ft/min provide adequate cuttings transport in typical muds
3. Cuttings transport efficiency increases as fluid velocity increases
4. The slippage of cuttings as they are transported induces shear thinning of the mud around the cutting reducing the expected transport efficiency
5. Cutting size and mud density have a moderate influence on cuttings transport
6. Hole size, string rpm, and drill rate have slight effects on cuttings transport.
Those who have observed a solids tracer emerging over the shale shaker will realize the large spread of “cuttings” that occurs. Therefore, any calculated estimation of slip velocity will only be an approximation. The reason for this “spread” of solids is the particles ability to be carried by the drilling fluid. It is a function of its position in the mud stream and the size of the particle. Cuttings will travel up the annulus more efficiently if they travel flat and horizontally. If the cutting turns on its edge, it will slip more easily. Smaller cuttings are more prone to do this. Rotation of the drillpipe will result in a helical motion of the fluid, which will aid transport for those
cuttings nearest the pipe.
The rheological properties of the drilling fluid will affect cuttings transport, in as much as they affect the flow profile. Lowering the “n” value or an increases in the YP/PV ratio will generally flatten the flow profile and increase carrying capacity.
The slip velocity of a cutting in turbulent flow may be estimated using:
where: Vs = Slip Velocity (ft/min)
dp = Particle Diameter (inches)
pp = Particle density (lb/gal)
MD = Mud Density (lb/gal)
CD = Drag Coefficient
For these calculations, the particle density is found by multiplying the cuttings density (gm/cc) by the density of fresh water (8.34). The drag coefficient is the frictional drag between the fluid and the particle.
In turbulent flow, the drag coefficient is 1.5.
In laminar flow, the equivalent viscosity (m) will effect the slip velocity. In this case the slip velocity is
:
Equivalent viscosity is calculated as mentioned earlier.
