Yield Point (YP) is resistance of initial flow of fluid or the stress required in order to move the fluid. It can be simply stated that the Yield Point (YP) is the attractive force among colloidal particles in drilling fluid. As per Bingham plastic model, YP is the shear stress extrapolated to a shear rate of zero.
Figure 1 – YP is a shear stress at zero shear rate.
Yield point can be calculated by the following formula.
Yield Point (YP) = Reading at 300 rpm – Plastic Viscosity (PV)
A unit of YP is lb/100 ft2.
You can determine the Plastic Viscosity (PV) by this formula.
Plastic Viscosity (PV) = Reading at 600 rpm – Reading at 300 rpm
For example, you have these values from the viscometer.
Reading at 600 rpm = 56
Reading at 300 rpm = 35
Plastic Viscosity (PV) = 56 – 35 = 21 CP
Yield Point (YP) = 35 – 21 = 14 lb/100 ft2
The YP indicates the ability of the drilling mud to carry cuttings to the surface. Moreover, frictional pressure loss is directly related to the YP. Higher YP will result in larger frictional pressure loss.
For water base mud, the YP will be increased with the following items;
- High temperature – the high temperature environment tends to increase the YP in the water based mud.
- Contaminants such as carbon dioxide, salt, and anhydrite in the drilling fluids
- Over treatment of the drilling mud with lime or caustic soda
For oil based mud, the causes of increasing in YP are listed below;
- Drill solid – the more drill solid you have, the more the YP will be.
- Treatment CO2 in the mud with lime (CaO) – The lime (CaO) will chemically react with CO2 to form Calcium Carbonate (CaCO3) which will increase the YP.
- Low temperature – in the oil based system, the low temperate will increase the viscosity and the YP. Please keep in mind that this is opposite to the water based system.
Operational impacts of the YP are as follows;
Equivalent Circulating Density (ECD)
The ECD typically increases when the YP increases.
Usually the larger diameter hole to be drilled, the higher the YP must be to support efficient hole cleaning.