Yield Point (YP) is resistance of initial flow of fluid or the stress required in order to move the fluid. You can simply say that the Yield Point (YP) is the attractive force among colloidal particles in drilling mud.

Practically, you can calculate the YP by this following formula.

Yield Point (YP) = Reading from a viscometer at 300 rpm – Plastic Viscosity (PV)

A unit of YP is lb/100 squarer foot.

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 centi-poise

Yield Point (YP) = 35 – 21 = 14 lb/100 squarer foot.

The YP indicates the ability of the drilling fluid to carry cuttings to surface. Moreover, frictional pressure loss is directly related to the YP. If you have higher YP, you will have high pressure loss while the mud is being circulated.

In water-based mud, the YP will be increased with following items;

High temperature – the high temperature environment tends to increase the YP in the water base mud.

Contaminants in the mud – Contaminants such as carbon dioxide, salt, and anhydrite in the drilling fluids.

Over treatment with chemicals – Over treatment the drilling mud with lime or caustic soda.

In oil-based mud, the causes of increasing in YP are listed below;

Drill solid – the more drill solid you have, the more YP will be.

Treatment Carbon Dioxide (CO2) in the mud with lime (CaO) – The lime (CaO) will chemically react with Carbon Dioxide (CO2) to form Calcium Carbonate (CaCO3) which will increase the YP.

Low temperature – in the oil base system, the low temperate will increase the viscosity and the YP. Please keep in mind that this is opposite to the water base system.

Operational impacts of the YP on drilling operation are as follows;

Equivalent Circulating Density (ECD) – The ECD typically increases when the YP increases.

Hole Cleaning – When you drill a large diameter hole, the YP in the drilling fluid must be high in order to help hole cleaning efficiency. It is very difficult to say how high of PV should be because it depends on several factors. Especially, if you are drilling in the new field, you may need to start with general YP values, and then you will learn once you have experience in a particular field.