The viscosity of the mud is very important for the optimisation of various different mud functions.
The results of the fann viscometer test can be expressed in two different ways: The Bingham method by which Plastic Viscosity and Yield Point are determined and the Power Law method which results in the Power Index (n) and the Consistency Index (k). The Bingham method is used to determine treatment requirements at the mud.
The results of the power law method are used in pressure drop calculations and to determine the carrying capacity.
2.2.1 Plastic Viscosity (PV)
The PV is the difference between the readings at 600 and 300 RPM (R600 ' Rsod)-
Plastic viscosity is that part of the resistance to flow caused by mechanical friction.
The PV is mainly dependent on the amount of particles in the mud. Secondary effects on PV are caused by the shape of the particles and the liquid viscosity.
A lowest possible PV is essential for: low frictional losses
Due to sizing of the Fann viscometer the PV is directly expressed in cP
The Yield Point is calculated by substracting the PV from the fann reading at 300 RPM (R300 - PV), and expressed in lb/100 ft2. The Yield Point is that part of the resistance to flow caused by attractive forces between particles. The YP is a function of:
Clays suspended in water generally develop negative charges on the faces of the individual platelets and positive charges on the edges. Attraction between these charges leads to build-up of a card house type structure which results in a high YP.
Gel values are a measure of the build-up of gel structures in the mud under static conditions. Gel values originate from the same forces and parameters as the YP. Gel values are measured after 10 seconds and 10 minutes of static build-up of the mud. A reasonable 10" gel is essential to prevent immediate settling of solids when circulation is stopped.
A large difference between 10" gel and 10' gel indicates a slow but ongoing build-up of structure. This may result in highly gelled muds during a round trip and hence in high swab/surge pressures causing hole failure. Optimally the 10' gel value is 1.5 x the 10" gel value.
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