Viscosity

The viscosity of the mud is very important for the optimisation of various different mud functions.

The viscosity is measured with two different instruments: The Marsh funnel and the Fann viscometer.

  1. 1 THE MARSH FUNNEL VISCOSITY - expressed in seconds per quart (946 ml) - is determined with very simple equipment. The marsh funnel is used routinely on the rigs to establish whether changes in the mud properties occur. Further conclusions cannot be drawn from the marsh funnel viscosity results.
  2. 2 The Fann viscometer is a far more delicate instrument. It consists of a rotating cilinder and a bob (stator) which is connected to a spring. The cilinder is rotated at 600 and 300 rpm respectively and readings of the position of the bob are taken. Subsequently gel values are determined by rotating at a low speed.

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.

PV is increased by:

  • Increase in solids content due to penetration or addition of weighting agents.
  • Increase in solids content due to insufficient solids removal.
  • Addition of polymers (CMC HV, starch HV) to the mud.

A lowest possible PV is essential for: low frictional losses

- optimal hole cleaning.

The PV can be reduced by:

  • lowering the solids concentration (watering back, settling)
  • removal of solids (centrifuge, desander, desilter)
  • reduction of fluid viscosity (watering back).

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:

  • type of solids and surface charge associated with them
  • solids concentration
  • ionic concentration in the liquid phase.

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.

The YP is increased by

  • addition of clay particles to the mud
  • drilling through clay layers
  • addition of biopolymers to the mud
  • contamination of mud with e.g. salts, cement of gypsum.

The YP is decreased by

  • shielding of the positive clay charges with thinners (e.g. lignosulfonates)
  • reduction of solids content (solids removal, watering back)
  • chemical neutralisation of contaminants.

An optimal YP is essential for:

  • carrying capacity of the mud (rule of thumb YP + 0.75 x hole size (in))
  • stability of mud towards settling of solids (rule of thumb YP + 9 x spec gravity kg/I)
  • viscosity behaviour viz. hole cleaning capacity ot the mud

Gels

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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Responses

  • willow boyle
    Can a viscofier increase the mud viscosity?
    7 years ago
  • erling
    How big is the hole in a viscometer?
    3 years ago
  • italia
    How to calculate r600 and r300 from a mud report?
    1 month ago

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