Drilling Mud Viscosity

Fluid viscosity is the measure of friction between the layers of the fluid.Due to this friction fluid resists flow.Viscosity also tells about the thickness of the fluid.More viscosity means more thick fluid and more pressure will be needed to flow it.Viscosity depends on velocity and pattern of flow that's why absolute viscosity is difficult to measure and only changes in mud viscosity are important.
Main function of viscosity in drilling is to lift drilled cuttings.Annular Pressure losses also depends on viscosity.Higher the viscosity of drilling fluid higher will be the cutting carrying capacity(desired) but annular pressure losses and swab and surge pressure will also be higher(not desired).That's why a compromise is made and an optimum value of viscosity is chosen.

Field Measurement of Viscosity

In field, Viscosity is measured in two ways:-
  • Marsh funnel gives Marsh funnel viscosity
  • Rotary viscometer gives Plastic viscosity
Drilling mud viscosity

Marsh funnel Viscosity:- Marsh funnel works on the principle that thick fluid will flow slower than thin fluid. To measure viscosity first the marsh funnel is filled with drilling mud up-to a marked point (1500 ml).Then the time taken by the marsh funnel to discharge 946 ml (one quart) is noted and this value is taken as marsh funnel viscosity.Its unit is seconds/quart.
Changes in the value of marsh funnel viscosity tells that the mud quality is degrading and it needs treatment.This change in mud viscosity can be due to addition of drilled solids or influx from the formation. For fresh water marsh funnel viscosity is around 25-26 sec/quart.Though Marsh funnel viscosity don't have scientific support but it is the fastest way to check changes in mud quality.

Plastic Viscosity:- Plastic viscosity is measured by rotary viscometer which consists of two cylinders. Outer cylinder can rotate at various speeds while the inner cylinder is stationary and is connected to a dial through a spring. There is very less distance between the two cylinders that only very thin layer of fluid exists between them.When the outer cylinder is rotated it transmits torque to the layer of fluid between the cylinders which is transmitted to the inner cylinder which will deflect the dial and gives the reading of shear stress(torque) for that particular shear rate(RPM of outer cylinder)

Plastic Viscosity (PV) = Reading at 600 rpm – Reading at 300 rpm
Its unit is centipoise(cp).
viscositymeter
If the viscosity is to be increased viscosifiers such as bentonite,PHPA copolymers etc are added. If the viscosity is to be reduced water can be added.

Note:-Marsh funnel viscosity is measured regularly while drilling(every 30 min or 1 hour) both at suction pit(mud into well) and shale shaker (mud out) while plastic viscosity is measured by mud engineer once or twice per shift(or as per requirement)

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