Mud rheology refers to the mud of deformation and flow behavior of all forms of matter. Certain rheologic measurements made on fluids, such as viscosity, gel strength, etc. help determine how this fluid will flow under a variety of different conditions. This information is important in the design of circulating systems required to accomplish certain desired objectives in drilling operations.
Viscosity is defined as the resistance of a fluid to flow and is measured as the ratio of the shearing stress to the rate of shearing strain.
Two types of fluid characterizations are:
1. Newtonian (true fluids) where the ratio of shear stress to shear rate or
viscosity is constant, e.g. water, light oils, etc. and
2. Non-Newtonian (plastic fluids) where the viscosity is not constant, e.g.
drilling muds, colloids, etc.
The Baroid (Model 286) Rheometer is a coaxial cylindrical rotational viscometer, used to determine single or multi-point viscosities. It has fixed speeds of 3 (GEL), 100, 200, 300 and 600 RPM that are switch selectable with the RPM knob.
Additionally, the same switch set to the VAR position enables speed selection of between 3 and 625 RPM, by manual adjustment of the variable knob.
Viscosity Measurement Procedure
- Place a recently agitated sample in the cup, tilt back the upper housing of
the rheometer, locate the cup under the sleeve (the pins on the bottom of
the cup fit into the holes in the base plate), and lower the upper housing
to its normal position.
- Turn the knurled knob between the rear support posts to raise or lower
the rotor sleeve until it is immersed in the sample to the scribed line.
- Stir the sample for about 5 seconds at 600 RPM, then select the RPM
desired for the best.
- Wait for the dial reading to stabilize (the time depends on the sample’s
- Record the dial reading and RPM.
Mud Rheological Calculations
- Plastic viscosity (in centipoise-up): Plastic Viscosity = µp=600 RPM reading – 300 RPM Reading.
- Apparent Viscosity (in centipoise-cp): Apparent Viscosity=µa= 600 RPM Reading⁄2.
- Yield Point (in lb/100 ft²): Yield Point = Y. P. = 300 RPM Reading – Plastic Viscosity.
The Baroid Rheometer is also used to determine the Gel strength, in lb/100 sq. ft., of a mud. The Gel strength is a function of the inter-particle forces. An initial 10-second gel and a 10-minute gel strength measurement give an indication of the amount of gellation that will occur after circulation ceased and the mud remains static. The more the mud gels during shutdown periods, the more pump pressure will be required to initiate circulation again.
Most drilling muds are either colloids or emulsions which behave as plastic or non-Newtonian fluids. The flow characteristics of these differ from those of Newtonian fluids (i.e. water, light oils, etc.) in that their viscosity is not constant but varied with the rate of shear, as shown in Figure 2. Therefore, the viscosity of plastic fluid will depend on the rate of shear at which the measurements were taken.
Gel Strength Measurement Procedures
- Stir a sample at 600 RPM for about 15 seconds.
- Turn the RPM knob to the STOP position.
- Wait the desired rest time (normally 10 seconds or 10 minutes).
- Switch the RPM knob to the GEL position.
- Record the maximum deflection of the dial before the Gel breaks, as the
Gel strength in lb/100 ft². (lb/100 ft² x 5.077 = Gel strength in dynes/cm²).
This is the measure of the electro-chemical or attractive forces in the mud under flow (dynamic) conditions. These forces depend on (1) surface properties of the mud solids, (2) volume concentrations of the solids and (3) electrical environment of the solids. The yield point of the mud reflects its ability to carry drilled cuttings out of the hole.
YP = 300 RPM – Plastic Viscosity
- Obtain a recently agitated mud sample from each of mud tanks (1) and
- Using the Baroid Rheometer, obtain dial readings at 3, 300 and 600
- By means of the rheological calculations procedure, determine the
Apparent and Plastic Viscosities, Yield Point and initial 10 sec. and final
10-minute Gel Strength parameters.