Rheology

Rheology is the study of the flow and deformation of matter, particularly fluids. In the context of drilling fluids (commonly referred to as “mud”), rheology is crucial because it influences the efficiency and safety of drilling operations. Drilling fluids are complex mixtures that can include water, oil, clay, polymers, and various additives. Their rheological properties determine how they behave under different conditions, such as varying temperatures, pressures, and shear rates.

What Rheology Tells Us About Drilling Fluids

1. Plastic Viscosity (PV): Indicates the fluid’s resistance to flow. High viscosity can improve cuttings transport but may increase pump pressure.
2. Yield Point (YP): The minimum stress required to initiate flow. Important for preventing sagging of weighting materials.
3. Gel Strength: Indicates the fluid’s ability to suspend cuttings when circulation stops.
4. Thixotropy: The property of becoming less viscous under stress and returning to a more viscous state when the stress is removed. Important for maintaining fluid stability.
5. Flow Behavior: Understanding whether the fluid is shear-thinning or shear-thickening helps in predicting its behavior under different drilling conditions.

Measurement of Rheology

Fann VG (viscosity-gel) Meter: A standard viscometer used in the industry. It can measure at multiple speeds (e.g., 3, 6, 100, 200, 300, and 600 RPM) to provide a range of shear rates.

In this instrument, the test fluid is container in the annular space (shear gas) between an outer cylinder and the bob (inner cylinder). Viscosity measurements are made when the out cylinder, rotating at a known velocity, causes a viscous drag exerted by the fluid.

Plastic Viscosity and Yield Point Calculation:

Test a sample at 600 rpm and record the dial reading. Change the speed to 300 rpm and record the dial reading.

Determine the PV and YP using the fllowing equation

PV (cp) = θ₆₀₀ – θ₃₀₀

YP (lb/100 ft²) = θ₃₀₀ – PV

Where θ is the dial reading.

Newtonian Viscosity Calculation.

The Newtonian viscosity in centipoise (cP) may be read directly from the dial when the viscometer speed is 300 rpm and the rotor-bob-torsion spring combination is R1-B1-F1. Other springs may be used provided that the dial reading is multiplied by the “f” factor (spring constant) to calculate the viscosity.

To calculate Newtonian viscosities in centipoise with the Fann viscometer, use the following equation

μ= S x θ x f x C

Where,

S is the speed factor

Θ is the dial reading

f is the spring factor

C is the rotor-bob factor

μ is the Newtonian viscosity (cP)

Example : Using and R2-B1 combination at a speed of 600 rpm with an F5.0 spring, and a dial deflects to 189, the viscosity is

μ = 0.5 x 189 x 5 x 0.315 = 149 cP