Skip to content
← All procedures
rheology

Rheology (Viscometer)

Purpose

Determine the flow properties of drilling fluids using a rotational viscometer (e.g. Fann VG Meter). Results describe how the fluid behaves under shear, which governs hole-cleaning, ECD, and surge/swab pressures.

Equipment

  • Fann VG Meter or equivalent coaxial-cylinder rotational viscometer
  • Thermometer
  • Sample cup

Procedure

  1. Place a freshly collected mud sample in the viscometer cup to the scribed fill line.
  2. Immerse the rotor sleeve and bob assembly in the sample.
  3. Turn on the instrument at 600 rpm and allow the dial reading to stabilise.
  4. Record the 600 rpm dial reading.
  5. Reduce speed to 300 rpm, allow to stabilise, and record the reading.
  6. Continue reducing speed and record readings at 200, 100, 6, and 3 rpm.
  7. 10-second gel: set to 3 rpm for 10 seconds, then stop the motor. Wait exactly 10 seconds, then briefly restart and record the maximum deflection.
  8. 10-minute gel: repeat the gel step after the sample has been undisturbed for 10 minutes.

Calculations

  • Plastic Viscosity (PV): PV = R₆₀₀ − R₃₀₀ (cP)
  • Yield Point (YP): YP = R₃₀₀ − PV (lb/100 ft²)
  • Apparent Viscosity (AV): AV = R₆₀₀ ÷ 2 (cP)

Notes

  • Report all individual readings (600, 300, 200, 100, 6, 3 rpm), PV, YP, AV, 10-sec gel, and 10-min gel.
  • Record sample temperature; rheology is temperature-sensitive.
  • For OBM, ensure the sample is conditioned at the specified test temperature before reading.