drilling · mud properties
Plastic Viscosity & Yield Point (Bingham Plastic)
PV = θ600 − θ300; YP = θ300 − PV
click formula to derive ↑
Inputs
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Description
Computes plastic viscosity (PV) and yield point (YP) from a two-speed (600/300 RPM) rotational viscometer test using the Bingham plastic rheological model. PV and YP are the two most routine daily mud-property measurements, governing hydraulics, hole cleaning, and surge/swab behavior.
Variables
| Symbol | Unit | Description |
|---|---|---|
| PV | cP | Plastic Viscosity |
| YP | lbf/100ft² | Yield Point |
| θ600 | — | Fann (or equivalent rotational) viscometer dial reading at 600 RPM. |
| θ300 | — | Fann (or equivalent rotational) viscometer dial reading at 300 RPM. Must be less than the 600 RPM reading. |
Assumptions
- Mud rheology is reasonably approximated by the Bingham plastic model — highly shear-thinning muds are better described by a power-law or Herschel-Bulkley model
- Viscometer readings are taken at stable, repeated RPM settings per API RP 13B-1 procedure
- Readings are taken at a controlled, representative temperature (rheology is temperature-sensitive)
Limitations
- Bingham plastic PV/YP systematically overstates true yield stress and understates low-shear-rate viscosity compared to power-law or Herschel-Bulkley models — increasingly significant for modern low-solids and synthetic-based muds
- A two-point (600/300) fit ignores curvature visible in a full 6-speed viscometer sweep
- Does not capture time-dependent (thixotropic) gel-strength behavior — see a separate gel strength measurement for that property
Use Cases
- → Daily mud check / mud report: Compute PV and YP from routine rig-site viscometer readings as part of the standard daily mud report.
- → Hydraulics program input: Feed PV and YP into pressure-loss and ECD calculations that use the Bingham plastic rheological model.
- → Hole-cleaning and barite sag screening: Track YP trends to assess suspension capability for cuttings transport and to flag barite sag risk in low-YP, high-density muds.
Related Calculations
Region Notes
Permian Basin
Slickwater and hybrid fluid systems used in lateral drilling often run very low PV (5-15 cP) and YP (2-8 lbf/100ft²) compared to conventional water-based mud, requiring careful hole-cleaning monitoring.
Gulf of Mexico
Deepwater synthetic-based muds are typically formulated for YP in the 8-20 lbf/100ft² range to maintain barite suspension under the low shear rates common in large-diameter risers.
Global
API RP 13B-1 specifies the standard field test procedure for rotational viscometer readings; always confirm test temperature is reported alongside PV/YP, since both increase markedly at lower temperatures.
References
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