Plastic viscosity (PV) and yield point (YP) come from the 600 and 300 r/min viscometer dials: PV = R600 - R300 in cP and YP = R300 - PV in lbf/100 ft2. In a drilling fluid, PV tracks solids and base-liquid viscosity, while YP tracks the attraction between clay particles.
The same two readings give apparent viscosity, and the 6 and 3 r/min readings add the low-shear yield point. The dial route to pascals gives 6.7 % more than converting the reported lbf/100 ft2 value.
A worked example gives PV 16 cP and YP 14 lbf/100 ft2 (6.70 Pa), published muds run from PV 4 to 30 cP, and four out-of-range signals separate dilution from chemical treatment.
What Are Plastic Viscosity and Yield Point?#
Plastic viscosity is the slope of a drilling fluid's Bingham shear-stress line, and yield point is that line's stress intercept at zero shear rate, the stress the fluid needs before it flows. PV is the viscosity extrapolated to infinite shear rate.
PV follows the concentration of solids and the viscosity of the base liquid, and it represents the shear at the bit and in the drill string (IADC Drilling Manual). YP follows the electrochemical attraction between clay particles, so it rises with flocculation.
Apparent viscosity and the Bingham rheology model#
Apparent viscosity is half the 600 r/min reading, in cP, and describes the mud at 1,022 s-1 only. The Bingham model is one of three field models, with power law and Herschel-Bulkley (Jachnik, 2003), and the three are compared under drilling fluid rheology models. Apparent viscosity equals PV plus half of YP.
How to Calculate PV, YP and Apparent Viscosity from Viscometer Readings#
PV, YP and apparent viscosity follow from two dial readings: R600 minus R300 gives PV, R300 minus PV gives YP, and half of R600 gives apparent viscosity. The five formulas are listed below.
- PV (cP) = R600 - R300
- YP (lbf/100 ft2) = R300 - PV
- YP (Pa) = 0.511 x (R300 - PV)
- Apparent viscosity (cP) = R600 / 2
- LSYP (lbf/100 ft2) = 2 x R3 - R6, with LSYP set equal to R6 if R3 exceeds R6
R600, R300, R6 and R3 are the dial readings at 600, 300, 6 and 3 r/min. YP also equals 2 x R300 - R600, and R600 - R300 is PV, never YP.
Units of PV and YP: cP, lbf/100 ft2 and pascals#
PV is reported in cP, numerically equal to mPa.s, and YP in lbf/100 ft2, because one dial degree equals 1.067 lbf/100 ft2 (0.511 Pa) and the field reads the dial directly as lbf/100 ft2.
The API SI form of YP applies the dial factor of 0.511, while converting a reported lbf/100 ft2 value uses 0.4788 Pa per lbf/100 ft2 (NIST SP 811), so a pascal value names its route.
The table lists the units, factors and shear rates used on this page.
| Quantity | Oilfield unit (SI) | Factor | Note |
|---|---|---|---|
| PV and apparent viscosity | cP (mPa.s) | 1 cP = 1 mPa.s | exact |
| YP and LSYP | lbf/100 ft2 (Pa) | 1 lbf/100 ft2 = 0.4788 Pa | reported-value route |
| Dial reading | 1 degree | 1.067 lbf/100 ft2 = 0.511 Pa | API SI route |
| Route difference | ratio | 0.511 / 0.4788 = 1.067 | 6.7 % more Pa by the dial route (calculated) |
| Rotor speed | r/min to s-1 | x 1.7023 | 600 = 1,022; 300 = 511; 6 = 10.2; 3 = 5.1 s-1 |
Worked example from one set of dial readings#
An illustrative water-based mud at 120 degF (48.9 degC) reading 46, 30, 7 and 6 at 600, 300, 6 and 3 r/min has PV 16 cP, YP 14 lbf/100 ft2 (6.70 Pa) and apparent viscosity 23 cP.
The table gives each result, its calculation and its SI value.
| Result | Calculation | Value | SI |
|---|---|---|---|
| PV | 46 - 30 | 16 cP | 16 mPa.s |
| YP | 30 - 16 (check: 2 x 30 - 46) | 14 lbf/100 ft2 | 6.70 Pa (x 0.4788) |
| YP, API SI route | 0.511 x 14 | 7.15 Pa | |
| Apparent viscosity | 46 / 2 | 23 cP | 23 mPa.s |
| LSYP | 2 x 6 - 7 | 5 lbf/100 ft2 | 2.39 Pa |
| LSYP / YP | 5 / 14 | 0.36 | |
| YP / PV | 14 / 16 | 0.88 (lbf/100 ft2 per cP) | |
| Bingham reading at 3 r/min | 14 + 16 x 3 / 300 | 14.2, against 6 read |
The LSYP is 0.36 of YP, near the 0.30 average for water-based muds, and the Bingham line overstates the 3 r/min reading. Any other reading set runs through the rheology calculator for PV, YP and LSYP from viscometer readings.
How Are PV and YP Measured on a Viscometer?#
PV and YP are measured on a rotational viscometer that shears the mud at 1,022 s-1 (600 r/min) and 511 s-1 (300 r/min), at 120 degF (48.9 degC) for most wells (IADC Drilling Manual). High-temperature wells report at 150 degF (65.6 degC).
Water-based muds are tested to API RP 13B-1 (6th edition, January 2026) and nonaqueous muds to API RP 13B-2 (6th edition, July 2023, with Addendum 1 of April 2025). API RP 13D (8th edition, April 2026) covers the rheology and hydraulics calculations.
The Marsh funnel gives one flow time, not PV or YP. The full procedure is the viscometer test at 600 and 300 r/min.
Low-Shear Yield Point and True Yield Stress#
The low-shear yield point, LSYP = 2 x R3 - R6 in lbf/100 ft2, estimates the stress needed to start flow better than the Bingham YP, which overstates the true yield stress of most drilling fluids (IADC Drilling Manual).
LSYP is the most suitable estimate of yield stress from standard viscometer data, and vane measurements averaged 0.94 of LSYP in synthetic-based and 1.09 in water-based muds, with oil-based results inconclusive (AADE-03-NTCE-35, 2003).
Curve-fit yield stress averaged 0.57 of Bingham YP in synthetic-based mud, 0.50 in oil-based mud and 0.30 in water-based mud, inside an accepted range of 0 to 1 (same paper). The Herschel-Bulkley field method cited from API RP 13D, 5th edition (2006), takes yield stress as 2 x R3 - R6, the same form as LSYP.
Typical PV and YP Values by Fluid System#
PV and YP targets come from each well's mud program rather than a standard, and the published examples in the table span PV 4 to 30 cP and YP 10 to 20 lbf/100 ft2 (4.8 to 9.6 Pa).
The table compares sourced PV and YP values by fluid or application.
| Fluid or application | PV, cP (mPa.s) | YP, lbf/100 ft2 (Pa) | Condition | Source |
|---|---|---|---|---|
| 40 CFR 435 reference mud: C16-C18 internal olefin synthetic-based | 12 to 30 | 10 to 20 (4.8 to 9.6) | reference mud of 9.0, 11.5 or 14.5 lb/gal (1.08, 1.38, 1.74 sg) for discharged mud of 8.5 to 11, above 11 to 14 and above 14 lb/gal | 40 CFR 435, Appendix 8 |
| Polymer drilling fluids: low-solids PHPA, 9 lb/gal (1.08 sg) | 4 to 6 | 10 to 14 (4.8 to 6.7) | handbook range | Amoco Drilling Fluids Manual, 1994 |
| Low-solids PHPA, 12 lb/gal (1.44 sg) | 8 to 10 | 12 to 20 (5.7 to 9.6) | handbook range | Amoco Drilling Fluids Manual, 1994 |
| Mineral exploration core drilling | YP only | 10 to 20 (4.8 to 9.6) | target | Bulletin of the Mineral Research and Exploration |
| HPHT drilling fluids: 16 lb/gal (1.92 sg) | below 30 | below 15 (7.2) | tested at 425 degF (218.3 degC) | AADE-14-FTCE-44 |
Across 48,310 mud reports, YP ran higher at lower mud weights and lower at higher mud weights (AADE-03-NTCE-35, 2003).
What Raises and Lowers PV and YP: Viscosifiers, Thinners and Solids#
PV and YP respond to different causes: solids and base-liquid viscosity move PV, while clay flocculation, viscosifiers and thinners move YP. Bentonite and xanthan gum, two of the drilling fluid viscosifiers, raise YP and low-shear readings, and mixed metal hydroxide gives high YP with low PV.
The table compares the qualitative effect of each additive class or cause; doses sit on each material page.
| Additive class or cause | What it moves | Effect | Mechanism |
|---|---|---|---|
| Bentonite | YP | raises | edge-to-face association of dispersed platelets |
| Xanthan gum | low-shear readings | raises | rigid anionic chains form a weak network |
| Mixed metal hydroxide | YP | high YP and fragile gels, low PV | cationic particles bridge bentonite platelets; incompatible with anionic deflocculants |
| Lime as flocculant | YP | raises | flocculates dispersed clay |
| Thinners and deflocculants | YP | lowers in clay-based mud | neutralize positive clay edges |
| Weighting agents and drilled solids | PV | raises | solids concentration |
| Dilution and removal of solids in drilling fluids | PV | lowers | fewer solids per volume |
| Carbonate drilling fluid contamination | YP | raises, with progressive gels | deflocculants have little effect; lime or gypsum treats it |
Solids, dilution and base-fluid viscosity#
PV rises with drilled and added solids, such as low-gravity solids and weighting material, and with a viscous base fluid, and it falls with dilution and solids removal. Cold raises the viscosity of water-based and oil-based mud exponentially (Offshore magazine), and pressure matters in oil-based fluids but is negligible in water-based fluids (API RP 13D, 2003 edition).
Clay flocculation, thinners and contamination#
YP rises when freshly dispersed clay or a flocculant such as lime links clay particles, and it falls when a deflocculant neutralizes the positive clay edges. Deflocculants include lignosulfonates, polyphosphates, tannins such as quebracho and low-molecular-weight polyacrylates, the drilling fluid thinners and deflocculants. Overtreatment in nonaqueous mud drops the 6 and 3 r/min readings. Carbonate contamination raises YP with progressive gels while deflocculants have little or no effect (AADE).
Problems When PV or YP Is Out of Range#
PV or YP out of range shows up as slower hole cleaning, higher circulating pressure or a contamination signal. Cuttings concentration rises as YP falls and as PV rises (Busahmin et al., OALib Journal, 2017).
The four signals and what they point to are listed below.
- High PV with low YP: more cuttings left in the hole, a hole cleaning and cuttings transport problem.
- High PV and YP: higher annular pressure loss, raising the equivalent circulating density.
- Low 6 and 3 r/min readings in weighted mud: weighting material can settle, as in barite sag.
- Rising YP with progressive gels and Mf rising faster than Pf: a carbonate contamination signal.
The mud program sets the band, and readings are compared at the same reporting temperature.
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Where PV and YP Sit Among Drilling Fluid Properties#
PV and YP belong to the rheology family of drilling fluid properties, next to density, filtration, solids and chemistry, and their low-shear end connects them to gel strength and suspension. The full set of drilling fluid properties places rheology beside the other four families.
The yield point of steel and other metals in tensile testing is a different property. The plastic viscosity of fresh concrete and cement paste is a separate field.
Gels and funnel viscosity next to PV and YP#
Gel strength is the maximum 3 r/min reading after 10 s and 10 min of rest, and Marsh funnel viscosity is one flow time in s/qt; neither replaces PV and YP. The four neighboring readings are listed below.
- Gel strength: the 10 s and 10 min readings at 3 r/min.
- Marsh funnel viscosity: 946 mL (1 US quart) of fresh water in 26 +/- 0.5 s.
- Low-shear-rate viscosity: the Brookfield measurement at the suspension end.
- Drilling fluid rheology: the three rheology models.