Plastic Viscosity
Plastic viscosity (PV) in drilling fluid engineering is one of the two primary parameters of the Bingham plastic rheological model that describes the flow behavior of drilling mud — specifically the slope of the shear stress versus shear rate relationship above the yield point, expressed in centipoise (cP) and measured by the difference between the 600-rpm and 300-rpm Fann viscometer dial readings (PV = θ600 - θ300); plastic viscosity represents the contribution of fluid viscosity to overall drilling mud flow resistance, arising from the physical friction between solid particles suspended in the mud, the friction between solid particles and the continuous fluid phase, and the internal friction of the liquid phase itself, making PV the primary drilling fluid property that is increased by adding viscosifying solids and decreased by dilution or by mechanically removing fine solids through solids control equipment.
Key Takeaways
- Bingham plastic model describes drilling mud flow with two parameters: plastic viscosity (PV, the slope of the linear shear stress-rate relationship above the yield stress) and yield point (YP, the extrapolated shear stress at zero flow rate — the force required to initiate mud circulation); PV and YP are calculated from Fann viscometer readings as PV = θ600 - θ300 and YP = θ300 - PV (in lbs/100 ft2), where θ600 and θ300 are the dial readings at 600 and 300 RPM respectively; the Bingham model is a simplification of real drilling mud behavior (which typically shows shear thinning at low shear rates and non-Newtonian effects throughout the shear range), but PV and YP derived from the Bingham model remain the industry standard drilling mud parameters because of their direct relationship to the Fann viscometer readings that are measured routinely at the rig site.
- Elevated plastic viscosity from excess solids reduces drilling performance by increasing equivalent circulating density (ECD), increasing pump pressure requirements, reducing bit hydraulics efficiency, and increasing annular pressure losses that can fracture weak formations; PV increases when the concentration of fine solids (drilled cuttings smaller than shaker screen size, hydrated clay particles, weighting material aggregates, and barite fines) accumulates in the active mud system; an acceptable PV for most mud systems is typically in the range of 10 to 25 cP for moderate-weight mud (10 to 13 ppg) — higher than this range usually indicates excess solids content that is degrading mud performance and requiring dilution to control; PV increases of more than 5 cP between consecutive mud tests (measured 8 to 12 hours apart during active drilling) indicate that solids generation is exceeding solids control removal rate and the mud requires dilution treatment.
- Plastic viscosity control in field operations uses three approaches: mechanical solids removal through shale shakers, hydrocyclones, and centrifuges to physically separate fine solids from the active mud before they build up to unacceptable concentrations; chemical flocculation to aggregate very fine clay particles into larger particles that can be removed by centrifuge or allowed to settle in the active pits; and dilution with base fluid (water for WBM, base oil for OBM) to reduce the solids fraction in the mud — dilution is effective but expensive and increases the total mud volume requiring maintenance; the most cost-effective PV control strategy combines aggressive mechanical solids control (running all available shakers, hydrocyclones, and centrifuges at maximum capacity) with targeted dilution as a backup when solids control alone cannot maintain PV within target range.
- Temperature effect on plastic viscosity causes it to decrease as mud temperature increases — most drilling fluid components become less viscous with increasing temperature following approximately an Arrhenius temperature dependence — with PV measured at surface (typically 20 to 25°C) being significantly higher than the downhole PV at bottom-hole circulating temperature (which may be 60 to 150°C in deep wells); this temperature reduction of PV means that surface PV measurements overestimate the actual annular pressure loss (which is governed by the downhole viscosity) and can cause the engineer to over-treat the mud with thinners at surface when the downhole rheology is already acceptable; high-temperature viscometers (HTHP viscometers operating at up to 200°C and 500 psi) measure mud properties at simulated downhole conditions and provide more accurate input for annular hydraulics calculations in deep HPHT wells.
- Weighted mud PV management requires maintaining the PV within a tighter range than unweighted mud because barite additions (required to increase mud density) add high-density fines that increase PV, while the high-viscosity mud needed to suspend the barite also has elevated PV from viscosifier additions; the optimum PV for a weighted mud balances the need for adequate barite suspension (which requires minimum PV above approximately 15 to 20 cP for barite density 4.2 g/cc) against the ECD and pump pressure penalties of excessive PV; the Yield Point to Plastic Viscosity ratio (YP/PV) is the parameter that determines whether a drilling fluid achieves adequate cutting suspension and transport at acceptable circulating pressure — high YP relative to PV creates a shear-thinning profile that provides better suspension and transport efficiency per unit of flow resistance than an equivalent Newtonian fluid of the same PV.
Fast Facts
The Fann viscometer (model 35 and its variants), which provides the measurements used to calculate plastic viscosity in field mud engineering, was developed by engineers at Fann Instrument Company in the 1950s and became the universal standard for rotational viscometer testing of drilling fluids through its adoption in API Recommended Practice 13B-1 (for water-based mud) and 13B-2 (for oil-based mud). The Bingham plastic model used to interpret Fann viscometer readings into PV and YP was introduced to drilling fluid engineering by Bingham in 1922 and adapted for drilling mud applications by Marsh and collaborators in the 1930s, providing the two-parameter rheological model that remains in daily use on drilling rigs worldwide despite the availability of more sophisticated multi-parameter models (Herschel-Bulkley, Robertson-Stiff) that better represent the full non-Newtonian behavior of modern polymer-treated drilling muds.
What Is Plastic Viscosity?
Drilling mud does not flow like water. Water at a given temperature has a single, fixed viscosity that describes its flow resistance completely. Drilling mud, loaded with clay, barite, polymers, and drill cuttings, behaves as what engineers call a Bingham plastic — it requires a minimum force (the yield point) to start flowing at all, and once flowing, its resistance increases with flow rate at a rate described by the plastic viscosity.
The plastic viscosity captures everything about the mud that resists flow because of solid particles and fluid molecules rubbing against each other. More solids in the mud means higher PV. Heavier solids (denser barite) means higher PV. Finer particles that have a larger surface area per unit mass mean higher PV. The relationship is direct and unavoidable — any addition of solid material to the mud increases its plastic viscosity.
For the drilling engineer, plastic viscosity is both a diagnostic tool and a design parameter. A rising PV during a drilling interval tells you that fine solids are accumulating in the mud faster than the solids control equipment can remove them. A PV that is too high relative to the mud weight tells you the mud has too many colloidal fines that are degrading performance. A PV that is too low for a weighted mud tells you the barite may be poorly suspended and at risk of settling. Knowing how to interpret PV measurements and how to adjust them through solids control, dilution, and chemical treatment is fundamental knowledge for any drilling engineer responsible for mud performance.
Plastic Viscosity in Hydraulics and Solids Control
Annular pressure loss calculation using the Bingham plastic model requires PV and YP as inputs to calculate the frictional pressure gradient in the annulus — the Bourgoyne and Young method or the API-recommended Bingham method computes annular pressure loss as a function of annular flow velocity, mud density, PV, YP, and annular geometry; the resulting annular pressure loss added to the hydrostatic mud column pressure gives the equivalent circulating density (ECD) that the formation sees during active drilling; ECD must remain below the fracture gradient at all depths in the open hole below the last casing shoe to prevent lost circulation, making PV control a wellbore stability parameter as well as a pump efficiency parameter.
Solids loading calculation from PV and density measurements provides a rapid field estimate of the total solids volume fraction in the active mud — the retort analysis (measuring oil, water, and solids volumes in a standard mud sample by evaporation) provides the precise solids volume fraction, but PV combined with the total mud density can estimate the solids fraction using the empirical relationship PV = f(total solids fraction, solids particle size, viscous fluid properties); tracking the trend of PV versus mud density over a drilling interval allows the mud engineer to identify when solids fraction is increasing beyond acceptable limits before the retort result is available, enabling preemptive dilution to prevent PV from reaching the point where pump pressure or ECD constraints are violated.
Plastic Viscosity Across International Jurisdictions
Canada (AER / WCSB): WCSB drilling fluid programs specify PV targets and acceptable ranges for each section of each well as part of the mud program submitted to AER before drilling commences — typical WCSB water-based mud programs target PV in the range of 12 to 20 cP for vertical and build sections and 15 to 25 cP for horizontal lateral sections where higher ECD from narrower annular geometry makes PV control more critical; AER Directive 008 requires that mud properties including PV be recorded in the daily drilling report submitted to AER for all regulated wells, and the WCSB mud property records provide the historical dataset used by operators to benchmark mud performance between wells drilled in the same play or formation by different contractors.
United States (API / BSEE): API RP 13B-1 (Standard Procedure for Field Testing Water-Based Drilling Fluids) and RP 13B-2 (Standard Procedure for Field Testing Oil-Based Drilling Fluids) specify the standard Fann viscometer procedure for PV measurement used by all US drilling operations, with BSEE referencing API standards in its offshore well design regulations under 30 CFR Part 250 for GoM wells where the mud program is reviewed as part of the drilling permit; US drilling contractors and mud companies use the API PV measurement protocol in their daily mud reports that are archived in the operator's well files and available to BSEE inspection for GoM wells; the industry association the International Association of Drilling Contractors (IADC) includes PV measurement and interpretation in its Well Control training programs as part of the standard mud engineering curriculum.
Norway (Sodir / NORSOK): NCS drilling mud programs for Equinor, Aker BP, and other NCS operators specify PV requirements in the well program submitted to Sodir prior to each well's spudding, with NORSOK D-010 providing the minimum well integrity standards that include requirements for mud property monitoring including PV as a key indicator of mud quality; NCS drilling operations teams typically perform Fann viscometer measurements every four to six hours during active drilling to track PV trends and identify when solids accumulation requires treatment or dilution; Sodir's NCS drilling permit review includes assessment of whether the mud program's specified PV range and solids control equipment plan are adequate to manage the expected solids loading from the drilling interval being planned.
Middle East (Saudi Aramco): Saudi Aramco specifies PV targets and monitoring requirements in its drilling engineering standards for all well categories drilled in Saudi Arabia, with PV measurement frequency (every 4 hours minimum during active drilling through reservoir sections) and acceptable ranges (typically 8 to 18 cP for Arab Formation water-based mud programs) specified in Aramco's in-house drilling standards that supplement the API RP 13B guidance for Aramco-specific formation conditions; Aramco's large-volume Arab Formation development drilling program manages mud PV across fleets of rigs drilling simultaneously, with centralized mud engineering teams monitoring PV trends from multiple wells and coordinating solids control equipment maintenance schedules to maintain mud system performance across all active drilling programs in a given field area.