Shear Rate
Shear rate is the velocity gradient of a fluid flowing between two surfaces, expressed mathematically as dv/dy (the change in velocity with respect to distance perpendicular to flow) and measured in reciprocal seconds (s⁻¹); in drilling fluid rheology it describes how rapidly fluid layers slide past one another, governing viscosity behaviour across the range of flow conditions encountered from the bit nozzles to the annulus.
Key Takeaways
- Fann VG meter readings at 600, 300, 200, 100, 6, and 3 rpm correspond to shear rates of approximately 1022, 511, 341, 170, 10.2, and 5.1 s⁻¹ respectively, spanning the practical range from bit nozzle flow to low-circulation gel states.
- High shear rate viscosity (HSV), read at 600 rpm, governs pressure losses in the drill string and at the bit; low shear rate viscosity (LSV), read at 6 and 3 rpm, controls cuttings suspension and gel strength development during pipe trips.
- The relationship between shear rate and shear stress defines the fluid rheological model: Bingham plastic, power law, or Herschel-Bulkley; each model uses different shear rate data points for curve fitting.
- Surge and swab pressure calculations during tripping are highly sensitive to low shear rate behaviour; inaccurate low-shear viscosity data leads to underestimating swab-induced formation influxes in narrow-margin wells.
- Drilling fluid engineering targets a flat rheological profile across the shear rate spectrum to balance annular hole cleaning at high shear rates with cuttings suspension capacity at low shear rates.
Fast Facts
The conversion from Fann VG meter RPM to shear rate uses the factor 1.7023 s⁻¹ per RPM (based on the standard R1 rotor and B1 bob geometry). A 600 rpm reading therefore equals 600 x 1.7023 = 1,021.4 s⁻¹. This factor applies to all standard Fann 35 viscometers and equivalent instruments used worldwide in oilfield drilling fluid laboratories.
Tip: When comparing shear rate data between laboratories, always confirm the viscometer geometry (rotor/bob/sleeve combination) and the RPM-to-shear-rate conversion factor used, as non-standard geometries alter the conversion constant and can produce misleading rheology comparisons.
What Is Shear Rate
Shear rate quantifies how fast adjacent fluid layers are moving relative to one another within a flowing system. Imagine two parallel plates separated by a thin fluid film: if the upper plate moves at velocity v while the lower plate remains stationary, the velocity changes linearly from v at the top to zero at the bottom across distance y. The shear rate is the ratio v/y, expressed in s⁻¹. In real wellbore geometry, shear rate varies continuously across the annular gap and through the drill string, reaching maximum values at the pipe wall and zero at the flow centreline for Newtonian fluids.
For non-Newtonian fluids like drilling muds, viscosity changes with shear rate. This shear-thinning or thickening behaviour is why a single viscosity number is insufficient to characterize drilling fluid performance. A mud that is adequately viscous at the low shear rates of the annulus may become excessively thick under the high shear rates at the bit, generating unacceptable circulating pressure losses.
How Shear Rate Works
The Fann VG meter rotates a sleeve around a stationary bob suspended in the drilling fluid. The torque required to maintain rotation at each speed is read from a spring dial and converted to shear stress in lb/100 ft². Shear rate is fixed by the rotational speed and the geometry. The six standard speeds (600, 300, 200, 100, 6, 3 rpm) provide data across three orders of magnitude of shear rate, capturing both high-energy flow zones and near-static conditions.
Bingham plastic model uses the 600 and 300 rpm readings to compute plastic viscosity (PV = theta600 minus theta300, in cP) and yield point (YP = theta300 minus PV, in lb/100 ft²). The power law model uses the 600 and 300 rpm readings differently, computing the flow behaviour index n and the consistency index K. The Herschel-Bulkley model incorporates yield stress plus power law behaviour and requires additional low-shear readings at 6 and 3 rpm for accurate fitting.
In the annulus, average shear rate can be estimated from the annular velocity and the hydraulic diameter. At typical pumping rates, annular shear rates range from 50 to 200 s⁻¹ in the drillpipe-to-casing annulus and 100 to 400 s⁻¹ in the drill collar section. Bit nozzle shear rates reach 10,000 to 50,000 s⁻¹. Understanding these zones allows mud engineers to design fluid rheology that performs across all three environments simultaneously.
Low shear rate viscosity (LSRV), sometimes measured at 0.06 and 0.09 rpm using a six-speed Fann meter or a Brookfield viscometer, characterizes the fluid's ability to suspend barite sag in deviated wellbores. Barite sag, the gravitational settling of weighting material in static or slow-circulation conditions, is a direct consequence of insufficient low-shear viscosity and can cause wellbore control problems by creating density windows between the light and heavy mud columns.
Shear Rate Across International Jurisdictions
In the Western Canada Sedimentary Basin, AER and BC OGC regulated drilling programs routinely specify minimum and maximum rheology bands for both WBM and OBM systems used in Montney, Duvernay, and deep Triassic formations. WCSB mud engineers commonly run full six-speed Fann measurements twice daily on active drilling wells. Canadian Association of Drilling Contractors (CAOEC) well control standards require accurate rheology data for equivalent circulating density (ECD) modeling, which depends directly on proper shear rate characterisation across annular flow regimes.
In the United States, the American Petroleum Institute Recommended Practice 13B-1 (water-based fluids) and 13B-2 (oil-based fluids) standardise the Fann VG meter procedure and specify which RPM speeds must be measured and reported. BSEE regulations for Gulf of Mexico deepwater drilling require operators to maintain fluid rheology data as part of the well control documentation package. Narrow pressure window deepwater wells in the Gulf are especially sensitive to ECD control, where shear rate characterisation drives hydraulic program design.
On the Norwegian Continental Shelf, Sodir (Norwegian Offshore Directorate) references NORSOK D-010 well integrity standards, which require comprehensive rheological characterisation of all drilling fluids used in Norwegian waters. Norwegian operators Equinor, Aker BP, and Vaar Energi routinely apply Herschel-Bulkley models rather than Bingham plastic, because the additional low-shear parameters improve surge/swab accuracy in HP/HT wells on the Barents Sea shelf where temperature and pressure effects on viscosity are pronounced.
In the Middle East, Saudi Aramco drilling standards specify comprehensive rheology monitoring as part of mud reporting requirements on all Ghawar, Khurais, and Shaybah field operations. The extreme HP/HT conditions of Arab-D carbonate reservoirs, with bottomhole temperatures exceeding 300 degrees Fahrenheit and pressures above 10,000 psi, require shear rate measurements at elevated temperatures using pressurised HTHP viscometers rather than standard Fann VG meters at surface conditions, since temperature significantly reduces apparent viscosity at all shear rates.
Synonyms and Related Terminology
Shear rate is also referred to as the velocity gradient in fluid mechanics literature. Related rheological terms include shear stress, plastic viscosity, yield point, gel strength, and Herschel-Bulkley model. The ratio of shear stress to shear rate defines apparent viscosity at any given shear rate. Low shear rate viscosity is sometimes abbreviated LSRV. The Fann VG meter is also called a rotational viscometer or concentric cylinder viscometer.
FAQ
Q: Why do high shear rate viscosities matter for pump pressure but low shear rate viscosities matter for hole cleaning?
A: Drill cuttings move with the fluid at annular shear rates of 50 to 200 s⁻¹, where viscosity governs the drag force lifting cuttings toward surface. However, pressure losses in the drill string and bit nozzles occur at much higher shear rates (hundreds to tens of thousands of s⁻¹), where fluid viscosity controls friction losses and pump pressure requirements. A shear-thinning fluid keeps high-shear pressure losses manageable while maintaining sufficient low-shear viscosity to suspend cuttings in slower annular flow.
Q: What shear rate corresponds to the static condition when the pumps are off?
A: Truly static conditions approach zero shear rate, which is below the measurement range of standard Fann meters. Gel strength measurements at 10 seconds and 10 minutes approximate the initial and progressive gel structure at near-zero shear rate. LSRV measurements at 0.06 to 0.09 rpm (0.1 to 0.15 s⁻¹) provide the closest practical proxy for the static settling environment relevant to barite sag modeling.
Why Shear Rate Matters
Shear rate is the fundamental parameter linking fluid rheology to wellbore hydraulics. Every hydraulic calculation performed during a drilling program, including ECD, surge and swab, cuttings transport efficiency, and cementing displacement, relies on accurate shear rate characterisation of the drilling fluid. Errors in rheological data propagate directly into wellbore pressure predictions and, in narrow-window wells, into wellbore integrity risk. Conversely, well-characterised shear rate profiles allow mud engineers to optimise fluid composition, pump rates, and trip speeds to maximise drilling efficiency while maintaining well control margins throughout the entire wellbore depth.