Fracture Fluid Viscosity & Power-Law Rheology Calculator

Q: Why does API RP 39 specify shear rates of 170 s⁻¹ and 511 s⁻¹ for testing?

Standard Fann 35 and 50 viscometers spin at 100 rpm (170 s⁻¹) and 300 rpm (511 s⁻¹). A shear rate of 170 s⁻¹ closely simulates the average shear experienced by a crosslinked fluid slowly moving through an open hydraulic fracture.

Q: What is the role of chemical breakers in crosslinked gels?

After pumping terminates, encapsulating enzyme or oxidative breakers (such as ammonium persulfate) break the polymer backbone and hydrolyze borate/zirconate crosslinks, reducing gel viscosity back down to near water (1–2 cP) so the fluid flows back cleanly without plugging proppant pack permeability.

Model shear-thinning non-Newtonian behavior of linear guar, crosslinked borate gels, and viscoelastic surfactants using the Ostwald-de Waele power-law equation.

Power-Law Rheological Parameters

Fluid System Type:

Nominal Shear Rate γ̇ (s⁻¹): API RP 39 standard = 170 s⁻¹ or 511 s⁻¹

Flow Behavior Index n' (dimensionless): n' < 1 indicates pseudoplastic shear-thinning

Consistency Index K' (lbf·sⁿ/ft²):

Bottomhole Reservoir Temp (°F):

Elapsed Exposure Time (min): Breaker & thermal degradation

Apparent Viscosity & Rheology Output

Apparent Viscosity μ_app (at γ̇):

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Shear Stress τ (lbf/ft²):

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Low-Shear Viscosity (at 10 s⁻¹):

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High-Shear Viscosity (at 511 s⁻¹):

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Thermal Degradation Factor:

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Proppant Suspension Rating:

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Frequently Asked Questions

Why does API RP 39 specify shear rates of 170 s⁻¹ and 511 s⁻¹ for testing?

Standard Fann 35 and 50 viscometers spin at 100 rpm (170 s⁻¹) and 300 rpm (511 s⁻¹). A shear rate of 170 s⁻¹ closely simulates the average shear experienced by a crosslinked fluid slowly moving through an open hydraulic fracture.

What is the role of chemical breakers in crosslinked gels?

After pumping terminates, encapsulating enzyme or oxidative breakers (such as ammonium persulfate) break the polymer backbone and hydrolyze borate/zirconate crosslinks, reducing gel viscosity back down to near water (1–2 cP) so the fluid flows back cleanly without plugging proppant pack permeability.