Low shear-rate viscometry is needed to predict drilling mud performance.

Many companies use the Fann viscometer to study the rheology of drilling muds. However, the Fann equipment is unable to make measurements at low shear rates, which must be characterized to predict suspension properties. With the advent of modern rheometers, it is easy to make low shear-rate measurements to assess the carrying capacity of drilling fluids, even when they are still.
Measuring geometry
In a recent study, all measurements on the drilling mud were made at 76°F (25°C) on a Bohlin CVO rheometer. The measuring system used was a coaxial cylinder, DIN C25, with a bob diameter of 25 mm. This geometry was selected because the drilling mud contained relatively large particles and was of low viscosity. The gap between the inner and outer cylinder for this coaxial DIN is significantly larger than the average particle size (about 50 microns). The intrinsic large surface area of the inner cylinder provides enhanced stress sensitivity for low-viscosity fluids, and this becomes more significant at ultralow shear rates.
A cone-and-plate measuring system would not have been suitable because the particles would jam in the gap, typically 50 to 150 microns. Gap size generally needs to be about 10 times the particle size. A parallel plate could have been used, but a large measuring gap would be needed to accommodate the particles, and there was the possibility that the sample would not have stayed in the gap due to its inherent low viscosity. Also, large gap parallel plates produce nonuniform shear rates across the measuring gap, which means absolute viscosity data cannot be obtained.
Unsheared yield stress
Yield stress, or the stress at which viscosity begins to decrease, was measured on an unsheared drilling fluid sample (Figure 1). The rise in viscosity below the yield stress is due to sample elasticity. The Bohlin CVO rheometer is able to measure this viscosity rise at low stress due to its ability to measure low rotational speeds. Mechanical bearing rheometers are unlikely to detect this region of the yield stress measurement. In the controlled stress mode, the rheometer directly measures yield stress, thus it can apply a stress and produce an effective zero shear. On the other hand, a dedicated, controlled-rate Fann viscometer calculates yield stress by extrapolating back to zero shear. In other words, it assumes a certain type of rheological behavior (e.g., Bingham), which may not be an accurate representation of the actual flow properties at low shear.
The yield stress values recorded were relatively low, emphasizing the need for sensitive measurements at low shear. The Bohlin CVO rheometer is able to do this as it has an ultralow mass air bearing that provides an almost inertia-free measurement.
The Bohlin yield stress program is unique - it defines the yield stress at the point where the sample actually flows, or where the viscosity starts to decrease. Some rheometers determine the yield stress as the first point of movement, but this is rarely an accurate representation of yield stress, as this first movement point is merely elastic deformation.
Sheared yield stress
The yield stress measurement was repeated, but this time the sample was presheared and rested prior to the measurement (Figure 2). The second measurement gave a lower value of yield stress, suggesting the sample had not recovered its structure after preshear.
Equilibrium flow curve
Figure 3 shows a plot of shear stress and viscosity as a function of shear rate. The Bohlin software produces an equilibrium flow curve by making a step stress measurement, greatly enhancing the quality of the viscosity data obtained at low shear rates.
The sample in Figure 3 appears to be shear thinning, especially at low shear rates. In one experiment that took less than 5 minutes to complete, nearly six orders of magnitude in shear rate were covered. The lowest shear rate was more than 1,000 times lower than the minimum shear obtainable on conventional viscometers. Moreover, it was more representative of the shear rates that the sample is subjected to when it is at rest in pipes or wells. The viscosity at these shear rates can be used to predict sedimentation and dispersion stability.
Creep and recovery curve
The low shear data can be further explored using a creep and recovery measurement (Figure 4). The creep (retardation) curve is defined as the deformation under constant stress, and the recovery is a measure of the elastic recoil (recoverable compliance) when the stress is at zero.
Shear stress of 0.05 Pa was applied during the creep test. Shear rate of 0.00059/sec was calculated from the slope of the creep curve. Viscosity was calculated to be shear stress divided by shear rate, or 84 Pa-sec (84,000 cp). The approach to viscous flow can be calculated as dln(J)/dln(t). This is a measure of the steady-state condition in the limiting linear region of the retardation curve. A value of 1.0 indicates 100% viscous flow, while a number below 1.0 indicates elasticity in the sample is retarding the viscous flow. In this case, dln(J)/dln(t) = 0.3244.
The measured recoverable compliance (Joc, elasticity) measured when the stress is turned off (and equal to 0) was measured to be 3.2/Pa, and the calculated recoverable compliance (Joc) was 1.4/Pa.
The creep and recovery curve in Figure 4 shows that the sample has a viscoelastic response at low shear stress. In other words, its flow is retarded by elasticity. The suspension properties of the drilling mud ultimately will depend on the magnitude of the viscosity from the creep data. In this case, the calculated viscosity is 84.0 Pa-sec (84,000 cp) at a measured shear rate of 0.00059/sec. Without having other samples to compare it to, it may be difficult to predict the effectiveness of this sample of drilling mud in terms of suspension. The important parameter is the shear rate, which is low, as would be expected for a deformation process at gravitational stress. The Bohlin rheometer is able to simulate such conditions due to its sensitive air bearing and its high-resolution position sensor.
Stress sweep oscillation
The measurement of stress sweep oscillation is made to find the value of critical strain where the modulus starts to break down. Critical strain is a measure of the linear viscoelastic region of the sample and can be related to the dispersion properties. Longer linear regions (higher values of critical strain) are indicative of a well-dispersed and stable system. In Figure 5, the drilling mud exhibits a linear behavior up to a strain of 0.1. The elastic/storage modulus (G') is used in the definition of the critical strain.
The Bohlin CVO rheometer has been shown to be a capable instrument for characterizing the rheological properties of drilling fluids. The ultralow shear rates and the yield stress test are of paramount importance for predicting suspension properties of drilling fluids.

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