Bohlin Instruments. Rheological measurements on drilling mud and a crude oil
Keywords: Bohlin Instruments, Drilling, Rheology, Crude oil
Introduction
The theological properties of crude oil are known to be very dependent on temperature. In order to predict the performance of the crude in "real life conditions", it is important to know the viscosity at high and low temperatures and also to know the temperature where waxing occurs.
The rheology of drilling muds is also of significance and to date, many companies have used the Fann viscometer. However, the Fann is unable to make measurements at very low shear rates which must be characterised in order to predict suspension properties. With the advent of modern rheometers like the Bohlin CVO, it is very easy to make these low shear rate measurements and be able to accurately predict the performance of a drilling mud.
This report examines some of the typical tests for measuring the rheological properties of drilling mud and crude oil. Actual data and interpretation of results are also attached.
Methodology
AII measurements on the drilling mud were made at 25°C on a Bohlin CVO rheometer. The measuring system used was a coaxial cylinder (DIN C25 - bob diameter of 25mm) measuring geometry. This type of geometry was selected as the drilling mud contained relatively large particles and was also of low viscosity. The gap between the inner and outer cylinder for coaxial DIN types is significantly larger (in this case) than the average particle size (~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 ultra low shear rates.
A cone & plate measuring system would not have been suitable because the particles would jam in the gap (gap on a cone & plate is typically 50 to 150 microns). It is generally accepted that the average particle size needs to be approximately 10 times the average particle size. A parallel plate could have been used, but a large measuring gap would be needed (to accommodate the particles) and there is the possibility that the sample would not have stayed in the gap, due to its inherent low viscosity. Also large gap parallel plates produce non-uniform shear rates across the measuring gap which means absolute viscosity data cannot be obtained.
Measurements on the crude oil sample were made over a broad temperature range also using the coaxial cylinder geometry.
A summary of the parameters used in the measurements are given below:
Drilling mud
Yield stress
Accelerated stress ramp 0,025 Pa to 5.0 Pa.
Experiment repeated with a preshear of 25 Pa for 30 seconds.
Viscometry
Step stress ramp 0.025 Pa to 25.0 Pa.
15 seconds delay time and 10 seconds integration time at each stress.
Creep and recovery
Creep Stress = 0.05 Pa for 120 seconds
Elastic Recovery measured over 120 seconds.
Stress sweep oscillation
Stress sweep 0.025 Pa to 5.0 Pa.
Oscillation frequency 1.0 Hz.
Crude oil
Oscillation
Temperature sweep 45°c to -10°c.
Ramp rate 2c per minute.
Strain = 0.001
Frequency = 0.1 Hz
Measurement interval = 30 seconds.
Results - drilling mud
The printouts from all the measurements are given below.
Figure 1. The first measurement shows a yield stress experiment on an unsheared sample. The yield stress can be defined in this case as the stress at where the viscosity begins to decrease. The rise in viscosity below the yield stress is due to sample elasticity (structure). The Bohlin CVO rheometer is able to measure this viscosity rise at low stress, due to it's ability to measure very low rotational speeds (NB: mechanical bearing rheometers are unlikely to detect this region of the yield stress measurement). It is important to understand that in the controlled stress mode, the rheometer directlymeasures yield stress as it can apply a stress and produce an effective zero shear. A dedicated controlled rate viscometer (e.g Fann) calculates yield stress by extrapolating back to zero shear. In other words, it assumes a certain type of rheological behaviour (e.g Bingham) which may not be an accurate representation of flow properties at low shear.
Figure 1 Drilling mud - yield stress
The yield stress values recorded (see below) are relatively low and it emphasises the need for sensitive measurements at low shear. The Bohlin CVO rheometer can do this as it has an ultra low mass air bearing which provides an almost inertia free measurement which is absolutely essential for these type of yield stress tests.
The Bohlin yield stress program is quite unique as it defines the yield stress at the stress where the sample actually flows i.e. where the viscosity starts to decrease. Some rheometers determine the yield stress as the first point of movement but this is not often an accurate representation of yield stress, as this first movement point is simply elastic deformation!
Figure 2. The yield stress measurement was repeated but this time the sample was presheared and then rested prior to the measurement.
Figure 2 Drilling mud - yield stress after shear
The repeat measurement gives a lower value of yield stress which suggests the sample has not recovered all it's structure after preshear.
Figure 3. The graph is 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 and this enhances particularly the quality of the viscosity data at low shear rate.
Figure 3 Drilling Mud - Viscornetry Profile
The data below shows the sample to be shear thinning especially at low shear rates. In one experiment that took less than five minutes to complete, nearly six orders of magnitude in shear rate have been covered. Note the low shear rates are below 10.3 s 1. This shear rate is over 1000 times lower than the minimum shear obtainable on conventional viscometers. Moreover it is more representative of the shear rates that the sample is subjected to when it is at 'rest' in pipes or drill ducts. The viscosity at these shear rates can be directly used to predict sedimentation and dispersion stability.
Figure 4. The low shear data can be further explored using a creep and recovery measurement. 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.
Figure 4
From the creep and recovery curve the following parameters were obtained:
Shear stress 5.0E-2 Pa Viscosity 8.4E+1 Pasdln(J)/dln(t) 0.3244 Shear rate 0.00059 s- 1Joc 3.2E+01/Pa Jor 1.4E+01/Pa
An explanation of each parameter is given below:
- 1.
Shear stress
- 2.
Shear rate
- 3.
Viscosity
- 4.
dlnJ/dlnt
- 5.
Jot 6. Joc
– applied shear stress during the creep test.– this is calculated from the slope of the creep curve.– this is found from 1. divided by 2.– this is a measure of the steady state condition in the limiting linear region of the retardation curve and denotes the 'approach'to viscous flow. A value of 1.0 indicates 100% viscous flow. A number below 1.0 indicates that elasticity in the sample is retarding the viscous flow.–this is the measured recoverable compliance (or elasticity) measured when the stress is turned off (i.e = zero).– is a calculated recoverable compliance.
0.0 10.0 20.0 30.0 40.0 t;.Ls 60.0 70.0 80.0 90.0 100]
The above creep and recovery curve shows the sample has a viscoelastic response at low shear stress, i.e. it's flow is retarded by elasticity. The suspension properties of the drilling mud will ultimately depend on the magnitude of the viscosity from the creep data. In this case the calculated viscosity is 84.0 Pas (84,000.00 cps) at a measured shear rate of 0.00059 s-1. Without having other samples to compare it with, it may be difficult to predict just how effective this sample drilling mud will be in suspension terms. The really important parameter is the shear rate which is very low, as would be expected for a deformation process at gravitational stress. The Bohlin rheometer is able to simulate such conditions due to it's sensitive air bearing and it's high resolution position sensor.
Figure 5. The curve below is a stress sweep oscillation. The measurement is made to find the value of critical strain, i.e. the strain where the modulus starts to breakdown. The value of the critical strain is a measure of the linear viscoelastic region of the sample and can be related to the dispersion properties. Longer linear regions (i.e. higher values of critical strain) are indicative of a well-dispersed and stable system. In this case the drilling mud exhibits a linear behaviour up to a strain of 0.1 (10%) - the G'(elastic/storage modulus) is used in the definition of the critical strain.
Figure 5 Drilling Mud - Linear Region 1m 10m 7 1' 10
Results - Crude Oil
Figure 6. The measurement below shows a temperature sweep on crude oil.
The graph has plotted G'(ELASTIC), G"(VISCOUS) and delta (PHASE ANGLE) as a function of temperature. At high temperatures the sample is viscous dominated but at sub zero temperatures the theological properties have reversed and the sample is elastic dominated. The crude oil actually goes through a wax point(7.7c) and this point is defined in rheological terms as when the elastic and viscous components are equal to each other i.e. the phase angle is equal to 45 degrees.
Figure 6 - Crude Oil - Temperature Sweep - 30.0 - 20.0 - 10.0 0.0 5.0 250 35.0 45.0 55.0
Figure 7. The plot below shows the variation in dynamic viscosity with temperature. At high temperatures the oil shows a small change in viscosity with temperature but as the wax point is approached the viscosity dramatically rises.
The viscosity at temperatures above 35°C is in the order of 60cps to 20 cps. The Bohlin rheometer is able to make accurate oscillatory measurements in this region due to its ultra low inertia motor and sensitive air bearing.
Figure 7 Crude Oil - Temperature Sweep
Conclusion
The Bohlin CVO Rheometer has been shown to be a very capable instrument for characterising the rheological properties of crude oil and drilling fluids. The most significant test on the crude oil is to look at the effect of temperature on viscoelastic properties and for the drilling mud, the ultra low shear rates and the yield stress test are of paramount importance for predicting suspension properties.
