Relating Chemical and Physical Properties of Heavy Oil
Amy Hinkle and Mike Batzle
Colorado School of Mines, Golden, Colorado (presented by Merrick Johnston)
The physical properties of heavy oils are still poorly understood. The viscosities measured for different heavy oils can vary by orders of magnitude. However, viscosity and density do not appear to be directly related for heavy oils as they are for lighter oils. Since heavy oil is a viscoelastic material, the shear modulus and the viscosity will be coupled (Han, 2005; Tschoegl, 1989). Understanding what controls heavy oil viscosity will provide insight into what controls heavy oil shear modulus. It has been suggested that the key to understanding the large variations in heavy oil viscosity may be heavy oil chemistry. This paper looks at potential chemical controls on heavy oil viscosity and shear modulus.
It has been suggested in the literature that variations in asphaltenes content may be the explanation for the large spread of viscosity values observed in heavy oils (Al-Mamaari et al., 2006; Argillier, 2001; Henuat, 2001). If asphaltenes control heavy oil viscosity, they should also control heavy oil shear modulus. To test this theory, two suites of measurements were completed on seven heavy oil samples from around the world. Viscosity and shear modulus were measured at low frequencies on a rheometer. Shear modulus measurements were also made at ultrasonic frequencies using a standard ultrasonic pulse technique. The findings suggest that viscosity and shear modulus depend on combined asphaltenes and resins content rather than asphaltenes content alone. The details of these results are outlined in this paper.
Theory and Method
Two categories of measurements were completed for this work. Low frequency viscosity and shear modulus measurements were collected in a range from .01 to 100 Hz on a rheometer. High frequency shear modulus measurements were collected in a range of 0.5 to 1 MHz using a standard ultrasonic pulse technique.
In the rheometer experiments, a small amount of heavy oil is loaded between the Peltier plate and the heating plate of a rheometer. A sinusoidal torsional stress is applied to the heavy oil on one end and the resulting sinusoidal strain is measured on the other side. The resulting strain will have the same angular frequency as the applied stress but will exhibit a phase lag. If the material being measured is perfectly elastic, the stress would be perfectly in phase with the strain and the phase lag would be 0 degrees. For a perfectly viscous fluid, the stress would be exactly out of phase with the strain and the phase lag would be 90 degrees. For a viscoelastic material, such as heavy oil, the phase lag will fall somewhere between 0 and 90 degrees.
The shear modulus, or G’, is the ratio of the stress in phase with the strain to the strain magnitude. The loss modulus, or G’’, is the ratio of the stress 90 degrees out of phase with strain to the strain magnitude (Tschoegl 1989). In addition, the viscosity, η’, is the ratio of stress in phase with rate of strain to rate of strain, and η’’ is the ratio of stress 90 degrees out of phase with rate of strain divided by rate of strain (Tschoegl 1989). The complex viscosity is then described by the following equation:
Therefore, this experiment provides a way to measure viscoelastic parameters such as shear modulus (G’), loss modulus (G’’), and complex viscosity (η*).
Another way to determine the shear modulus is to measure the travel time of an ultrasonic pulse through the heavy oil and use this to calculate compressional and shear wave velocities of the material. Once the velocities are determined, they can be related to shear and bulk modulus using the following equations:
where Vs = shear wave velocity, Vp = compressional velocity, ρ = fluid density, K = Bulk Modulus, and G’ =shear (or storage) modulus.
Once the physical properties of viscosity and shear modulus of the heavy oils had been measured using the previous techniques, the samples also had to be characterized chemically. Crude oils can be characterized geochemically using a process called SARA fractionation. The crude oil can be separated into four components based on solubility classes. These four components are saturates, aromatics, resins, and asphaltenes, hence, SARA fractionation. Heavy oils tend to be rich in the high molecular weight components, which are resins and asphaltenes.
Six heavy oil samples were measured under the previously described conditions: three samples from Canada, one sample from Venezuela, one Alaskan sample (Ugnu), one Utah sample (Asphalt Ridge), and one west Texas samples (Uvalde). SARA analysis was performed for each of the six heavy oil samples by Humble Geochemical. Oil densities were also determined by dividing mass by volume of the heavy oils. The Canadian, Venezuelan, and Alaskan heavy oil was donated from various companies. However, the Asphalt Ridge and Uvalde samples came from rocks collected at the outcrop and the oil had to be extracted. All of the samples were dead oils, or gas-free.
Complex viscosity and shear modulus measurements made on the G2 rheometer show a high dependence on temperature. Shear modulus and viscosity values increase orders of magnitude as the temperature is decreased linearly. Shear modulus and viscosity values decrease with increasing frequency, but the frequency dependence is less pronounced than the temperature dependence.
The viscosity also varies strongly between different heavy oil samples. The API gravities of six of the heavy oil samples fall in a range between 8 and 11.5 degrees. The viscosities of these same heavy oils, however, vary from 190 to 10,140 Pa-s. The viscosity is not strongly correlated with the API gravity for these heavy oils.
Since the viscosity of these heavy oils is not dependent on these fluids’ density, or API gravity, another possibility is that the viscosity is controlled by the heavy oil’s chemical make up. One chemical control that has been suggested in the literature is asphaltenes content (Al-Mamaari et al., 2006; Argillier, 2001; Henuat, 2001). In order to test this possibility, viscosities for each sample corresponding to 20 degrees Celsius and one Hertz frequency were plotted versus asphaltenes content. The correlation between viscosity and asphaltenes content is not as good as the literature implies.
Since asphaltenes do not correlate strongly with viscosity, viscosity was then plotted with respect to resins content. This relationship was better, but it could not explain the extremely high viscosity of the Uvalde, Texas heavy oil sample. It appears that above a threshold weight percent of asphaltenes, viscosity is dominantly controlled by asphaltenes content. However, the threshold value of weight percent asphaltenes is probably significantly higher than what is found in the current literature (Argillier, 2001; Henuat, 2001).
These findings suggest that for most heavy oils, it will be important to consider both resins and asphaltenes content. In light of this conclusion, viscosity was plotted versus combined resins and asphaltenes content. The correlation for this relationship was improved from the previous two relationships which did not consider both asphaltenes and resins.
The ultrasonic shear modulus measured for these oils ranges from 0.2 to 0.9 GPa at a constant temperature of -7 C. Additionally, the measured shear modulus of the same oil increases by as much as six times as the temperature drops 30 degrees. It is difficult to differentiate between noise and the shear wave signal above about 20 degrees Celsius. For this reason, the shear data collected resides in a temperature range between -25 and 20 degrees Celsius.
For the shear modulus which was measured on the same rheometer as the viscosity, the dependence of this shear modulus on combined asphaltenes and resins content is expected and easily demonstrated. The next question is whether the shear modulus measured ultrasonically will also depend on combined asphaltenes and resins content. In order to answer this question, the shear modulus extracted from ultrasonic velocities were plotted with respect to resins content, asphaltenes content, and combined resins and asphaltenes content. The best correlation for both ultrasonic data sets was seen with combined asphaltenes and resins content.
One chemical characterization measurement that correlates well with asphaltenes and resins content is molecular beam mass spectrometry (MBMS). A principal component analysis was performed on the MBMS data. The first five principle components explain more than 95% of the variance seen in the MBMS data. A multivariate statistical analysis was run using the first five principal components of the MBMS data as predictors and combined asphaltenes and resins content, viscosity, or shear modulus as the predicted values. The ability to predict combined asphaltenes and resins content, shear modulus, and viscosity using the MBMS data was satisfactory (See Figure 6). This is an attractive possibility for predicting heavy oil shear modulus and viscosity since the MBMS measurements are quick and the sample size required is extremely small.
There are several important concepts that were developed during the course of this research. Heavy oils are viscoelastic materials and this means shear modulus and viscosity will be coupled for heavy oils. The parameters that control heavy oil viscosity will also control heavy oil shear modulus. This was proven to be true based on the similar trends in shear modulus and viscosity data.
Chemistry was shown to be essential to understanding variations in viscosity and shear modulus. In literature, the viscosity is often said to be controlled by asphaltenes weight content. However, almost all of the published papers studied a single heavy oil sample where saturates, aromatics, and resins contents were constant while asphaltenes content varied (Argillier et al. 2001; Henuat et al. 2001, Luo and Gu 2005). In this thesis various naturally occurring heavy oil samples were studied. The results indicated that viscosity depended on combined asphaltenes and resins concentration.
In the petroleum industry, viscosity mapping of heavy oil fields is currently based on API gravity measurements. As was demonstrated in this thesis, and has been reported by other researchers (Al-Mamaari et al. 2006), there is no good correlation between API gravity and viscosity for heavy oil. Combined resins and asphaltenes content could be used to better map viscosity variations across heavy oil fields. Combined asphaltenes and resins could also be helpful for predicting shear modulus variations in heavy oil fields.
Since MBMS are quick and easy to collect, an empirical relationship could be established between MBMS measurements and available heavy oil shear modulus measurements. If enough samples are used to establish the relationship, it should be possible to estimate shear modulus for heavy oils for which no shear modulus data is available.
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AAPG Search and Discovery Article #90075©2008 AAPG Hedberg Conference, Banff, Alberta, Canada