Physicists propose a new analytical expression for calculating the viscosity of crude oil

Its accuracy is significantly higher compared to other known expressions—it describes a wide range of oil samples in a wide temperature range. The research was supported by the Priority 2030 strategic academic leadership program.

The research group of the Laboratory of Information Technologies in Physical Materials Science comprises Chair of the Department of Computational Physics and Modeling of Physical Processes Anatolii Mokshin, Associate Professor of the Department of Computational Physics and Modeling of Physical Processes Bulat Galimzyanov, and Engineer Maria Doronina.

The viscosity of crude oil determines its fluidity and ability to seep through geologic rocks. To correctly predict the viscosity of crude oil, reliable and universal expressions that are applicable over a wide range of temperatures and pressures are needed. Such expressions must take into account the fact that, from a physicochemical point of view, oil is a multiphase and multicomponent system.

To determine the temperature dependence of crude oil viscosity up to amorphization temperatures, scientists of Kazan University proposed a universal expression based on the concept of temperature scaling, which was developed by KFU physicists earlier. The accuracy of the proposed expression is significantly higher compared to other known expressions for describing oil viscosity.

"The concept of temperature scaling implies the introduction of such a temperature scale, different from the Celsius, Kelvin and other scales, according to which the values of key temperatures, such as glass transition temperature, melting point, take the same values for any systems. In this case, temperature as a physical parameter will be a dimensionless value," Anatolii Mokshin says.

The concept can be used in a wide variety of tasks where temperature is one of the parameters.

"In this work, the concept was used to obtain an analytical expression that would be able to correctly reproduce the temperature dependence of viscosity of such extremely complex real physical systems as crude oil," the Professor emphasizes.

In the course of the study, oil samples produced in various fields in Russia, China, Saudi Arabia, Nigeria, Kuwait and the North Sea were considered. It should be noted that samples from Ashalchi and Kuakbash fields of the Republic of Tatarstan were studied.

By the way, for the first time scientists of Kazan University used the regression analysis method to determine the correspondence between the parameters of the proposed viscosity model and various physical characteristics relevant to oil viscosity, such as amorphization temperature and API (American Petroleum Institute) index, which characterizes the density of oil in relation to the density of water at a certain temperature.

"Regression analysis allows us to solve the problems of establishing the relationship between different characteristics. Thus, scientific research of a particular system very often involves working with a large set of different physical characteristics of this system, where there is a need to establish general regularities, or rules, according to which these characteristics can be interrelated with each other," Mokshin specifies. "If the characteristics under consideration are not of the same type and their number is quite large, then a strict analytical conclusion and the establishment of a relationship between them or between some of them is difficult. Therefore, in this case, methods of statistical analysis of data, including the method of regression analysis, are very effective. The essence of this method is to find an analytical expression, usually a polynomial expression, according to which the characteristics under consideration can be interrelated."

Thanks to the method of regression analysis, it was possible to find out how the density of oil, which is directly determined by the proportion of paraffin and asphaltene fractions, the proportion of dissolved salts, water and other factors, affects the ability of oil to amorphize, that is, to solidify.

"For example, differences in the amorphization temperatures of crude oil samples produced from different, although close, fields in the Republic of Tatarstan can be more than 70 degrees. Similar results are obtained in the case of samples from Nigeria. Obviously, such a strong difference in amorphosis ability is due to differences in the composition of these oil samples. At the same time, samples produced in the fields of China, Saudi Arabia, Kuwait and the North Sea, have a fairly high amorphization temperature of minus 65-70 degrees, which may be due to their high structural heterogeneity," explains Dr. Mokshin.

According to him, the results of the research have an important fundamental significance—first of all, the problem is solved related to the elucidation of physical mechanisms determining the viscous flow of extremely complex multiphase and multi-component systems—such as crude oil. In addition, the work touches upon the problem associated with the amorphization of oil, that is, the determination of such physical conditions under which oil, in fact, should be able to be transformed into a solid. It follows that the results should also have practical significance. For example, knowledge of how viscosity changes with temperature should allow the selection of optimal conditions for the production and transportation of crude oil.

More information:
Unified scaling model for viscosity of crude oil over extended temperature range
www.sciencedirect.com/science/ … ii/S0016236124021677

Provided by Kazan Federal University