Solvent selection based on the study of the rheological properties of oil

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Solvent selection based on the study of the rheological properties of oil

Today in the world there are many production reagents, solvents, for example, various petroleum solvents, xylenes, gas condensate, etc. used in the process of oil recovery. They are applied widely in such methods of production as cyclic pumping of solvent in a production well with the subsequent selection of liquid, VAPEX and its variation [Nikolin, 2007; Batler, 2010; Roschin et al, 2013]. In addition, solvents like surfactants are often added to the suction of well pumps, making it easier to lift the liquid and helping prevent loss of asphaltene-resin-paraffin substances in the tubing and surface equipment, reducing the viscosity of crude oil and weakening its thixotropic properties [2-12]. In the Mining University a reagent-solvent with higher viscosity has been developed, having good dissolving and dispersing properties in relation to the resin and asphaltene, forming three-dimensional structures in the heavy oil [Roschin et al, 2013]. The presence of asphaltenes, resins and waxes leads to anomalies in the flow of highly viscous oil in porous medium, and the thixotropic structure of coagulation and coagulation-crystallization types are characterized by shear stress destruction, which complicates the fluid influx into the production well [Gafarov, Shamaev, 2005; Zinoviev et al, 2014]. The use of reagent-solvent allows for the reduction of the viscosity of crude oil, facilitating its production and transportation.

In the Russian oil industry, a wide range of solvent-reactants are used. Petroleum ether is a light benzine which is obtained by distilling a light fraction of distilled oil. It is used as a solvent for flushing out of wells and treating of hole-bottom region. Depending on the temperature of distillation, light and heavy ethers can be distinguished. An ether is considered light if it is obtained at temperatures below 70єC and heavy if obtained between 70-100єC. Petroleum ether Nefras S2-80/120 showed high efficiency when added to waxy oil from the Petrukhnovskoe field [Roschin et al, 2013]. Also it should be emphasized that white spirit (Nefras-S4-155/200) is the product of the straight distillation of crude oil (light fraction benzine) with a high content of aromatic hydrocarbons (up to 16%). It is used as a component of solvent mixtures in the treatment of bottomhole formation zones of reservoirs and the borehole.

Xylene is a mixture of ethylbenzene and three xylene isomers. Xylene is obtained during the aromatization of petroleum fractions. Ortho-xylene is obtained from the rectification of xylene. Xylene is actively used as a solvent in the treatment of hole-bottom region of pay zones with high viscosity oil. It is also used to dissolve resin and asphaltene deposits in producing wells and oil pipelines. According to the results of several laboratory experiments, xylene has proven to be an effective solvent when added to samples of high viscosity oil.

Also, there are a number of alcohols used as components of various mixtures during the treatment of bottomhole formation zones of reservoirs. Isopropyl alcohol is a product of the hydrogenation of acetone or propylene hydration. This alcohol reduces the interfacial tension at the «reagent - oil» frontier in the compositions used in the treatment of hole-bottom region. It is also used as a component of solvent-reactants. Nefras-A-130/150, is widely known as a petroleum solvent with the highest weight content of aromatic components of the benzene series. It is most often used in the treatment of bottomhole formation zones and removing asphaltene and wax deposits.

Temperature and reagent effect on the rheological properties of oil

For the experiment heavy oil was selected from a formation in the Samara region (viscosity at 20°C is 35200 mPa•s), with the mass content of asphaltenes and resins up to 75%. The hydrocarbon-bearing formation is characterised by fracture-porous carbonate rock. For research of the rheological properties of a sample of high-viscosity oil dynamic tests were carried out at various temperatures on the Rheotest RN 4.1 device in the Mining University (St. Petersburg). The study of the thixotropic properties was carried out using specific procedures described in papers [Devlikamov et al, 1975; Rogachev, Kolonskih, 2006; Roschin et al, 2013; Zinoviev et al, 2014].

Figure 1 displays the plots of the shear stress versus shear rate at different temperatures, forming the typical hysteresis loop area which indicates the presence of oil thixotropic properties at a given temperature.

Figure 1. The dependence of shear stress versus shear rate for the sample of heavy oil at different temperatures

rheological heavy oil field

The diagram shows that even at 70°C oil has thixotropic properties. The experiment at temperatures below 35°C is not possible due to limitations of the range of operation of the Rheotest device.

To investigate the influence of produced reagent-solvent that is a mixture of unsaturated fatty acids and aromatic hydrocarbons on the thixotropic properties of heavy oil, an experiment was carried out with the oil with the addition of a solvent at a concentration of 3% by weight. Figure 2 shows the experimental results obtained at 35°C.

Figure 2. The dependence of shear stress versus shear rate for clean oil and the oil with the addition of the reagent-solvent in a concentration of 3% wt at 35°C

While conducting experiments the shear stress of thixotropic structure destruction in highly viscous oil was determined. Methodology for conducting the experiments described in the papers [Roschin et al, 2013; Zinoviev et al, 2014]. Figure 3 presents the results of adding the solvent in the investigated high-viscosity oil.

Figure 3. The dependence of the effective viscosity and shear stress versus temperature for clean oil and the oil with the addition of the reagent-solvent in a concentration of 3% wt

Figure 3 clearly shows that the produced solvent reduces viscosity of heavy oil (almost 4 times at 15°C and 2 times - at 90°C) and shear stress of thixotropic structure destruction at various temperatures. The designed solvent has dissolving and dispersing actions in relation to the coagulation structure formed by the asphaltenes and resins due to the content of xylene and fatty acids.

The combination of thermal exposure (steam flooding) and the addition of the reagent-solvent can significantly reduce viscosity and shear stress in flow of oil in the reservoir, and as it moves along tubing and flowlines.

We would like to acknowledge the contribution of Dr. A.V. Petukhov and Dr. M.K. Rogatchev for their assistance during the conduction of the experiments. Finally, we would like to thank University of Mines (Saint Petersburg, the Russian Federation) which provided laboratory equipment support and samples for this research.

References

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[2] Roschin P.V., Petukhov A.V., Vasquez Cardenas L.C., Nazarov A.D., Khromykh L.N Issledovanie reologicheskikh svoystv vysokovyazkikh i vysokoparafinistykh neftey mestorozhdeniy Samarskoy oblasti. Neftegazovaya Geologiya. Teoriya I Praktika, 2013, vol. 8, no. 1, http://www.ngtp.ru/rub/9/12_2013.pdf [Research of the rheological properties of high-viscosity and high-paraffinic oilfields of the Samara region. «Oil and gas geology. Theory and practice», 2013, vol. 8, No. 1.].

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