A study of the effectiveness of chemical additives on the characteristic flow of crude oils

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Introduction
Crude oil viscosity is an essential physical property that influences and controls the oil flow through porous media and pipes 1 . Generally, the internal resistance of the fluid to flow is defined as the viscosity. The oil viscosity has a crucial function of the pressure, temperature, gas solubility and gas gravity 2,3 . Whenever possible, oil viscosity requires to be calculated by laboratory measurement at reservoir pressure and temperature 2,3 . According to reports from the International Energy Agency, half of the world's importable oil resource is represented by heavy crude oil represents 4 . Heavy oil is considered as petroleum, which has a density equal or lower than 20 API; however, if petroleum has 10 API or less, it will be considered as extra-heavy oil or bitumen 5 . The export of oil to the world markets requires important technology in the production chain. Additionally, the pipeline is the most convenient way to transport crude oil continuously and economically. Nevertheless, it is challenging to transport crude oil through the pipes due to the increased internal friction of crude oil inside it by viscosity (it is a major cause of the low flow capacity) 6 . Crude oil is exported through conveyor pipes for long distances up to thousands of kilometers. Basically, the pumps are used to pump crude oil inside the conveyor pipes. The rate of export of crude oil per day depends on the viscosity of crude oil, the capacity of the pump, and the number of pumps along the transport distance of crude oil. High molecular weight polymers are used to increase the efficiency of pumping crude oil. This technique is essential from the economic side, and this is achieved by injecting polymers with crude oil in a turbulent flow condition. Additionally, it gives good results when pumping crude oil for long distances without loss of pressure 7 . Crude oil is defined as a complex mixture of hydrocarbons associated with other organic matters in small quantities 8 . Non-hydrocarbon compounds include compounds containing oxygen, sulfur, nitrogen, and a small number of minerals (no more than 5%). Crude oil consists of carbon elements (83 -87%), hydrogen (10 -14%), sulfur (0.1 -3%), oxygen (1.5%) and nitrogen (0.1%), as well as heavy metals such as vanadium, mercury, cadmium, Nickel and others in small quantities 9 . Hydrocarbons components contain paraffin & circular paraffin and aromatic, where the proportions of these components vary from one oil to another according to the geological formation of the reservoir 10 . Hydrocarbon and non-hydrocarbon components are crucial in modifying (increasing and decreasing) crude oil viscosity 11 . It is necessary to increase the flow rate of crude oil in pipelines to avoid any additional costs and time spent in building new pipelines, which in consequence obtain the same improvement in the required flow. In the literature, several laboratory experiments have been conducted to determine the best polymer with a concentric ratio that reduces viscosity. Consequently, this leads to an increase in the flow rate of crude oil without affecting the chemical properties of crude oil. Many researchers conducted practical experiments to reduce the viscosity of crude oil in order to increase the rate of flow through the pipelines, from these experiments, the researchers (Hazlina Husin, Azlinda Azizi and Afuza Husna) used polymer, surfactant, chemical additive and emulsification in the petroleum industry to increase pump efficiency by reducing the viscosity of crude oil 12 . Other researchers used (TEA) to increase the flow and reduce the viscosity of crude oil, they used Niger crude oil to evaluate the efficiency of triethanolamine (TEA) as an additive by measuring the pour point and viscosity of processed and unprocessed crude oil 13 . While others studied the effect of polyacrylamide (PAM) on increasing the rate of flow of crude oil, they concluded that the concentration of the additive and the diameter of the pipe have a role in the increase or decrease in the flow rate of crude oil 14 .

Materials and the laboratory measurements
In this research, we studied the effect of three polymeric materials on a sample of Kirkuk crude oil (API = 34.8) to determine the efficiency of pumping crude oil in to conveying pipes. The molecular weight polymers Carboxymethyl cellulose (CMC-MW.T = 300.75) & Polyvinyl chloride [-C2H3Cl-] (PVC-M.WT=160000) and Sodium dodecyl sulfate [CH3(CH2)11SO4Na](SDS-M.WT = 288.38) are dissolved in the Toluene at a temperature of (30C o ). Then added to the crude oil by concentrations ranging from (100-200 ppm) with a temperature range between (30-60 C o ). We have used U-tube Viscometer for measuring the viscosity, electronic balance, water bath in addition to the thermometer. A laboratory unit was designed and constructed to test a suitability oil viscosity. In the beginning, the first test was measured without using additives, and then, the measurement was conducted for different additives with different concentrations. The kinetic viscosity was calculated according to equation (1): = (1) Where k is a constant given by the manufacturer (0.065mm/s 2 ), t refers to the time required for fluid to fall through the tube (s), and Ø is the inner diameter of the tube (m). A pipeline system device was designed for: 1) measuring the pressure difference between the two ends of a tube; and 2) reducing the drag and friction coefficient before and after the addition of polymers in different concentrations and different levels of temperature. It is performed depending on the following laws: 1) taking in to consideration that the length of the tube in the pipeline system is (3m); 2) surface friction of the tube is (Ԑ=0.0000064 m), which was found from special tables; and 3) the speed of fluid was (2.021m/s).

Polymer effect on viscosity
We can observe from Table 1 that the viscosity of crude oil decreases and increases at different levels of temperatures 30, 35, 40, 50 and 60 C 0 when adding polymer (PVC). These changes (decrease and increase) in viscosity of crude oil are occurred in different proportions depending on the percentage of the polymer (PVC) added at the same and different temperatures. Consequently, in crude oil, there is a direct relationship between the temperature and the speed of polymer degradation. However, other circumstances have an inverse relationship. As shown in Figure 1. the viscosity of crude oil gradually decreases and then increases at the same temperature by increasing the percentage of polymer (PVC) at fixed rates. The best explanation of this case indicates that the ratio of oxygen compounds gradually decreases in the mixture. This occurs because it degrades with increasing the polymer (PVC), and then reduces the viscosity of crude oil. In the end, most of the polymer is not degraded due to the absence of oxygen compounds in the mixture, and therefore the viscosity of crude oil increases.  The compounds that attack polyvinyl chloride are the oxygen compounds and these compounds dissolution PVC. The hydrogen represents the main part of the reaction in both PVC-PVC and PVC-solvent 16 . At some point, it can be assumed that the speed of the frontal reaction is equal to the speed of the reverse reaction, which can be represented in the following equation.
Oxygen compounds in crude oil include tetrahydrofuran, cyclohexanone, Cyclopentanone and dimethylformamide. These compounds are characterized by high molecular weights (which increase the viscosity of crude oil), and they are dissolution the PVC polymer. The polymer dissolution depends on the type of the oxygen compound so that whenever the compound is electronically stable, the following actions occur: 1) the polymer PVC decomposition speed increases; 2) the decomposition time decreases; and 3) the speed of diffusion increases 16 . As shown in Table 1, as observed at 60 C o , crude oil viscosity increases when it is added in different ratios from the polymer (PVC). The explanation for this situation is that when the temperature increases, the speed of the oxygen compounds decomposition in the polymer (PVC) significantly increases. Therefore, as noted in Figure 1, the speed of the equilibrium state will increase. Furthermore, for this reason, all oxygen compounds will deplete and crude oil viscosity will increase at all additions of the (PVC) ratio. Moreover, one can observe from the two Tables 2 and 3 that the viscosity of crude oil increases at temperatures (30, 35, 40, 50 and 60 C o ) when adding the polymer (CMC) or (SDS). This increase in viscosity of crude oil occurs at different rates depending on the percentage of polymers (CMC-SDS) added at the same and different temperatures. Accordingly, as illustrated in Figures 2 and 3, there is no degradation between polymers (CMC or SDS) and oxygen compounds or any relationship between the temperature and the speed of polymer degradation in crude oil. Therefore, polymer (PVC) is the best polymer that reduces the viscosity of crude oil by 12.63% at (100 ppm and 30 C o ). On the contrary, at 35, 40 and 50 C o , the viscosity decreases by 4.12%, 2.41% and 2.08%, respectively.

Polymer effect on Reynolds number
As seen in Tables 4 and 5    However, this change gradually decreases because the polymer does not decompose when its concentration is high. Furthermore, the amount of oxygen compounds decreases gradually because these compounds decompose in this type of polymer (see Table 6 and in Figure 6).

Polymer effect on friction factor and pressure drop
The relationship between the friction coefficient and the pressure drop with polymer concentrations increases linearly when the polymer concentration increases at different temperatures as observed in the Tables (7, 8, 9 and 10) and in the Figures (7, 8, 9 and 10).   Table 9. Effect of polymer (SDS) concentration on the friction factor of crude oil at a velocity of 2.021 m/s  The relationship between friction coefficient and pressure drop with polymer concentrations is linearly reduced to a certain extent and then linearly increases. This relationship depends on the availability of oxygen and polymer compounds together in crude oil at temperatures (30, 35, 40 and 50 C o ). However, at a temperature (60 C o ) this relationship increases linearly when the polymer concentration increases. Finally, the best value for the friction coefficient and pressure drop is obtained at (100 ppm) and temperature (30 C o ) as observed in Tables  11 and 12, and in Figures 11 and 12.

Polymer effect on drag reduction
As observed in Tables 13 and 14, the relationship between the concentration of CMC and SDS polymers with drag reduction of crude oil is that the drag reduction reduces with increasing the concentration of polymer. However, Table 15 shows the concentration of PVC polymer increases linearly, and the best drag reduction occurs at concentration of (100 ppm) and (30 C o ) (see Figures 13, 14 and 15).

Conclusion
In this article, we evaluated the effect of the concentration with additives on the kinematic viscosity of crude oils. Additionally, we studied the influence of solvents that reduces the viscosity of heavy oil with significance to their functions in the flow rate of crude oil. Based on the findings, we conclude: 1. PVC is the best polymer that reduces the viscosity of crude oil and reduces the friction factor of the fluid at the temperature of (30 C O ) and the concentration of (100 ppm). Additionally, it gives the best drag reduction for the fluid in the practical experiments. After that the viscosity and friction factor of the fluid increases linearly because the proportion of oxygen compounds decreases gradually in crude oil. This type of polymer reduces and dissolves oxygen compounds only and does not affect the chemical properties of crude oil.
2. The best effect of the polymer (PVC) is on heavy crude oil more than light crude oil because the heavy crude oil contains more proportion of oxygen compounds.