The Physical And Chemical Characteristics Of Crude Oil

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Crude oil is a complex mixture of various organic compounds, comprising of different hydrocarbons and heteroatomic compounds (Simanzhenkov and Idem, 2003). In addition to carbon and hydrogen, petroleum might contain sulphur, oxygen and nitrogen, and metals such as nickel, vanadium, iron, and copper. Crude oil physical and chemical compositions vary with location, depth and age (Rahimi and Gentzis, 2006). Physical composition of crude oil varies from black liquids of high viscosity to straw-colored liquids that vary from viscous of free flow (Nji, 2010).
Crude oil is classified commercially by API gravity into light, medium, heavy and extra heavy or bitumen. Accordingly, light oils have an API gravity >31.1° and medium crude oils are those
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During oil production, density does not cause massive problems. However, most problems associated with oil production are associated with viscosity (Silset, 2008).
Heavy Oil characteristics
Viscosity of heavy crude oils ranges from about 20 cp (0.02 Pa.s) up to more than 1,000,000 cP (1,000 Pa.s). This extremely high viscosity results in almost immobility of heavy oils under real reservoir conditions. As a result, primary oil recovery is extremely low (Borregales, 1979). Sub-categorization of heavy oils into extra-heavy and bitumens is based upon the viscosity of heavy oil at reservoir conditions. Extra heavy oils are reported to have a viscosity lower than 10,000 cP (mPa.s) while natural bitumens are of a viscosity higher than 10,000 cP (mPa.s) (Huc, 2010).
Viscosity of heavy oils is directly dependent on the temperature and the oil gravity. Viscosity is expected to increase with a decrease in temperature and API gravity. This is due to the fact that when a decrease in temperature takes place, phase change occurs. Heavy oils change its phase drastically from a low viscosity liquid phase to higher quasi-solid and eventually to the glass phase (Chopra et al., 2010; Han et al.,
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The main reason behind the heavy nature of heavy oil is an internal balance between a relatively high portion of high molecular weight, non paraffanic complex compounds and a lower portion of low molecular -weight volatile compounds. Disturbing this internal balances influence oil mobility and results in deposition of asphaltene constituents. For those reasons, disturbing the internal balance is considered the main reason behind most of the problems associated with heavy oil production (Speight, 2009). This emphasizes the connection between heavy oils’ physical properties and chemical properties and the importance of understanding the chemical composition of heavy crude

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