Specific Gravity of Crude Oils

Specific gravity is important commercially because the crude oil price depends partly on this property. The specific gravity is expressed most often in degrees API (see Chapters 1 and 4). 

During loading and unloading of crude oil tankers, the specific gravity of the crude is measured to confirm it meets the specifications for the case where payment is made on a barrel basis, or when the volume is converted into weight if the transaction is based on a price per ton (Hayward et al., 1980). 

Within the same geographicai region, the crude specific gravity varies from one reservoir to another, in Saudi Arabia, for example, the crude from the Ghawar field has an average standard specific gravity on the order of 0.850 (34° API) while the specific gravity of the crude from the nearby Safaniyah field is 0.893 (27° API). 

Within the same reservoir, we also observe variations of specific gravity from one well to another for example, 0.848 (38.4° API) and 0.861 (32.8° API) in the Ghawar field. 

The specific gravities of crudes fall generally between 0.800 and 1.000 as shown in Table 8.1, even though crudes having specific gravities outside this range exist 0.787 (48.2° API) for crudes in Barrow South, Alaska, USA and Santa Rosa, Venezuela and 1.028 (6° API) for the crude from Bradley Canyon, California, USA. 

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A tank 100 ft in diameter and 20 ft tall is filled with crude oil to within 2 ft of the top of the tank. One accident scenario is that a 6-in-diameter line connected to the bottom of the tank might break loose from the tank, allowing crude oil to drain out. If a 30-min emergency response time is required to stop the leak, estimate the maximum amount of crude oil (in gallons) leaked. The tank is vented to the atmosphere, and the specific gravity of crude oil is 0.9. 

Accurate determination of the density or specific gravity of crude oil is necessary for the conversion of measured volumes to volumes at the standard temperature of 15.56°C (60°F) (ASTM D1250 IP 200 Petroleum Measurement Tables). The specific gravity is also a factor reflecting the quality of crude oils. 

In the petroleum industry it is customary to express the specific gravity of crude oils in terms of API gravity. The relationship between API gravity and specific gravity measured at 60° F is expressed by the equation... 

A relief device must be designed for a vessel to relieve 1800 gpm of crude oil liquid in the event that a discharge line is blocked. The MAWP is 250 psig, and a maximum backpressure of 50 psig is expected. The specific gravity of the oil is 0.928, and its viscosity is 0.004 kg/m s. 

Effect of Oil Composition. Experimental data on the gas solubility in numerous crude oils indicate that the solubility increases as the specific gravity of the oil decreases. A low liquid gravity indicates the presence of appreciable concentrations of low molecular weight liquid hydrocarbons. Consequently there is a greater chemical similarity between the gas and the oil and a greater gas solubility is to be expected. 

Table 7.4. Specific Gravity and API Gravity of Crude Oil and Selected Products...

In oil bearing formations, the presence of polar chemical functions of asphaltenes probably makes the rock wettable to hydrocarbons and limits their production. It also happens that during production, asphaltenes precipitate, blocking the tubing. The asphaltenes are partly responsible for the high viscosity and specific gravity of heavy crudes, leading to transport problems. 

Crude oil base Speclflc gravity of heavy gasoline cut, Specific gravity of residue (BP > 350°C)... 

Estimating the general nature of crude oils by measurement of two specific gravities. j... 

Table 8.1 gives the average specific gravities of some typical crude oils. 

An oil tanker has collided witli a freighter at sea. A rupture on tlie side of the tanker lias released 450,000 gallons of crude oil (specific gravity = 0.88) into a rough sea. Estimate the spill area of oil resulting from the accident. 

Specific gravity is used to calculate the mass of crude oils and its products. Usually, crude oils and their liquid products are first measured on a volume basis, then changed to the corresponding masses using the specific gravity. 

A correlation index is a useful criterion for indicating the crude class or type. The following relationship between the mid-hoiling point in Kelvin degrees (°K) and the specific gravity of a crude oil or a fraction yields the correlation index (Bureau of Mines Correlation index). 

Increased temperature lowe.rs the specific gravity of both the oil to be treated and the water that must be separated from it. However, depending on properties of the crude, it may either increase or decrease the difference in specific gravity, as shown in Fig 7. In most cases, if treating temperature is less than 200°F, the change in S.G. with temperature can be neglected. 

Fig. 7—Heating oil-water emulsion towers tie specific gravities of both liquids. But S.G difference may increase or decrease, depending upon crude oil characteristics.

VGC (viscosity-gravity constant) an index of the chemical composition of crude oil defined by the general relation between specific gravity, sg, at 60°F and Saybolt Universal viscosity, SUV, at 100°F ... 

Figure 1.2 Specific gravity of petroleum fractions. (Plotted from data in J. B. Maxwell, Crude Oil Density Curves, Data Book on Hydrocarbons, D. Van Nostrand, Princeton, NJ, 1957, pp. 136-154.)...

SGP T TWSEC vise VISCL Vt VTGOl VTWO wc wd specific gravity of production water at system temperature T system temperature, °F time for water drop in oil phase to fall, s viscosity of liquid, gas or oil at system temperature T, cP viscosity of liquid or oil at system temperature T, cP terminal velocity of particle, ft/s particle velocity in oil phase, ft/s terminal velocity of water particle in the oil phase, ft/s BS W of treated desalter outlet crude oil, percentage by volume Barrels of dilution water per day mixed with the crude oil fluid upstream of the desalter... 

Outside the United States, refining capacity is cited in metric tons per year. Precise conversion from one unit of measure to the other depends upon the specific gravity of the crude oil, but an approximate relation is 1 barrel per day equals 50 tons per year (tpy). 

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