Measurement Units and Conversion Factors

Depending on the purpose of the measurement-and on regional or national preferences - oil, gas, gas liquids and their products may be measured in terms of volume, weight or thermal energy. For example:

• Petroleum engineers, particularly those working in the Western Hemisphere, measure oil and gas volumes to answer questions like, "How much oil or gas do I have in my reservoir? How much can be produced during the life of the field? What is the daily production rate?"
• In contrast, ship owners would want oil shipments to be measured in weight to avoid overloading their tankers.
• Marketers, on the other hand, are interested in the value of the products to their customers. When they sell oil and gas products for fuel, they charge on the basis of thermal energyunits rather than volume or weight.

Crude Oil Measurement

Crude oil, as shown in Figure 15, is measured in units of volume, weight and thermal energy.

The standard volume unit for crude oil measurement, the 42-gallon barrel ("bbl"), dates back to the 1860s, when Pennsylvania producers actually stored and transported petroleum in wooden barrels. To this day you may hear a petroleum engineer say, "This field has reserves of 1 billion barrels and we expect to produce it for 20 years at a design rate of 150,000 bbl/day." In countries that use the SI or "metric" system, oil volumes may be measured in metric tonnes.

In Europe and especially the former Soviet Union, crude oil is measured in terms of weight and expressed in metric tonnes ("mt"), where one mt equals 2,204 lb. Although crude oils vary in density, a good "average" volume-to-weight conversion is 7.33 bbls/mt. Based on this conversion factor, a Russian engineer would refer to production not as 150,000 bbl/day but as 20,464 mt/day.

The thermal energy content or heating value of crude oil depends on its composition, but generally averages about 6 million BTU per barrel, where a BTU (British Thermal Unit) is the amount of energy required to increase the temperature of one pound of water by 1°F. Thus, a barrel of oil that sells for $48 would provide energy at an equivalent cost of:

($48/bbl) ÷ (6 million BTU) = $8/million BTU (per barrel of oil).
(For those familiar with the SI unit of energy, the Kilojoule (kJ),1 BTU = 1.055 kJ.)

Figure 15: A barrel of crude oil and its measurement units in volume, weight and thermal energy units.

Oil Quality: API Gravity

A crude oil's density is an important measure of its overall quality. This is because lighter oils are generally easier to produce and refine than heavy oils, and therefore tend to have higher value.

Oil density is sometimes expressed in terms of its specific gravity, but more often is given as API gravity.

• The specific gravity (S.G.) of a liquid is defined as the density of that liquid divided by the density of fresh water. Fresh water has a density of 62.4 pounds per cubic foot. An oil with a density of 53 pounds per cubic foot, therefore, would have a specific gravity of (53/62.4), or 0.85. Fresh water, by definition, has a specific gravity of 1.0.
• The American Petroleum Institute (API) has developed a special measure that expresses oil density in terms of API gravity, or ºAPI. It is related to the specific gravity as follows:

S.G. = 141.5/(131.5+ºAPI)

or

ºAPI = (141.5/S.G.) - 131.5

From these relationships, we can determine that fresh water, with a specific gravity of 1.0, has an API gravity of 10 degrees, while our 0.85 S.G. oil above has an API gravity of 35 degrees; almost all crude oils are lighter than water and so they will have higher API gravities. Figure 16 shows the correlation between specific gravity and API gravity for various crude oil and condensate samples.

Figure 16: Correlation between specific gravity and°API gravity. Note the the API gravities of crude oils from different fields, including Lagunillas (Venezuela), Prudhoe Bay, (Alaska), Ghawar (Saudi Arabia), Ninian (offshore UK), and the very light condensate produced from the Arun Field (Indonesia).

Natural Gas Measurement

Natural gas is typically measured in terms of its volume at surface conditions and in thermal energy units. It is measured by weight only when it is in the liquid state (LNG).

Because gas is compressible, its volume varies significantly with changes in temperature and pressure. In order for gas volume measurements to have any meaning, they have to have some standard frame of reference. For this reason, the industry has established standard conditions for referring to all gas volumes (Table 2).

The standard cubic foot and the standard cubic meter are the most widely used (1 m³=35.31 SCF). Some European countries use conditions called "Normal" which are for pressure 760 mmHg (14.696 psia or 1 atmosphere) and temperature 0 °C. It is important that these conditions are clearly defined at the time of signing any gas contract.

Figure 17 demonstrates the compressibility of natural gas. The gas has a volume of 1 unit (ft³ or m³) in the reservoir at 4,000 psia pressure. When it is produced to surface conditions of temperature and pressure it expands to 238 units, and when it is compressed into a high-pressure pipeline at 1,000 psia pressure it becomes 3 units. In all three conditions the engineer will refer to the gas volume as 238 standard units (ft³ or m³ ).

Figure 17:Compressibility of natural gas. This Figure shows the effect of pressure and temperature on natural gas volumes in the reservoir, pipeline and at surface conditions.

Because the cubic foot and cubic meter are too small for practical use, the industry uses larger standard quantities with appropriate symbols (Table 3). Thus, rather than saying that a gas well produces at a rate of 10,000,000 SCF/day, we say that the well produces at 10 MMCF/day.

The heating value of natural gas depends upon its compositi

troleum Product Measurement

LNG (liquefied natural gas) is typically measured in metric tonnes or cubic meters. The normal conversion to volume units in the liquid state is that 1 mt equals 2.12 m³ or 79.5 ft³. When 1 tonne of LNG is vaporized to become natural gas it expands about 625 times to equal 1380 m³ or 48,700 ft³ at atmospheric conditions.

LPG and petroleum products are generally measured in gallons in the United States and certain other Western Hemisphere countries, and kiloliters or metric tonnes in Canada, and in Europe and other Eastern Hemisphere countries. Their conversion from volume to weight will depend on the density of the product. Thus, one metric tonne of propane is equal to 521 gallons or 1.97 kiloliters, while one metric tonne of fuel oil is equal to 281 gallons or 1.064 kiloliters.

LPG must be stored at an elevated pressure or be refrigerated in order to remain in the liquid state, and so it is measured at these high-pressure and/or low-temperature conditions rather than at "atmospheric" conditions (Figure 18).

As is true for oil and natural gas, the heating value of a petroleum product depends upon its composition. For example, the heating value of propane is 91,500 BTU/gal, while that of heavy fuel oil is 150,000 BTU/gal. However, because each product has a standard, relatively narrow industrial specification, their individual heating values will not vary significantly and so they are usually traded on the basis of weight (e.g., $/mt) or volume (e.g., $/gallon).on. Pure methane has a heating value of 1,010 BTU/SCF [35,663 BTU/m³], while propane has a heating value of 2,516 BTU/SCF. A gas that contains 50% methane and 50% propane will have a heating value that is midway between 1,010 and 2,516 BTU/SCF.

Gas Liquids and Pe

Figure 18: Propane must be stored at a pressure of 200 psi or be cooled to -44°F (-42°C) in order to remain in the liquid state. This photo shows an outdoor grill with a storage tank in which liquid propane is maintained at 200 psia pressure and may be exposed to a temperature up to 100°F (38°C) and still remain a liquid.

Energy Content and Equivalency

From the standpoint of meeting heating requirements, is it better to have a tonne of crude oil or a tonne of coal or a tonne of LNG?

To answer this question, we can look at the heating values of crude, gas and products as shown in Table 4:

Thermal Energy Equivalency:

Using the thermal energy relationships from Table 4, we can relate the value of any hydrocarbon to a common measure based on thermal energy equivalency. The normal unit is either the barrel of oil equivalent (BOE) or tonnes of oil equivalent (TOE).

Hydrocarbon Price Equivalency:

We can also relate the price of the various hydrocarbons on the basis of their thermal energy equivalency.

In reality, as shown in Figure 19a and Figure 19b, each of these hydrocarbons has its own regional or international market prices. In the major consuming countries, gas approaches oil equivalent prices, but in countries like Qatar, which have large isolated gas reserves, gas prices are much lower relative to oil. Because of a large oversupply, coal prices have been much lower than the oil equivalency price shown above-on the order of $35/tonne for many years-but has increased in recent years, especially on the spot markets. Industrial companies can easily switch the burner tips of their boilers back and forth to burn natural gas or fuel oil to take advantage of regional price differences. The residential, commercial and power generating customers do not normally have that flexibility.Figure 19b. This figure shows the average annual natural gas prices in three regions (LNG delivered into Japan, UK Heren Index, and Henry Hub price in the USA) and BTU equivalent price for crude oil (Brent crude price divided by 6).(Source: BP Statistical Review of World Energy, June 2014. Current prices (2014) are estimated based on EIA and IEA publications)

Conversion Table and Electronic Calculator

Table 5 below lists some of the more widely used factors for converting crude oil, petroleum products and natural gas into their various equivalents. To do your own quick unit conversions, you may use the

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