Most of the customary units used in petroleum engineeringare not used in the SI metric system. This articlediscusses the customary units, proposes the correspondingSI equivalent, and shows the conversion factors. Whenfeasible, the new unit has about the same numerical valueas the customary one. Subsequent articles will show the use of these andadditional units in specific areas of expertise.


The language of petroleum engineering contains terms thathave special meaning in the analysis of information andthe communication of ideas. Some of these terms havebeen coined specifically for petroleum, while others havebeen borrowed. Through long-term usage these termshave become familiar and useful even though theirscientific basis, or source, may be dubious. This introduces a "Catch 22." Attempting to eliminatethese terms suddenly is difficult. It jeopardizes and delaysacceptance. On the other hand, to delay implementing newterminology unduly is also counterproductive. It is muchlike learning a new language or expanding our vocabulary;we do not toss away old words when teaming new ones. SPE recognizes the problem and is attempting toconvert to SI terminology in an orderly manner over a rathershort time period. Some units are labelled "allowable,"meaning they are temporarily acceptable but their use isdiscouraged. Although this policy satisfies no onecompletely, it is probably an intelligent compromise underthe circumstances.

Length, Area, and Volume

The basic unit is the meter (m). In most applications, thelength prefixes can he limited to kilometer (km), millimeter(mm), and, occasionally, micrometer ( m). The basicconversions for the meter are as follows.

1 ft=0.3048 m, 1 m=3.281 ft.

1 km=0.621 mile, 1 mile=1.609 km.

1 km = 1000 m, 1 m=1000 mm=1 (E + 06) m.

1 micron=1 m, 1 mil=25.4 m.

Depending on the units used in the original land survey, the hectare(ha) or km is the common unit for measuringlarge areas of land, as in reservoir engineering andgeology. The m is the most common unit in other applications.

1 sq ft=0.0929 m, 1 m = 10.76 sq ft.

1 sq mile (section)=2.59 km.

1 ha= 10 000 m, 1 acre=4047 m

The cubic meter is the most common volume term, although the cubic decimeter (dm 3) or liter (L) is usefulfor small volumes. Gone is that old familiar liquid volumeterm, the API barrel (bbl), equal to 42 U.S. gal. Exceptfor its familiarity, this unit of measurement possesses no redeeming features.

1 m3 = 35.31 cu ft = 6.29 bbl = 1000 L.

1 cu ft=0.028 32 m 3=7.481 U.S. gal=28.32 L.

1 bbl=0.159 m3 = 5.615 cu ft = 42 U.S. gal

=35 U.K. gal.

Standard Gas Volumes. It is common to specify a gasvolume or rate in "standard" volumes at a specified temperature and pressure. This will be discussed in alater section.


The second is the basic time unit, but any other unit maybe used. The use of the minute as a time unit is discouraged because of abbreviation problems. In the SI metric systemthe abbreviations are second (s), minute(min), hour (h), and year (a). Notice that "a" is used for annum instead of"y" for year.

Mass, Force, and Weight

This is an area where the ambiguity of the traditional customary unit system has led to confusion (and sometimes error). The SI metric system resolves this ambiguity byusing different units for force and mass. These twoquantities are related by the equation

(mass)(acceleration)force= .(conversion factor)


P. 1801^

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