Petroleum Formations

Petroleum Formations

Because the development of well construction technology has had a rather

fragmented past and many practitioners are not trained as either reservoir

or production engineers, it is worthwhile here to provide an elementary

description of the targets that the drilling of a well is supposed to reach.

Petroleum Fluids

Petroleum is a mixture of hydrocarbons consisting of about 11 to 13% (by

weight) hydrogen and 84 to 87% carbon. Chemically, "crude" petroleum

may include several hundred compounds, encompassing practically all openchain

and cyclic hydrocarbons of single, double, and triple bonds.

A description of these mixtures by composition was abandoned early in

industry history with the exception of very generic divisions that denote

important distinguishing content (such as, paraffinic or asphaltenic crudes).

Instead, bulk physical properties such as density and viscosity have been

used to describe crude behavior.

Specifically, the phase and thermodynamic behavior has been reduced to the

simplifying division of crude petroleum into (liquid) oil and (natural) gas.

While such a description is apparent and relatively easy to comprehend

given a temperature and pressure, crude petroleum content is generally

referred to as volumes at some standard conditions (for example, 60°F and

atmospheric pressure). With the definition of pressure and temperature, a

volume unit also clearly denotes mass.

Oil, then, consists of higher-order hydrocarbons such as C6+ with much

smaller and decreasing quantities of lower-order hydrocarbons, while gas

consists of lower order hydrocarbons—primarily methane, and some

ethane—with much smaller amounts of higher-order hydrocarbons.

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