Page 122 - Hydrocarbon Exploration and Production Second Edition

Reservoir Description 109

This section will concentrate on the hydrocarbons, but will also explain the
significance of the other compounds in the processing of the fluids.
Petroleum fluids vary significantly in appearance, from gases, through clear
liquids with the appearance of lighter fuel, to thick black, almost solid liquids. In
terms of weight percent of crude oil, for example the carbon element represents
84–87%, the hydrogen element 11–14% and the other elements typically less than
1%. Despite this fairly narrow range of weight percent of the carbon and hydrogen
elements, crude oil can vary from a light-brown liquid with a viscosity close to that
of water, to a very high viscosity tar-like fluid.
The diversity of the appearance is due to the many ways in which the carbon
atoms are able to bond to each other, from single carbon atoms to molecules contai-
ning hundreds of carbon atoms linked together in linear chains, to cyclic arrange-
ments of carbon atoms. It is the ability of carbon molecules to combine together in
long chains (catenate) which makes organic (i.e. carbon containing) compounds far
more numerous than those of other elements, and the basis of living matter.
The various arrangements of carbon atoms can be categorised into ‘series’ which
describe a common molecular structure. The series are based on four main catego-
ries which refer to

the arrangement of the carbon molecules
- open chain (which may be straight chain or branched)
- cyclic (or ring)
the bonds between the carbon molecules
- saturated (or single) bond
- unsaturated (or multiple) bond.

6.2.1.1. The alkanes
The largest series is that of the alkanes or paraffins, which are open chain molecules
with saturated bonds, and have the general formula C n H 2n+2 .
Figure 6.12 shows the way in which the molecules are visualised, their chemical
symbol and the names of the first three members of the series. The carbon atom has
H H H

H C H H C H H C H

H H C H H C H
Methane H H C H
(CH )
4
Ethane
(C H ) H
2 6
Propane
(C H )
3 8
Figure 6.12 Examples from the alkane (para⁄n) series.