Page 126 - Hydrocarbon Exploration and Production Second Edition

Reservoir Description 113

largely unidentified. Gaseous nitrogen reduces the calorific value and hence sales
price of the hydrocarbon gas. Natural gas containing significant quantities of
nitrogen must be blended with high calorific value gas to maintain a uniform
product quality.
Oxygen compounds are present in some crude oils, and decompose to form
naphthenic acids upon distillation. These may be highly corrosive.
Carbon dioxide (CO 2 ) is a very common contaminant in hydrocarbon fluids,
especially in gases and gas condensate, and is a source of ‘sweet’ corrosion problems.
CO 2 in the gaseous phase dissolves in any water present to form carbonic acid
(H 2 CO 3 ) which is highly corrosive. Its reaction with iron creates iron carbonate
(FeCO 3 ):
Fe þ H 2 CO 3 ) FeCO 3 þ H 2
The corrosion rate of steel in carbonic acid is faster than in hydrochloric acid!
Correlations are available to predict the rate of steel corrosion for different partial
pressures of CO 2 and different temperatures. At high temperatures the iron carbonate
forms a film of protective scale on the steel’s surface, but this is easily washed away at
lower temperatures (again a corrosion nomogram is available to predict the impact of
the scale on the corrosion rate at various CO 2 partial pressures and temperatures).
CO 2 corrosion often occurs at points where there is turbulent flow, such as in
production tubing, piping and separators. The problem can be reduced if there is
little or no water present. The initial rates of corrosion are generally independent of
the type of carbon steel, and chrome alloy steels or duplex stainless steels (chrome
and nickel alloy) are required to reduce the rate of corrosion.
Other compounds which may be found in crude oil are metals such as vanadium,
nickel, copper, zinc and iron, but these are usually of little consequence. Vanadium, if
present, is often distilled from the feedstock of catalytic cracking processes, since it
may spoil catalysis. The treatment of emulsion sludges by biotreatment may lead to
the concentration of metals and radioactive material, causing subsequent disposal
problems.
Natural gas may contain helium, hydrogen and mercury, though the latter is
rarely a significant contaminant in small quantities.

6.2.1.6. Classification of crude oils for refining
There are a total of 18 different hydrocarbon series, of which the most common
constituents of crude oil have been presented – the alkanes, the cycloalkanes and the
arenes. The more recent classifications of hydrocarbons are based on a division of the
hydrocarbons into three main groups: alkanes, naphthenes and aromatics, along with
the organic compounds containing the non-hydrocarbon atoms of sulphur, nitrogen
and oxygen.
As a general guide, crude oil is commonly classified in the broad categories of
paraffinic, naphthenic (meaning that on distillation the residue is asphalt rather than
a wax) or intermediate. These classes act as a guide to the commercial value of
the refined products of the crude oil, with the lighter ends (shorter carbon chains)
commanding more value. Figure 6.16 indicates a first-stage fractional distillation of
crude oil.