Page 107 - Geology and Geochemistry of Oil and Gas

WATER DRIVE 79
(c) bromine — does not have a genetic relationship to oil; however, many oilfield
waters characteristically contain high concentrations of bromine;
(d) boron — supplementary hydrochemical indicator, which is more common in the
bicarbonate type of waters;
(e) ammonium — indirect hydrochemical indicator; and
(f) barium and strontium — supplementary indicators, which do not have genetic
relationship to petroleum.
The presence of organic components is the most typical feature of oilfield water.
The presence of liquid and gaseous hydrocarbons in water facilitates reduction
processes (especially near the hydrocarbon accumulations) and the appearance of
various organic substances in water.
The reduction of sulfate ions results in the formation of hydrogen sulfide. The
generated hydrogen sulfide diffuses into the surrounding rocks and reacts with the
oxides of iron, forming pyrite and siderite. The rocks even change in color from
reddish and greenish to gray and dark-gray. These indications are considered as
favorable in exploration. The presence or absence of sulfates in the subsurface water is
not an indicator: concentration of sulfates in water depends not only on the reduction
2
processes but also on the input of the SO 4 ions from the surrounding rocks. The
sulfate reduction is possible at rather high temperatures (300–500 1C). The writers
believe that the sulfate reduction may occur at lower temperatures, at a slow rate, in
the low-pH environments. Microorganisms can reduce sulfates at temperatures even
as low as 70 1C. For example, Desulfovibrio desulfuricans in injection waters can
reduce sulfates to sulfides and convert ‘‘sweet’’ oilfield waters into ‘‘sour’’ ones.
Oilfield waters are highly reduced. The extent of the reduction is evaluated from
the amount of oxidizing substance, potassium iodate or permanganate. The oilfield
waters are also enriched in volatile and non-volatile phenols and fatty and
naphthenic acids. Up to a temperature range of 120–150 1C, liquid hydrocarbons are
barely soluble in water. Currently, the only compound that was clearly identified was
benzene, which is typical for water in the accumulations of highly aromatic
condensates. When water is moving, gaseous hydrocarbons soluble in water form, in
terms of their concentration, front and back effects. Same effects are typical for all
other geochemical indicators.
Oilfield waters contain iodine, bromine, boron, aluminum, and mercury. Gas and
gas-condensate accumulations usually have elevated amounts of iodine in brines.
High-salinity brines contain bromine, whereas alkaline water is enriched in boron.
Formation water of gas accumulations contains mercury. Various nitrogen-rich
substances (such as amines and complex heterocyclic compounds including pyridine)
are also found in brines.

4.3. WATER DRIVE
Subsurface water is encountered in a solid state (‘‘ice’’), liquid state, and steam (at
high temperature). Interstitial water may be free to move or is bonded (residual,
irreducible water). Free water is subdivided into gravity water and capillary water.