Page 422 - Fundamentals of Enhanced Oil and Gas Recovery

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10.5.2.4 Nitrate-Reducing Bacteria
It is well established that nitrate or nitrite is effective in controlling souring
[138,346 360]. This increased the interest in nitrate or nitrite-reducing bacteria
(NRB) in oil fields [345,346,361,362]. NRB can be classified in two main classes of
heterotrophic NRB (hNRB) and sulfide-oxidizing NRB (SO-NRB) [361]. Various
NRB have been isolated from oil fields [186,333,334,350,363 367]. More details
about the NRB and their action can be found in Section 10.12.14 and also literature
[6,345].

10.5.2.5 Iron-Reducing Bacteria (IRB)
IRB have been isolated from several oil reservoirs [177,286,334,368,369]. Further
details about IRB is available in literature [6,345].

10.5.3 Microorganisms Based on Metabolic Processes

Considering the metabolic processes, microorganisms are classified as aerobic and
anaerobic. Through the metabolism, which is a chemical process, a living organisms
gain energy and nutrient to continue living [63]. Aerobes employ oxygen (O 2 ) as the
terminal electron acceptor while the anaerobes incorporate inorganic compounds
such as sulfate, nitrate, or carbon dioxide [63]. The mentioned four compounds sup-
ply the metabolic needs for almost all reservoir microorganisms [63].
Volk et al. [22] mentioned the higher potential of anaerobic processes in MEOR
considering the particular microbiology of the oil field waters, which is vastly biased
toward the anaerobic microorganisms. For example, the studies on the oil fields in
Western Canada revealed none of the oil field waters contain high numbers of aerobic
microorganisms [370,371]. Aerobic condition is not common in the petroleum reser-
voirs. The aerobic microorganisms would have consumed the existed oxygen already.
The other point is that iron and sulfur, which are commonplace in oil reservoirs,
deplete the free oxygen too [22].

10.5.3.1 Aerobic Microorganisms
Aerobes metabolism is the most energy efficient of the mentioned four processes and
can reproduce at a rapid rate [63]. This chemical process can be represented as
oil 1 O 2 1 bacteria-CO 2 1 H 2 O 1 more bacteria (10.4)

In a few early filed trials, aerobic bacteria were utilized. Injection of oxygen as
well as nutrients to stimulate the hydrocarbon metabolism was an early method for
additional oil recovery [24,372,373]. In this respect, several researchers provided evi-
dences linking microbial activity with oil recovery [18,275,276,374 386]. Most of
the underground wells lack oxygen. Injection of oxygen to the well may result in
metal corrosion and damages to the equipment and downhole piping [24]. Moreover,