Page 313 - Fundamentals of Enhanced Oil and Gas Recovery

Microbial Enhanced Oil Recovery: Microbiology and Fundamentals 301

temperature optima from high-temperature environments [132], and halophilic and
halotolerant microorganisms recovered from salt crystals with a relatively low salt
tolerance [133 135]. They modified the criterion as considering range (minimum
and maximum growth limits) or the ability to survive for prolonged periods of time
at the in situ reservoir condition. However, they mentioned the difficulties associ-
ated with evaluating the growth limits of slow-growing recovered microorganisms
due to the probability of resulting false negatives because of extended incubation
period.
Several studies have reported microorganisms isolated from the petroleum reser-
voirs or effective ones in degrading the petroleum. Table 10.1 lists some of the men-
tioned microorganisms along with the corresponding taxonomy and isolation source.
The taxonomic details are acquired from the SILVA [136].

10.5.2 Microorganisms Based on Action
Based on their actions, microorganisms for MEOR can be classified as the main cate-
gories of:
1. Methanogens;
2. Sulfate-reducing bacteria (SRB);
3. Fermentative microorganisms;
4. Nitrate-reducing bacteria (NRB); and
5. Iron-reducing bacteria (IRB);
The mentioned classes are explained briefly in following.

10.5.2.1 Methanogens
Methanogenesis is a microbial metabolism through which methane is generated by
microorganisms called methanogens. This is a common process in the oil fields and
the corresponding first report was prepared in the early 1950s [268,269] and then was
further elucidated especially by Russian scientists [270 277]. Through methanogen-
esis, methanogens metabolize the substrates of hydrogen and CO 2 , methylamines, ace-
tate, and dimethysulfides and produce methane as a biogenic gas [6]. All methanogens
belong to the domain Archaea, phylum Euryarchaeota [278]. These microorganisms
are distributed in four classes and five orders of Methanobacteriales, Methanococcales,
Methanomicrobiales, Methanosarcinales, and Methanopyrales [278]. The ecological niches
of methanogens are widely distributed. All the mentioned orders have been isolated
from oil reservoir except Methanopyrales [6]. Based on the utilized substrate by the iso-
lates from the oil fields, the main categories of methanogens include [6]
1. Hydrogenotrophic methanogens [277,279 288];
2. Methylotrophic methanogens [148,289 293]; and
3. Aceticlastic methanogens [147,171,271,283,294 300].