Page 312 - Fundamentals of Enhanced Oil and Gas Recovery

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10.5.1 Microorganisms Based on Origin
Based on the origin, microorganisms are categorized as:
1. Indigenous (autochthonous) and
2. Exogenous (allochthonous or foreign).
Indigenous microorganisms have been originally in place and are not artificially
transferred or injected. Various studies have reported isolation of different microor-
ganisms, however in some cases there is suspicion that the microbes may have an
external source like injection of seawater [125]. Without direct injection of target
microorganisms, waterflooding may introduce some microorganisms to the forma-
tion. Even reinjecting the produced water, which has been exposed to the surface
condition, may introduce surface microorganisms to the reservoir [6]. In addition to
inoculation with the surface microorganisms, it is likely that the injected water
(as brine of seawater) change the reservoir geochemistry even permanently. For
example, presence of oxygen or sulfate may change the structure of the pristine
microbial consortia [6]. There is a common fact that MEOR is always applied on
mature fields. A mature field is defined as a field that has passed its production peak
and production is declining. It is unlikely that the microorganisms present in such
fields are the real indigenous ones and represent the biome presented before start of
oil production [22]. Even in fields with no history of waterflooding, still there is a
possibility of introduction of new microorganism species during drilling or well
equipment operations. Damaged tubings might be another potential source thor-
ough fluid leaking. Petroleum industry frequently uses biocides to avoid bacterial
induce problems such as souring and corrosion problems. Although the microbial
colonies will grow and flourish again in the treated area, the microbial dynamic is
not original anymore [22]. Stringent sampling methods should be employed to iso-
late the bacteria from the oil reservoirs. Some expounded and detailed procedures
have been proposed [126 128]; however, they are too expensive to be frequently
used for sampling the hydrocarbon reservoirs [6]. Considering the economic issues,
Magotetal. [129] specified that wellhead sampling is the only way of collecting
samples from petroleum reservoir, which should deal with several sources of con-
tamination. In order to have the best judgment about the origin of the recovered
microorganisms, Magot [130] proposed two key criteria:
1. Comparing the isolate’s growth optima with the in situ conditions in the oil reser-
voir and
2. Comparing the global distribution of the strain’s phylotype in oil reservoir samples
from all over the world.
Youssef et al. [6] argued the first criterion by mentioning some counterexamples.
The authors mentioned reported existence of thermophilic isolates with much lower
temperature optima than their ecosystem [131], thermotolerant isolates with a low