Page 440 - Fundamentals of Enhanced Oil and Gas Recovery

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produce lactate [6]. Microorganisms including Clostridium, mixed Acidogens,
Dsulfovibrio, and Bacillus generate bioacids such as carboxylic acids of various molecular
weights, low molecular weight fatty acids, formic acid, propionic acid, (iso)butyric
acid, etc. [22,24]. Bacteria such as Clostridium produce acetate and butyrate, which
will produce corresponding bioacids [688]. Lactobacillus sp. and Pediococcus sp. are the
other bacteria reported to be acid-producing [686]. Moreover, CO 2 and H 2 S biogases
are the other microbial products dissociation of which into the water makes acids
[22]. Different studies suggest that aerobic hydrocarbon degraders can produce a mix-
ture of bioacids and bioalcohols, which furtherly would be converted to methane by
the methanogenic consortia [275,276,375,689,690].

10.10.4 Biosolvents

A possible microbial action is partial oxidization of hydrocarbons to biosolvents such
as alcohols, aldehydes, and fatty acids [691]. As it was mentioned before, biosolvents
can partially dissolve the carbonate rocks and consequently improve the reservoir per-
meability and porosity. The other action by the biosolvents, which improve the per-
meability, is dissolving heavy components from pore throats. Biosolvents can reduce
the oil viscosity through dissolution of asphaltene and heavy components existing in
the oil. In addition, dissolution of biosolvents in oil could reduce the viscosity too
[17]. The other advantage of biosolvents is they have cosurfactant effects and are capa-
ble to reduce the IFT between oil/water and oil-rock [22,24]. Solvent can alter the
rock wettability at the rock 2oil interface [17]. Good solvent candidates for MEOR
application include mainly lower alcohols (methanol, ethanol, 1-propanol, propan-2-
diol, 1-butanol, which are water soluble), volatile fatty acids, and ketones such as pro-
panone (acetone) and butanone [15,22,692]. As it was mentioned before, members of
the genera Bacillus and Clostridium are the most commonly used microorganisms to
produce bioacids, biogases, and biosolvents [6,88,114,397,408,567,685,687].
Clostridium spp. can produce alcohols of ethanol and butanol and also acetone as bio-
solvents. Bacillus spp. produce alcohols of ethanol and 2,3-buanediol [6]. In addition,
LAB can produce CO 2 too [6]. Some important reported biosolvent generating bacte-
ria are Zymomonas mobilis, Clostridium acetobutylicum, Klebsiella, Arthrobacter, and
Clostridium pasteurianum [24,77].
Patel et al. [31] mentioned that biosolvents are unlikely to be produced in large
enough volumes for direct injection into the wellbore. Considering the current tech-
nology, the best practice seems to be stimulation of the indigenous or injected micro-
organisms to generate the biosolvents rather than direct injection of generated
biosolvents in laboratory.