Page 445 - Fundamentals of Enhanced Oil and Gas Recovery

Microbial Enhanced Oil Recovery: Microbiology and Fundamentals 433

It is well proofed that many microorganisms are capable to degrade hydrocarbons
both aerobically and anaerobically [19,388,691,755 757]. Some commercial processes
use proprietary mixtures of hydrocarbon biodegraders to prevent paraffin and asphaltene
deposition [81,615,691,748,753,758,759]. Some microorganisms can attach to the long-
chain hydrocarbons and break them to smaller-chain ones (the proportion of low-
carbon number alkanes to high-carbon number alkanes increases), which owns generally
lower viscosity and consequently better mobility [6,81,759 766].Youssef et al. [6]
mentioned that the conversion process of long-chain alkanes to the short-chain ones is
unclear and there is no microorganisms known to catalyze such a reaction. The other
assumed mechanism for hydrocarbon biodegradation is the partial hydrocarbon transfor-
mation to aldehydes, alcohols, and fatty acids, which could serve as biosolvents or bio-

surfactants [691]. Temperatures higher than 80 C, which is the common condition for
many oil reservoirs, inhibit the oil anaerobic degradation in oil reservoirs [10,119,129].
An excellent review on biodegradation in petroleum reservoir by Head et al. can
be found in literature [119]. Further details about aerobic [743] and anaerobic [738]
hydrocarbon biodegradation can be found elsewhere.

10.11.2 Lowering the Entrapped Oil Viscosity
Two main microbial activities can reduce the oil viscosity [121,767]:
1. Microbial production of metabolites such as biogases, which alter the physical
properties of the oil [119,351,443,768], and
2. Microbial biodegradation of heavy oil components to the lighter ones
[24,338,443,757,760,769,770].
Microbial metabolisms produce carbon dioxide, hydrogen, nitrogen, and methane
gases [24]. In addition, carbon dioxide may be produced through the reaction of met-
abolic acids with carbonate rocks. Provided that sufficient amount of gas is produced
by the microorganisms, it can be absorbed in oil and resulted in a viscosity decrease.
As it was mentioned before, some microorganisms can attach the long chain hydrocar-
bons and break them to smaller chain ones, which owns generally lower viscosity and
consequently better mobility [6,81,759 766]. Biosolvents can reduce the oil viscosity
through dissolution of asphaltene and heavy components existing in the oil [24].
Briefly speaking, biosolvents such as alcohols, ketones, and short-chain hydrocarbons
as well as bioacids and biogases can reduce the oil viscosity [771].

10.11.3 Increasing the Water Viscosity
As it was mentioned before, an increase in the water viscosity promote the sweep effi-
ciency. Another favorable effect is that it may contribute to miscibilization of
entrapped oil and surrounding water. Metabolically generated bioproducts capable to
increase the water viscosity are biofilms, biopolymers, long-chain alcohols, and fatty