Page 444 - Fundamentals of Enhanced Oil and Gas Recovery

432 Afshin Tatar

properties and employing multiple mechanisms is an effective strategy for oil recovery
[77,88,693,737]. Microorganisms can produce a broad range of biochemicals includ-
ing biosurfactants and bioemulsifiers, biopolymers, biomass, bioacids, biosolvents, and
biogases. As it was mentioned before, several parameters affect the subsurface micro-
bial life including [119 121]:
1. Chemical factors such as nutrient composition, electrolyte composition, redox
potential (activity of electrons (Eh)), and activity of hydrogen ions (pH);
2. Physical factors such as pressure, temperature, salinity, pore size, and pore geome-
try, porosity, permeability, dissolved solids; and
3. Biological factors such as cytotoxity of the microbial metabolites and also specific
type of microorganisms.

10.11.1 Hydrocarbon Metabolisms and Biodegradation
Owing to their structural properties, hydrocarbons exhibits low chemical reactivity
and their degradation demands special biochemical reactions [738]. Oxygen plays the
main role in aerobic hydrocarbon degradation, which is the terminal electron acceptor
and provide highly reactive oxygen species. Microbial degradation of alkanes and
other aliphatic oil components has been investigated since the beginning of the 20th
century [739 742]. In the 1990s, the anaerobic alkane degradation was discovered
[743]. As a matter of fact, anoxic or anaerobic condition is dominant in many natural
environments including the oil reservoirs [443,744]. Hydrocarbon biodegradation in
anoxic condition is probably an evolutionary metabolic trait of microorganisms [738].
It seems that microbial degradation of long-chain n-alkanes under anaerobic condi-
tions is more realistic and relevant for MEOR [121]. Anaerobic hydrocarbon biodeg-
radation can affect the geochemistry of the petroleum reservoirs and also is applicable
for bioremediation of contaminated sites [738].
Paraffin available in the crude oil may induce formation damages. Different meth-
ods including chemical (such as using solvents, dispersants, surfactants, and wax crystal
inhibitors) and physical (such as physical scarping, downhole electrical heating, using
hot hydrocarbons or water) methods are alternatives to remediate this problem to
keep the wells producing [453,745 751], which are usually expensive [6].Asitwas
mentioned before, the most common microbial approach to deal with the paraffin
deposits is stimulation of in situ hydrocarbon metabolism [6]. Based on literature, the
wells treated with hydrocarbon degrading microorganisms exhibit less paraffin deposi-
tions on the production equipment such as sucker rods and do not require as frequent
treatment with hot oil [615,691,752,753]. This would significantly decrease the opera-
tional costs and increase the economic lifetime of the producing wells [412].
However, some studies have reported the mentioned method is ineffective or incon-
clusive results were acquired [748,754].