Page 310 - Fundamentals of Enhanced Oil and Gas Recovery

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performed between 1971 and 1982, reporting successful results [31,104]. The field trials
performed in the mentioned countries employed mixed anaerobic or facultative anaero-
bic bacteria, which were selected based on their capability to produce adequate volumes
of biochemicals and also biomass [16]. Youssef et al. [6] mentioned the improvement in
the technology of MEOR as using mixture cultures adapted to the nutrients and reser-
voir condition such as temperature and pressure and also incorporating larger volumes
of nutrients [105 114]. By the end of the 20th century, MEOR was proved as a scien-
tific and interdisciplinary approach for promoting oil recovery [16].
In 2003, Van Hamme et al. [19] stated that more than 400 MEOR field projects
have been done so far just in the United States. Based on Khire and Khan [43,115],
MEOR projects has been applied on over 400 wells in the same country. In addition,
this recovery method has been tested on more than 1000 wells in numerous oil fields
in China [32]. Based on Thomas [116], an estimated amount of 2.5 million oil barrels
per day were produced in 2007 using EOR method of which the role of MEOR was
negligible. On the other hand, based on a report by Chinese Ministry of Land and
Resources (www.mlr.gov.cn), nearly 50 billion oil barrels in onshore Chinese oil fields
have the potential to be treated by MEOR [32]. Youssef et al. [6] specified that the
residual oil recovery in many MEOR field trials increased by 10% 340% for 2 8
years [78,92,99,101,106,108,117].

10.5 MICROBIAL ECOLOGY

Microbes are single-celled organisms existing everywhere in the nature,
including the hydrocarbon reservoirs [20]. In other words, it is possible to find
microbes everywhere in biosphere [118]. Several groups of microorganisms includ-
ing bacterial and archaeal communities have been isolated from oil fields, which are
different in physiological and metabolic abilities and phylogenetic affiliations [6].
Microbial life may extend the biosphere up to 4 km below the surface [118].Single-
celled bacteria presented in the oil reservoirs live in the aqueous phase and take in
water through the cell membranes [63]. Accessible water is the most important
resource for microorganisms to survive [63]. More than two-thirds of a bacterial cell
is composed of water. Water provides the nutrients and various vitamins essential for
cell construction. Moreover, the unwanted waste products are removed via hydra-
tion [63]. Many bacteria found in the oil reservoirs are heterotrophic, i.e., they are
not capable to produce their own food and must gain energy and carbon via con-
suming organic substances [63]. As it was mentioned, the microorganisms are in
aqueous phase, thus, the only way is to obtain the nutrient from sources, which are