Page 423 - Fundamentals of Enhanced Oil and Gas Recovery

Microbial Enhanced Oil Recovery: Microbiology and Fundamentals 411

oxygen is an electron acceptor compound that may cause imbalances in the microbial
environment [31]. The undesired effects of the air injection into the reservoir and
doubts in their effectiveness dispirited their utilization [23]; however, still there are
supporters for aerobic MEOR applications [18,387].

10.5.3.2 Anaerobic Microorganisms
As it was mentioned before, most of anaerobes available on oil reservoirs utilize car-
22
2
bon dioxide (CO 2 ), sulfate (SO ), or nitrate (NO ) [63]. Methanogens incorporate
4 3
CO 2 for their metabolic processes and produce methane (CH 4 ):
oil 1 CO 2 1 bacteria-CH 4 1 more bacteria (10.5)

This biodegradation process is the least energy efficient of the mentioned four
metabolic processes [63].
22
SRB employ the sulfate (SO ) as the terminal electron acceptor. Through this
4
metabolic process, sulfate is reduced to hydrogen sulfide, which is a notorious biopro-
duct due its deleterious corrosive effects on equipment and pipelines.

oil 1 SO 22 1 bacteria-CO 2 1 H 2 S 1 more bacteria (10.6)
4
2
Nitrate-Reducing Bacteria (NRB) employ nitrate (NO ) as the terminal electron
3
acceptor for the metabolic process, which is called denitrification [63].Thisprocess is
the most energy efficient among the anaerobic processes and involves the stepwise reduc-
2
2
tion of nitrate (NO ) to nitrite (NO ), nitric oxide (NO), and nitrogen (N 2 ) [63]:
3 2
2
oil 1 NO 1 bacteria-CO 2 1 N 2 1 more bacteria (10.7)
3
In case of injecting adequate amount of nitrate, the microbial activity shifts to
denitrification. As nitrate reduction is more energy efficient, NRB can outcompete
the SRB so that H 2 S production will reduce. More details are provided in
Section 10.12.14.
Based on literature, the capacity of some bacteria to anaerobically metabolize
hydrocarbons by microorganisms was not recognized until the late 1980 [388]. Far
ahead, researchers reported that microorganisms can anaerobically degrade the oil in
the subsurface within the reservoir [389,390]. More details are provided in
Section 10.11.1. Most of the field trials with the high percentage of success have used
anaerobic microorganisms [24]. There is an active area of research on anaerobic extre-
mophiles such as the ones tolerant to extreme salinity (halophiles), pressure (piezo-
philes or barophiles), and temperature (thermophiles) to develop better adaptation to
the reservoir harsh condition [391 394].