Page 441 - Fundamentals of Enhanced Oil and Gas Recovery

Microbial Enhanced Oil Recovery: Microbiology and Fundamentals 429

10.10.5 Biogases
Microorganisms can produce biogases of H 2 (which is then rapidly consumed by fur-
ther microbial activity), H 2 S, N 2 ,CH 4 , and CO 2 by fermenting the carbohydrates
[15,22,24].H 2 is produced in large volumes in anaerobic environments; however, it is
then quickly consumed by methanogens (reducing CO 2 to CH 4 ), SRB (reducing sul-
fate to sulfide), homoacetogenic bacteria (reducing CO 2 to acetic acid), and NRB
(reducing nitrate to N 2 ) [6,17]. As it was mentioned before, the other source for CO 2
can be the bioacids reaction with rock minerals. The biogenic gases can enhance oil
recovery via different mechanisms of:
1. Reducing the oil viscosity;
2. Repressurization of reservoir [144,407];
3. Swelling the oil [693];
4. Altering pH of the formation water [694];
5. Changing IFT [22];
6. Increasing permeability by dissolving the carbonate rocks [24];
7. Bioplugging via precipitation of inorganic minerals such as calcium carbonate
(CaCO 3 ) due to the metabolite CO 2 [22,695,696]; and
8. Changing pour point of oil [22].
Biogas production is reported to be an important mechanisms for oil recovery
[697]. Several laboratory experiments using different microorganisms such as
Enterobacter sp. [697], Clostridium strains [698], Vibrio sp. and Bacillus polymyxa [124],
Streptococcus sp. [699], Staphylococcus sp. [699], Clostridium acetobutylicum [114,408],have
shown the effect of biogas production on enhanced oil recovery. The produced gas
can be absorbed by the oil and consequently decrease its viscosity. Lower viscosity
facilitates the oil displacement. The other function of generated biogases can be the
repressurizing the reservoir [144,407]. The induced pressure will exert extra forces to
expel oil out of the pores [23,700]. The other beneficial effects of biogases are swell-
ing the oil volume, displacing the immobile oil, and increasing the permeability by
dissolving the carbonate rocks [24]. Induction of biogenic CO 2 into the formation
water and consequent pH reduction might promote oil producibility, especially in
fractured carbonate rocks [694]. It is unlikely that the generated biogases enhance oil
recovery through miscible displacement due to limited generated volumes [24,407].
Production of biogases may be stopped at some points when the gas is swollen
enough. This let production of more valuable components [10]. 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. produces gases of CO 2 and H 2 and Bacillus spp. produces
CO 2 . In addition heterofermentative LAB produce CO 2 too [6]. Several different
methanogens generate methane. Formation of mineral precipitates is another