Page 459 - Fundamentals of Enhanced Oil and Gas Recovery

Microbial Enhanced Oil Recovery: Microbiology and Fundamentals 447

which out-compete the SRB [37,354,877]. The mechanisms through which nitrate
addition can control souring are [6,863,878 882]
1. Competition for electron donors between the SRB and NRB;
2. Promoting the redox potential and consequently inhibition of SRB;
3. H 2 S oxidation by NRB; and
4. Production of incompletely reduced nitrogen compounds such as NO 2 , which
inhibit the sulfate reduction pathway.
In different cases, it is probable that one mechanism dominate the others or multiple
mechanisms do simultaneously [863]. As it was mentioned before, NRB are categorized
as hNRB and SO-NRB. The former outcompete the SRB for common electron
donors due to the fact that nitrate or nitrite reduction is more favorable than sulfate
reduction in energetical point of view. In other words, nitrate reduction to nitrogen or
ammonia provides more free energy than sulfate reduction [688]. This dictates greater
molar growth yields compared with SRB [37]. The mechanism of SO-NRB is differ-
ent. With nitrate or nitrite as the electron acceptor, SO-NRB oxidize the hydrogen sul-
fide to sulfate or sulfur and do not effect on the SRB growth [6,874,881].Youssef etal.
[6] specified the importance of SO-NRB in decreasing the sulfide concentration
reported by several laboratory experiments [318,865,881,883,884].
Sulfurospirilum spp. is reported to be capable for both hNRB and SO-NRB meta-
bolisms [885]. Thiomicrospira sp. strain CVO and Arcobacter sp. strain FWKO both are
reported to be SO-NRB [363]. Other microorganisms capable to reducing nitrate are
Denitrovibrio acetiphilus, Proteobacteria, Campylobacter sp. strains NO3A, NO2B, and KW,
Garciella nitratireducens, belonging to cluster XII of the Clostridiales, moderately ther-
mophilic members of the genus Geobacillus [333,361,364,878,886]. Gittel [887] men-
tioned that although the activity of hNRB and SO-NRB was not specifically assessed,
the recovery of sequences affiliated with representatives of both types of nitrate redu-
cers, including members of the Epsilonproteobacteria (Sulfurospirillum spp.,Arcobacter spp.)
and the Deferribacterales (Deferribacter spp.), is promising for future studies.

10.13 FIELD TRIALS

The Socony Mobil Research laboratory performed the first MEOR field trial
in the Lisbon field, Union County, Arkansas in 1954 and reported marginal success
due to the increase in the wells oil follow rates [16,31,76 88] and the analyses men-
tioned the complexity of using microorganisms. However, Volk and Liu [32] men-
tioned that the pioneering field studies were performed in the United States in the
1930s and 1940s by Claude ZoBell et al. at the Scripps Institute of Oceanography in