Page 458 - Fundamentals of Enhanced Oil and Gas Recovery

446 Afshin Tatar

10.12.14 Souring Due to the Presence of
Sulfate-Reducing Bacteria (SRB)
SRB plays a very negative role in MEOR processes [15,21,129]; however, recently
some positive roles are reported [304,862] as they are capable to reduce the oil viscos-
ity, replenish the declining pressure of reservoir, transform the heavy oil to light oil
via the effect of produced acids, and gas (H 2 S) as well as their wide availability in
global oil reservoirs [304]. In addition, it is reported that SRB nonselectively plug the
porous media [31], which adversely affect the oil recovery.
Reservoir flooding by the seawater or brine containing a high level of sulfate can
be the starting point for souring due to sulfide production [6,37,352]. It is worthwhile
to mention that SRB can survive extended starvation in sea water at both reservoir
and surface temperatures [15]. Suitable condition for SRB to produce hydrogen sul-
fide can be provided by [6,37,863]
1. Supplying nitrogen, sulfate, and phosphorous sources by the injected water;
2. Reducing the reservoir temperature by the injected water, which is cooler; and
3. Presence of the electron donors (organic acids and hydrocarbons) in the reservoir.
The deleterious effects of presence of hydrogen sulfide or in other words souring
can be summarized as [318,354,362,370,864 866]
1. Corrosion of pipelines and equipment;
2. Inducing additional costs to refine oil and gas;
3. Increasing the health risks as H 2 S is highly toxic; and
4. Plugging the reservoir due to accumulation of sulfides minerals.
The microbially-influenced corrosion (MIC) may be the most important detri-
mental effect of the SRB. The cost related to MIC is hundreds of millions of dollars
per year [867,868]. The role of SRB on corrosion of ferrous metals has been reviewed
by several researchers [310,315,554,867,869 871].
Biocides such as bronopol, formaldehyde, glutaraldehyde, benzalkonium chloride,
cocodiamine, and tetrakishydroxymethyl phosphonium sulfate are generally used to
control the H 2 S concentration [6,872]. Employing biocides is associated with some
problems such as the need for high concentrations to achieve the desired results
[863,873,874] and health concerns for the operators [6] as well as hazardous to the
environment. However, there have been some efforts to evolve green and biodegrad-
able biocides to remediate the SRB problem [875]. Addition of nitrate, nitrite,
molybdate, and inorganic nutrients to the oil formation is suggested as an alternative
to inhibit sulfate reduction and also stimulate the indigenous microorganisms to pro-
duce CO 2 [876]. As it was mentioned before, it is well established that nitrate or
nitrite is effective in controlling souring [138,346 360]. This increased the interest in
NRB in oil fields [345,346,361,362]. In case of using as the bacterial electron accep-
tor, nitrate provides more energy than sulfate, thus, the growth of NRB is enhanced,