Page 447 - Fundamentals of Enhanced Oil and Gas Recovery

Microbial Enhanced Oil Recovery: Microbiology and Fundamentals 435

It has been reported that the Bacillus licheniformis BNP29 is a suitable strain for selec-
tive plugging and fulfill the mentioned four criteria [545].In contrast, SRBnonselec-
tively plugs the porous media and adversely influence the oil recovery. This shows the
importance of stimulating the proper bacteria type. As it was mentioned, the other
probable plugging agent is the inorganic biomass. Literatures indicate the capability of
microorganisms to enhance precipitation processes generating solid sulfides [784,785]
and carbonates [786 789]. The mentioned processes can be influenced by water chem-
istry, surface charge, nutrients, pH, fluid flow, and microbial physiology [17,618,619].

10.11.5 Dissolution of Some Parts of Reservoir Rocks

Bioacids and biosolvents such as acetone and ethanol can dissolve some parts of the car-
bonate rock to increase the porosity and improve the permeability and make it easier to
access the hidden oil during the flooding [31,391]. Carbonate dissolution has been
regarded as the reason for higher porosity [76,685]. It is possible to metabolically pro-
duce the bioacids and biosolvents using microorganisms. Acetic and propionic bioacids
are particularly used for this aim [15]. Siegert et al. [10] mentioned bioacids produced
by the microorganisms hydrolyzes the carbonates. Core flood experiments has proved
that carbonate dissolution may mobilize the entrapped oil from carbonate rocks [8].
This method is also effective in sandstones cemented by carbonate minerals [66].

10.11.6 Wettability Alteration
Wettability has a key role in controlling the location, flow, and distribution of fluids in
a reservoir as well as in multiphase flow problems such as oil migration from the
source rocks and EOR [790,791]. Favorably changing the wetness state can improve
the spontaneous imbibition of water. This would promote the waterflooding perfor-
mance and consequently the oil recovery. Microbial activities can induce the desired
alteration by different mechanisms in which produced biosurfactants, biopolymers,
and biomass and even enzymes are involved such as [66]:
1. Direct attachment of the microorganism to the matrix surface. In such a condi-
tion, the contact angle is governed by a heterogeneous mixture of mineral and
bacterial surface properties. This mechanism is applicable only in large pores that
can be accessible by the bacteria. The pore size should be at least 1 μm;
2. Absorption of produced biochemicals on the mineral surface. Microbial metabo-
lites such as biosurfactants can be absorbed on the mineral surface and potentially
change the wettability by altering the surface to be more or less hydrophobic.
There is no pore-size limitation for this mechanism; and
3. Coating the minerals by biopolymers or exopolysaccharides. This mechanism is
somehow analogous to the direct attachment of the microorganisms to the mineral
surface. As the polymers are associated with cells, this mechanisms is corresponded