Page 443 - Fundamentals of Enhanced Oil and Gas Recovery

Microbial Enhanced Oil Recovery: Microbiology and Fundamentals 431

growth of biomass may adversely affect the well injectivity [24]. In the porous media,
biomass will occur in a web-like structure, hence, they would be called bioweb at
a certain stage of growth too [708]. There is a wealth of laboratorial evidences
[423,425,639,709 713] and also some reported field applications [646,708,714] that
in situ growth of bacteria and consequent biomass development can significantly
reduce the permeability.
When the microorganisms grow within the reservoir, the molecules on their
surfaces keep them attached to the substrates near where they feed. As the result, a
biofilm is created that prevent the oil to be drawn into the porous zones [715].
Biofilms can be introduced as the populations of bacterial cells attached to a surface
enclosed in an organic polymeric matrix [715 718]. As it was mentioned
before, exopolysaccharides-bound clusters of cells commonly comprises the
biofilms [22].
The biomass displace the oil by its growth. Considering the microscopic sweep
efficiency, the biomass growth in the large pores diverts the fluid flow to the smaller
pores because the large pores receive most of the nutrients [6]. This has been proved
by pore size distribution analysis of fused-glass columns [719] and sandstone cores
[720]. Moreover, biomass can act as the plugging agents and contribute to perme-
ability profile modification. It should be mentioned that biopolymer production is
necessary in addition to biomass development for significant permeability reduction
in fusedglass andsand-packed columns [6,401,719,721 729]. The other favorable
effect is its viscosity and pour point reduction [24].Itisreportedthatbiomass can
play a role in oil emulsification and desulfurization [24].There is afactthatbacteria
tend to develop attached to a surface [715,730]. Biofilms are capable to alter the
physical and chemical characteristics of the surfaces [731,732], thus it is possible
they favorably amend the wettability of the reservoir rock. Some attempts have been
done regarding starving the bacteria to reduce their size and increasing their pene-
tration depth into the formation. After that the nutrient are provided for growing
the bacteria colonies to act as the biomass [420]. In comparison with the bacteria
suspended in the liquid phase, biofilms exhibits more resistance to the biocides
[733 735].

10.11 MEOR MECHANISMS

At first, it should be mentioned MEOR takes place through a combination of
several mechanisms and multiple biochemical processes rather than a single one
[21,43,392,736]; however, it is possible that one mechanism do the most contribution.
Youssef et al. [6] specified that utilizing a consortium of microorganisms with different