Page 446 - Fundamentals of Enhanced Oil and Gas Recovery

434 Afshin Tatar

acids [771]. Several studies have proved the ability of bioproducts generated by micro-
organisms to increase the water viscosity [15,24,768,772].

10.11.4 Selective Plugging To Modify the Permeability Profile
In some reservoirs, there are highly permeable streaks, which reduce the overall water
flooding sweep efficiency. As it would be explained in further details in the following
sections, there is a fact that biomass preferentially plugs the large pores [719,720],which
conduct the majority of the fluid flow. This would divert the flow to the much smaller
pores, which results in permeability reduction. Based on the selective plugging concept,
microorganisms can block the highly permeable channels (thief zones) and direct the
floodwater to the less permeable zone with immobilized water to enhance oil recovery.
The permeability profile modification will increase the sweep efficiency and subse-
quently more oil will be produced. Laboratory experiments have proved that permeabil-
ity reduction is higher in higher permeable channels compared with lower permeable
ones [773,774]. Permeability modification depends only on the microbial growth and it
is independent of the bacteria type. It is mentioned that generation of biopolymers is
more effective than biomass accumulation [15]. Moreover, emulsions can become ade-
quately thick to promote the channeling for oil recovery [775]. In fact, permeability
reduction occurs as the result of combination of pore throat plugging due to occlusion
of bacterial cells and biofilm formation, which retards the fluid flow [17]. Considering
the microbial activities, plugging may occur through three different mechanisms of
1. Physically via flourish of highly viscus organic matter such as biopolymers and bio-
films [22,31,776] and emulsions [31,775];
2. Biologically via formation of biomass [22,777]; and
3. Chemically via precipitation of inorganic minerals such as calcium carbonate due
to the metabolite CO 2 [22,695,696].
Myriad studies have reported about the microbial induced selective plugging
[629,639,695,696,712,714,778 783]. Based on Fink [783] experiments, after inject-
ing a mixture including biopolymer and bioacid producing bacteria into the Berea
Sandstone core, the permeability was decreased from 850 to 2.99 mD and 904 to
4.86 mD. Four criteria are proposed in literature to conduct and effective selective
plugging by biomass [777]. These criteria suggest
1. The cells must be transported through the rock matrix;
2. Necessary nutrient for growth should be supplied;
3. The bacteria community should adequately grow and/or generate bioproducts for
selective plugging; and
4. The growth rate must be under control and should not be so rapid that it chokes
the wellbore.