Page 454 - Fundamentals of Enhanced Oil and Gas Recovery

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processes barotolerant microorganism will be necessary rather than the barophilic
ones. Barotolerant microorganism can grow at severe pressures but their optimal
growth does not depend on the high pressures. The pressure tolerance is dependent
on the prevailing biophysical condition [825]. Microbial growth at high pressures
depends on energy source present, inorganic present, Eh (redox potential), pH, and
temperature [25]. Salts such as NaCl and also divalent cations such as Mg, Mg 21 , and
21
Ca , which are common in in petroleum reservoirs, can confer a greater pressure
tolerance to some marine organisms [25,826 828].

10.12.5 Salinity
The importance of aqueous phase on microbial growth and metabolisms was debated
in Section 10.5. The origin of the water found in mostly marine. As it was mentioned
before, water is injected to the reservoir during EOR processes.
Reservoir water may contains high dissolved salt contents as high as 0.1 % to satu-
ration [829]. Sodium chloride (NaCl) accounts for up to 90% of the total dissolved
solids found in the reservoir brine [84]. Capability to tolerate the NaCl is an
essential key factor for microorganisms to be used in MEOR [25]. Donaldson et al.
[25] specified more likely candidate halophiles for MEOR processes would be those
capable to grow over a wide range of salinities, often referred to as moderate halo-
philes. A study by Zahner et al. [39] testified successful MEOR at formation salinities
as high as 140,000 ppm TDS. However, as it was mentioned earlier, most of the
microorganisms utilized for MEOR cannot function in salinities greater than
100,000 ppm [23]. Grula et al. [698] reported about a significant reduction in solvents
and gases production by Clostridia sp. at NaCl concentrations as high as 5% w/v.
However, there are reports regarding isolation of halophilic methanogens among
which one microorganism can grow optimally at NaCl concentration of 15% w/v
[272,830,831]. It is found that moderate homophiles can dominate the extreme halo-
philes at high salinities and limiting nutrient concentrations, which is commonly the
condition for MEOR processes [25,832]. Fujiwara et al. [833] stated that salinity along
with pH affects the enzymatic activity and change the membrane thickness and cellu-
lar surface. Salinity of the brines acquired from the oil fields spans in range of a few
thousand to 463,000 ppm TDS [834]. In addition, it is likely that the salinity gradient
be different in the range of the same formation [84]. Donaldson et al. [25] mentioned
that there is positive relationship between the microorganisms’ growth at high sali-
nities and their ability to grow at high temperatures, the condition, which is common
in oil reservoir [835 838]. Extensive waterflooding of the high saline reservoir with
low saline water may reduce the overall salinity and make the reservoir suitable for
MEOR.