Page 453 - Fundamentals of Enhanced Oil and Gas Recovery

Microbial Enhanced Oil Recovery: Microbiology and Fundamentals 441

the proteins acting as the catalyst in chemical reactions. Based on the temperature

ranges the microorganisms can survive, they can classified as psychrophiles (,25 C),

mesophiles (20 45 C), thermophiles (45 80 C), and hyperthermophiles ( . 80 C).

Further details about the microbial thermophily can be found in literature [812 814].
For example, the upper limit temperature tolerance level of Leuconostoc mesenteroides is
40 C [4]. Some microorganisms are reported to be able to survive only up to 80 C

[24], some at 115 C [24] and even higher at 121 C [122]. A study by Maudgalya

et al. [23] reported the best temperature for MEOR projects is less than 93 C. In

addition, Zahner et al. [39] reported the best temperature for MEOR projects is less
than 93 C, which supports the Maudgalya et al. [23] statement. Certain hyperthermo-

philes may be present at reservoir extreme temperature condition [125,150,339,815];
however their indigenous nature is questionable [6,339,815].
10.12.4 Pressure
Depending on the geographical area, the pressure gradient varies between 0.43 and
1.0 psig/ft (0.973 and 2.262 MPa/100 m), however, is some areas the rate of pressure
changes increases with increasing the depth [816]. The considerable hydrostatic pres-
sure, which is in range of several tens of MPa will not prevent the microorganisms’
life but adversely affect the growth of microorganisms, which are adapted to the atmo-
spheric pressure condition [817 819]. Donaldson et al. [25] specified that extreme
pressures impose considerable effects on growth and metabolism of microorganisms.
Based on Schwarz et al. [820], increasing the pressure at ambient temperature will
result in a considerable decrease in the rate of hydrocarbon metabolisms compared
with that in atmospheric pressure. The most applicable pressure for EOR in produc-
ing wells is in range of 20 30 MPa [84]. Pressures lower tan 10 20 MPa generally
do not make a tremendous impact on the microbial metabolism [25]. For many meso-
philic microorganisms, a hydrostatic pressure in range of several tens of MPa will hin-
der the cell growth and it will be completely inhibited at about 50 MPa [821].Itis
reported that some bacteria can only survive at pressure up to 20 mPa [24].Itis
worthwhile to mention that pressure effects may depend on other physicochemical
factors such as pH, temperature, composition of culture media, and oxygen supply
[818,821,822]. In addition, microbial pressure maxima can be affected by the utilized
nutrients by tens of MPa [823]. Not only the pressure value but also the duration of
being exposed to the pressure will affect the microorganisms [821]. Moreover, an
increase in the overburden pressure will reduce the permeability in sandstones [824].
The authors observed that an overburden pressure of 3000 psig decreased the perme-
ability to the 59% 89% of the permeability measured under no overburden pressure.
Hover, above a certain compaction pressure, permeability will not change by further
pressure increase [25]. Donaldson et al. [25] mentioned that for most MEOR