Page 432 - Fundamentals of Enhanced Oil and Gas Recovery

420 Afshin Tatar

concentration of lipopeptides purified from Bacillus subtilis MTCC 1427 cultures
[450]. This indicates effectiveness of biosurfactants at much lower concentrations.
As the main function of surfactants, these biochemicals reduce the interfacial and
surface tensions, which results in flooding recovery [444]. The roles of biosurfactants
are chiefly (1) altering the surface and IFTs, (2) adsorbing on the immiscible inter-
faces, (3) emulsifying the crude oil, and (4) increasing the mobility of bacterial cells
[15,31,451]. Moreover, they can alter the wettability, clean the contaminated soil, and
promote the long alkyl chains degradation [15,443,444,451]. A sign of biosurfactant
generation is alteration of some characteristics of the crude oil such as viscosity, cloud
point, and pour point [6,452 455]. In MEOR trials, sometimes further compounds
such as different metal ions are mixed with biosurfactants. Because of polar interac-
tions between the biosurfactants and metal ions the action will be improved [456].
Beside several advantageous of biosurfactants, there are some drawbacks associated
with them in MEOR projects. There is a fact that both quantity and quality of bio-
surfactant are affected by nutrient supply and also environmental condition [457].
Biological cytotoxicity of metabolites such as the case for surfactin, which is reported
to exhibits antimicrobial activities [458], may make some constraints in microbial
growth in oil reservoirs [22]. Several microorganisms (including aerobic bacteria,
which are not proper for in situ MEOR purposes [412]) are reported to generate bio-
surfactants; however, only a few of them are proper candidates for MEOR purposes
to be able to sufficiently reduce the oil 2 water IFT [22]. It is likely that biosurfactants
may undergo biodegradation by other microorganism in mixed cultures under fer-
mentative, aerobic, nitrate-reducing, and sulfate-reducing conditions [459,460]. This
makes the surviving of the produced biosurfactants a challenging issue. Comparatively,
the soluble biosurfactant is a much more accessible nutrient for the microorganisms
than the crude oil [66]. Biosurfactant activity and solubility can be influenced by salin-
ity and pH [461,462]. The effective pH range is 4 10. It is found that at pHs less
than 4, many biosurfactants starts to precipitate [461,462]. This can be attributed to
the fact that their isoelectric point is near pH 5 4 [121]. It is reported that surfactants’
activity increases in NaCl concentration from 0% to 8% [462,463] while some authors
have reported no considerable impact on activity [464,465].
There are several types of biosurfactants with particular characteristics, which can
be produced by microorganisms [4]. Neu [466] categorized the biological surface
active compounds as
1. Biosurfactants;
2. Amphiphilic polymers (e.g., lipopolysaccharides and lipoteichoic acids); and
3. Polyphilic polymers (e.g., hydrophobic polysaccharides and emulsan).
The major categories of biosurfactants include [441,467 469] (see Table 10.2):
1. Glycolipids;
2. Lipopeptides and lipoproteins;