Page 311 - Fundamentals of Enhanced Oil and Gas Recovery

Microbial Enhanced Oil Recovery: Microbiology and Fundamentals 299

in contact with the aqueous phase. If the residual oil is the nutrient, the microor-
ganism will preferentially thrive in the oil water interface [63]. In case of injecting
the nutrients such as molasses the microbial growth is boosted but discouraged on
the oil water interface.
Several parameters affect the subsurface microbial life including [119 121]:
1. Chemical factors such as nutrient composition, electrolyte composition, redox
potential (activity of electrons (Eh)), and activity of hydrogen ions (pH);
2. Physical factors such as pressure, temperature, salinity, pore size, and pore geome-
try, porosity, permeability, and dissolved solids; and
3. Biological factors such as cytotoxity of the microbial metabolites and also specific
type of microorganisms.
Studying the microbial life in deep biosphere, Jørgensen and Boetius [118] stated
that temperature is the prevailing influential parameter. However, there are evidences

of bacteria life at an extreme temperature of 120 C [122]. Youssef et al. [6] mentioned
the three parameters of redox potential, temperature, and salinity as the most impor-
tant influential parameters on the oil reservoir microbial communities. Redox activity
is corresponding with availability of electron accepters and donors. Generally, oil
reservoirs are associated with low redox potential [6]. The electron donors in the oil
reservoirs are hydrogen, volatile fatty acids such as propionate, acetate, and benzoate
[123], and inorganic electron donors and electron acceptors include sulfate and
carbonate minerals in many reservoirs and also iron(III) in some reservoirs [6].
Presence of nitrate and oxygen as the other electron acceptors are not likely unless
they are added via injection [6]. Van Hamme et al. [19] specified that considering the
size and ability to grow under the harsh subsurface petroleum condition, only prokar-
yotes are the capable candidates for MEOR and yeasts, algae, molds, and protozoa are
not suitable microorganisms. The microbes utilized in MEOR are typically nonpatho-
genic hydrocarbon-utilizing microorganisms and can be found naturally in the oil
reservoirs [15,18,19,22,124]. These microbes may naturally inhabit the subsurface
reservoirs or injected.
Aerobic condition is not common in the petroleum reservoirs. The aerobic
microbes would have consumed the existed oxygen already. The other point is that
ferrous iron and sulfur, which are commonplace in oil reservoirs, deplete the free oxy-
gen too [22].
Microorganisms for MEOR can be classified based on the
1. Origin;
2. Metabolic processes; and
3. Action.
In the following sections, a brief explanation is provided.