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Microbial Enhanced Oil Recovery: Microbiology and Fundamentals 413

10.7 NUTRIENTS

Providing the nutrients is the main expense in MEOR trials. The nutrients are
very important as the right combination and quantity has a key role in MEOR suc-
cess. The main considerations to select the nutrients are the desired outcome and the
involving organisms [6]. The need to the carbon source can be satisfied either by
exogenous (usually sugar) or indigenous (crude oil itself) sources. Molasses (or black
treacle, is a viscous product resulting from refining sugarcane or sugar beets into sugar)
are the most commonly used carbon source as it is widely available, its injection to
the well is an easy process, and contains essential minerals and vitamins [24,84].
Updegraff and Wren [398] were the first researchers, who proposed using molasses as
the substrate. Although using ex situ carbon sources may induce more microbial activ-
ity, the carbon source might be expensive and it is of more interest in economical
point of view to utilize microorganisms that mainly consumes residual oil as their car-
bon source [22]. Moreover, this would be excellent for heavy oil production as this
process reduces the carbon chain of heavy oil and consequently increases the crude
quality [84,399,400]. However, in case of utilizing the in situ oil as the carbon source,
the generation of the by-products noticeably retards and the growth can be very slow
[25,84]. The other necessary nutrients are inorganic nitrates and phosphorous salts.
These compounds are usually provided by fertilizers such as ammonium phosphate
((NH 4 ) 3 PO 4 ), superphosphate (Ca(H 2 PO 4 ) 2 ), ammonium nitrate (NH 4 NO 3 ), and
sodium nitrate (NaNO 3 ) [25].
For selective plugging and modifying the permeability profile, biomass production is
the main MEOR solution. Supplying nitrate, which is the electron acceptor, can maxi-
mize the biomass generation [6]. The other example to show the influence of nutrients
on the microbial activity is Leuconostoc sp., which generates dextran (a type of biopoly-
mer) only when sucrose is supplied [401,402]. In cases intended to produce biosurfac-
tant, there should be a delicate balance between the carbon and nitrogen sources [6].
It is reported that limiting the nitrate source can promote the surfactin production by
Bacillus subtilis, biosurfactant by Candida tropicalis,and rhamnolipid by Pseudomonas sp.
[403 405]. As an example for nutrients, laboratory experiments have shown that
corn steep liquor (carbon source) along with diammonium phosphate )nitrogen and
phosphorus source) is an effective nutrient for microbial growth for efficient plugging
scenarios [406]. Finding the optimum nutrient blends to stimulate the target bacteria
is of great importance. For aerobic microorganisms, oxygen is the other essential
nutrient. As it was debated before, injectionofoxygenisassociated with someadverse
effects on the equipment and pipeline. The other issue is that oxygen solubility in
water is limited [407].