Page 429 - Fundamentals of Enhanced Oil and Gas Recovery

Microbial Enhanced Oil Recovery: Microbiology and Fundamentals 417

along with the corresponding nutrients are injected to the well with the hope that the
desired biochemicals will be generated within the reservoir [31]. The number of
6
5
injected microorganisms should be in the range of 10 10 /mL injected water [417].
An advantage corresponding with this method is the possibility to design a specific
nutrient package to stimulate the growth of injected microorganisms in the reservoir
[418]. One challenge associated with this method is the transport abilities of the exog-
enous microorganisms [6]. Mentioning conflicting recommendations on the utiliza-
tion of nonstarved versus starved cells [419,420], Youssef et al. [6] specified
microorganisms injection should be such that their adsorption to the reservoir rock
material is minimal. As a matter of the fact, the starved microorganisms are relatively
small and smaller cells are less likely for retention. Thus, utilizing starved cells provide
more efficient transport [421]. The other point about the starved cells is more effective
penetrating capacity to the porous material as it is proved by laboratorial studies
[422,423]. It is also possible to utilize microbial spores in this regard [424,425] (spores
are thick-walled and highly resistant to survive under undesired conditions. When the
condition becomes suitable the microorganisms will rise again) [6,426]. In applying this
approach, there is the presumption that the injected microorganisms will dominate the
microorganisms already present in the reservoir and has adapted themselves with the res-
ervoir condition and became fitted. Unfortunately, this is not always the case [31].
Another constraint with this approach is about the co-injection of nutrients and micro-
organisms. In reservoirs characterized with small pore throat sizes, the nutrient, which is
coinjected in solution, has faster propagation rate and may nourish and invigorate the
preexisting microorganisms in the reservoir. This makes it difficult for the injected
microorganisms to establish themselves over their indigenous counterparts [22].

10.10 PRODUCE BIOCHEMICALS AND THEIR ROLE IN MEOR

The main metabolically produced biochemicals by microorganisms beneficial
for MEOR processes are:
1. Biosurfactants and bioemulsifiers;
2. Biopolymers;
3. Bioacids;
4. Biosolvents;
5. Biogases; and
6. Biomass.
Each of the mentioned items are discussed in the following.