Page 455 - Fundamentals of Enhanced Oil and Gas Recovery

Microbial Enhanced Oil Recovery: Microbiology and Fundamentals 443

10.12.6 pH
A broad range of pH can be observed in reservoirs. This factor is considered one of
the main environmental factors that affect the microbial growth [84]. Microorganisms
grow superbly under slightly alkaline condition [10] and at low pH values the micro-
bial activities will be adversely affected [84]. In general, the optimal pH range for
microbial growth is 4.0 9.0 [84,839]. However, there are evidences of withstanding
extreme pH values less than 2.0 at high temperatures by Sulfolobus [840]. Moreover,
Donaldson et al. [25] mentioned about isolation of microorganisms capable of growth
at pH values as low as 1.0 and as high as 12.0. Before injecting microorganisms to the
reservoir, the growth capability under the reservoir condition such as pH should be
confirmed as it is done in several laboratory studies [559,560,841]. The pH induces
some effects on enzymatic activities and many enzymes are sensitive to pH [84,833].
Jenneman and Clark [842] stated that the prevalent pH range in the oil reservoirs may
not hinder the microorganisms’ growth; however, the pH gradients can influence the
specific metabolic processes required for some certain MEOR processes. pH can indi-
rectly affect the microorganisms’ growth and metabolism by influencing the solubility
of toxic materials [25].

10.12.7 Lithology
In MEOR activities, it is necessary to inject the nutrients (and in some cases microor-
ganisms) in to the reservoir. The effects of rocks and clays on retentions of microbial
cells and nutrients are of importance through the transport process [25,629,843,844].
Several different minerals build the oil reservoir rocks. Sedimentary rocks are the most
common in oil reservoirs; however, hydrocarbons might be found in ingenious and
metamorphic rocks too [25,816]. Sandstones and carbonates including limestones and
dolomites are the main categories of sedimentary rocks in which hydrocarbons can be
found [25]. Fractured carbonate reservoir contains most of the world’s oil [793].In
this type of reservoirs, the matrix blocks are mixed to oil-wet. Carbonates and silicates
do not significantly retard the microbial activity but adsorptive capacity of clays and
some other minerals present in the reservoir rocks may interfere with microbial pro-
cesses [25]. There are charges on the rocks surfaces, which can adsorb the microor-
ganisms and prevent their transport. Amongst the clays, montmorillonite and
kaolinites are the greatest and the least ion exchange capacities, respectively, and illites
exhibit intermediate capacity [25]. Via swelling, clays absorb water, which impose
some limitations for microorganisms’ migration [25].

10.12.8 Porous Media and Microorganisms’ Size
Scrutinizing several field trials, Maudgalya et al. [23] stated that a minimum reservoir
permeability of 75 mD has been the most suitable for MEOR projects. The lower