Page 442 - Fundamentals of Enhanced Oil and Gas Recovery

430 Afshin Tatar

mechanism through which produced biogases may improve oil recovery [17].CO 2
can result in oversaturation of the fluid with calcite (CaCO 3 ) and consequently its
precipitation, which may reduce the formation porosity and permeability and hence
modify the fluid flow path [701]. Observations of flow alterations in a hypersaline
petroleum reservoir after addition of nutrient [702] can be explained by this
mechanism.
As it was mentioned earlier, the process of methane production is referred to as
methanogenesis [25,395,396], which is strictly anaerobic [22]. Details of the methano-
genesis process by studies on laboratory microcosms can be found in literature [703].
Methanogenesis is the easiest way to recover the trapped oil; however, the produced
methane is relatively cheap [10]. Because of this, when it was assured that the oil has
swollen enough, this process may be stopped to recover more valuable components.
To inhibit methanogenesis, it is possible to use inhibitors such as 2-bromoethane-
sulfonic acid and methylfluoride for general and acetoclastic methanogenesis, respec-
tively [704]. On the other hand, it should be mentioned that to generate 1 kWh
energy, methane produces 0.569 kg CO 2 which is less than those for using oil
(0.881 kg) and coal (0.963 kg) [705]. This can reduce the CO 2 emission by more than
35% in case of energy generation by the biogenic methane. Briefly speaking, harvest-
ing energy from heavy oils and tar sand in form of gas rather than heavier hydrocar-
bons is more economic and environmentally friendly [22].

10.10.6 Biomass
Generally, microorganisms prefer to develop clusters and colonies in form of biomass,
which can be beneficial considering their plugging effect. The biomass is comprised
of bacteria, their produced exopolysaccharides, and water channels [15]. To be more
specific, they are composed of 27% microbial bodies and the rest (73% 98%) are
extracellular products (such as exopolysaccharides) and void space [618,636,637].
Bacteria multiply their mass at exponential rates [17]. Microbial plugging is usually
associated with supplying nutrients for microorganisms either injected or exogenous
[15,21,616,628,639]. The injected fluid including the microorganisms and/or nutri-
ents tends to flow through the more permeable pathways [706,707]. The provided
nutrients stimulate the microorganisms to grow and this would decrease the perme-
ability of region that once were highly permeable and consequently modify the per-
meability profile. In another words, most of the injected nutrients will stimulate the
bacteria available in the regions with high permeability to growth. Biomass can
accumulate in the highly permeable zones, referred to as the thief zones, and then
divert the waterflood to the oil-bearing zone [636]. The biomass, first, start to accu-
mulate in wellbore along high-permeable channels, which diverts the fluid flow
into the lower permeable zones. For this, cautions should be considered as the fast