Page 456 - Fundamentals of Enhanced Oil and Gas Recovery

444 Afshin Tatar

limit for effective microbial transport is reported to be 75 100 mD [842]; however,
some studies indicate microbial transportation in permeabilities lower than 75 mD
[646,845]. Moreover, the movement of microorganisms in a formation with perme-
ability of 30 mD is proved too [417]. There have been the problem of plugging in
some early MEOR experiments [72,846]. In a study, Davis and Updegraff [847] stated
that to avoid plugging problems, the diameter of the pore entry should be at least
twice the diameter of the injected microbial cells. In his patent, Hitzman [848] sug-
gested to use spores instead of vegetative cells as their size is smaller. Later, it was dis-
cussed that spores could induce plugging and it was suggested to incorporate UMB
due to their further smaller size [422]. Some years later, Jack et al. [849] concluded
that the microorganisms to be injected should be small and the ideal one is with a size
less than one-fifth of the pore throat size of the target formation. Bacteria can be
found in different morphologies such as rods, curved rods, cocci, tetrads, chains, etc,
and have typical dimensions of 0.5 10.0 μm in length and 0.5 2.0 μm in width [25].
This indicates that pore dimensions less than 0.5 μm pose severe restriction on micro-
bial activity. It is suggested that the pore considerable bacterial activity was reported in
media in which the interconnections of pores have at least 0.2 μm diameters [850].In
accordance to the permeability calculations by Stiles [851], Gray et al. [66] suggested
that reservoirs with porosities less than 6% would be suitable cases for microbial plug-
ging. Comparing the shales with sandstones, the former has much smaller pore-throat
(less than 0.2 μm compared with up to 13 μm for sandstone) [852]. In such systems,
the microorganisms may not be able to easily transport within the matrix and also the
rate of nutrients diffusion will be slow too. Sheng [24] stated that pore geometry and
size can influence the chemotaxis although it has not been proven in reservoir condi-
tion. In addition to the transfer of microorganisms, porous media characteristics such
as porosity, permeably, and pore size can affect the microbial growth and metabolism
and also size and number of the bacterial cells [25,853 855].

10.12.9 Oil Gravity
Successful MEOR trials have been reported for the oil gravity range of
3
0.82 0.96 g/cm [39]. Pautz and Thomas [856] reported an API oil gravity range of
34 40 for several MEOR project performed all over the world.

10.12.10 Depth

Deep reservoirs are usually associated with high temperature, pressure, and salinity and
also poor permeability, which adversely affect the MEOR efficiency [23,24]. Deep
reservoirs are not favorable candidates for MEOR. The depth itself does not impose
limitation on microbial growth; actually, its effect on the temperature and pressure
influence the microbial growth and metabolism [25]. For the several MEOR projects