Page 306 - Fundamentals of Enhanced Oil and Gas Recovery

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bioreactors, facilities, and purification practices compared with lower yield of the
induced production are the main drawbacks [24,31]. In MEOR, the metabolically
generated hydrogen sulfide induces souring and significant damages to equipment
and piping due corrosion [19,24,37,38]. However, in contrast, some study has
reported that MEOR reduced the reservoir souring [39]. Nitrate reducer microbe
can contribute to reduce souring [24].
The inconsistency between the lab results and the field trials has been a primary
reason for the MEOR not to become a popular technology [23]. The other reason
preventing MEOR becoming a routine and accepted procedure for enhancing oil
recovery may be the lack of scientific understandings and knowledge about the basis
and details of the different MEOR approaches [32]. New advanced technologies will
provide acceptable explanations for this fact and remove this barrier. Cheap oil sup-
plies and low-price oil in the last decades in addition to inexpensive simple CO 2
injection EOR have hindered considerable investments on investigation of tertiary
recovery methods including MEOR. Youssef et al. [6] specified 96% of all the studied
MEOR projects have been successful. The ever growing field trials and laboratory
experimental studies and issued patents regarding MEOR indicate the potential of this
method to become an important and reliable tool in EOR.

10.3 RECOVERY EFFICIENCY

The recovery efficiency can be expressed as [40]:

E r 5 E d 3 E v (10.1)
where E r is the recovery efficiency. E d denotes the microscopic oil displacement effi-
ciency expressed as the fraction of the total volume of the oil displaced from a unit
segment of rock and E v is the volumetric or macroscopic sweep efficiency expressed
as the fraction of the total reservoir that is contacted by the recovery fluid.
Trapping of oil in the porous media depends on fluid/rock interactions (reflected
by wettability), fluid/fluid interaction (reflected by interfacial tension (IFT)), and pore
structure [30]. The remaining oil in the reservoir is often located in difficultly accessi-
ble area such as small pores and dead-end pores and is trapped by capillary pressure
[15,41 43]. Capillary forces in the porous media are governed by the combined effect
of the IFTs between the rock and fluids, the pore size and geometry, and the wetting
characteristics of the system [44]. Viscous forces denote the pressure gradients associ-
ated fluid flow within the porous media [30]. It is possible to show the effect of vis-
cous and capillary forces on the trapping of oil within the porous media using a