Page 175 - Primer on Enhanced Oil Recovery

Microbial enhanced oil recovery (MEOR) 165

strata, but also to prevent the deposition of solid hydrocarbons. For this process to
work, the provision of the bioactive zone with air or oxygen is imperative.
Microorganisms are able to use hydrocarbons of different classes of simple and
complex structure. Almost all hydrocarbons that make up oil can be subjected to
microbial digestion. Nevertheless the digestion speed depends on the exact chemis-
try and even on the phase state. For instance, microorganisms multiply and process
nutrients most quickly and effectively on solid n-paraffins while the processes are
somewhat slower in liquid and gaseous n-paraffins.
It is known that increasing the concentration of carbon dioxide in the gas mix-
ture will help to reduce the amount of precipitation of calcium and magnesium
salts. The increase in carbon dioxide content in the oil reservoir can be achieved by
nutrient flooding of the reservoir with a bioreagent containing carbohydrate com-
pounds which is decomposed by microorganisms during fermentation to CO 2 For
this purpose molasses or whey can be used.
The use of biotechnologies associated with the selective action of microorgan-
isms can be beneficial to improve the efficiency of waterflooding of oil fields and
to increase the sweep efficiency in the reservoirs with very high heterogeneity.
Microorganisms quickly clog the high permeability zones, where a large amount of
injected fluid enters. The expected efficiency and selectivity of blocking highly per-
meable interlayers of biomass of microorganisms is the highest when both microor-
ganisms and nutrients are injected into the oil reservoir. At the same time, the
biopolymers, formed as a result of nutrient digestion by the microorganisms,
increase viscosity of the formation water. This increases oil displacement by
advancing finger-free water front.
In some cases, waterflooding of oil reservoirs may be accompanied by function-
ality activation of sulfate-reducing bacteria. In this case, in nutrient containing
water injection it is necessary to focus on the factors limiting the activity of sulfate-
reducing bacteria (SRB).
The main problems that can be encountered in the implementation of microbio-
logical methods, in addition to sulfate reduction processes, are related to the trans-
port of metabolic products into the reservoir, ensuring the necessary concentration
of microorganisms for their growth and development, as well as the possibility of
optimizing the required microbiological activity.

13.1 Applicability criteria

The geological and physical conditions of oil reservoir considered for the microbio-
logical treatment must meet certain requirements for the temperature, pressure,
salinity and mineralogical composition of connate water. Biotechnologies can be
applied most effectively at fields with terrigenous and sandy oil reservoirs. The for-
mation should have sufficiently high porosity (more than 20%) and reservoir per-
2
meability (more than 0.04 µm ). The oil strata temperature should be less than

100 C. All criteria are summarized on Fig. 13.3Highly mineralized connate water