Page 172 - Primer on Enhanced Oil Recovery

162 Primer on Enhanced Oil Recovery

Practice has shown that provision for production of biopolymers and bio-
surfactants at the surface level in special bioreactors, mixing the products with
water and injection into the formation is relatively costly and requires significant
investments. However, this approach has significant advantages. Due to high qual-
ity, effectiveness and stability of the bio-products, the bio-products can be applied
in the oil deposits with a relatively high reservoir temperatures and highly mineral-
ized waters.
The method of microbiological impact on the reservoir in situ is based on an oil
displacement rate enhancement. This enhancement is a result of the intensification
of living activity of microorganisms capable of digesting and converting complex
organic compounds. The digestion products are more chemically simple com-
pounds. The secretion products of microorganisms are liquids (fatty acids, alcohols,
solvents) and gases (CO 2 ,CH 4 ,N 2 ,H 2 ). All those metabolic products are good oil
properties modifiers and oil displacers. Certain groups of microorganisms are able
to form biopolymers and bio-surfactants within an oil reservoirs.
The mechanism of microbiological activity in a reservoir is complex and it is a
multistage process. Firstly, oxygen or air with high amount of easily digestible for
microbes materials are injected into the formation. In the reservoir after a nutrient
flooding, decomposition and oxidation of hydrocarbons injected by microorganisms
proceeds in the presence of oxygen. This results in the microbes secretion of fatty,
naphthenic and aromatic acids, alcohols, ethers and other solvents. The increase in
the biomass of microorganisms happens due to the correctly selected nutrient
medium.
A characteristic feature of oil, as it has been mentioned so far many times, is its
exceptional heterogeneity. Oil consists of organic compounds, markedly different in
chemical composition, structure and properties. Individual fractions of oil in vari-
able degrees are subjected to the microbial digestion and decomposition.
Secondly, oxidation products of petroleum hydrocarbons, due to oxygen injec-
tion, become food substrates for microorganisms deep in the formation, where oxy-
gen is absent. Produced fatty acids and alcohols are used by other microorganisms
types. The later stage produces acetic acid, carbon dioxide and hydrogen. In the
final phase, methane is formed from the intermediate decomposition products,
which is the final link in the chain of transformations of organic substances in the
formation. This demonstrates complex chain of processes at the beginning there
is a decomposition, consumption of more easily digestible organic compounds,
then a consumption of compounds more difficult to utilize. All this leads to the sit-
uation when in the reservoir there are mixtures of all processes products from
every stage.
The scheme of oil displacement during microbiological processes is shown in
Fig. 13.1.
The microbial cells activity is supported by the presence of enzymes organic
compounds that catalyse certain reactions. Organic substances, introduced by the
culture liquid (oxygen plus nutrients plus bacteria) into an oil reservoir, undergo a
chain of transformations under the influence of the functional activity of all,
injected and reservoir present, microorganisms.