ROLE OF GEOMICROBIOLOGY IN ENHANCED RECOVERY OF OIL: STATUS QUO

Abstract

The forthcoming decrease in availability of known, presently economical deposits of crude oil in the foreseeable future, makes it imperative that the search for new oil deposits be intensified and the present methods of oil recovery be improved or new ones introduced. From the work reported in the literature it is obvious that microbiology may play a significant role in both cases. Among many parameters influencing oil recovery, viscosity of the oil and the surface tension between the rock, oil and water are of great importance. Microorganisms growing in the reservoir produce gases and surfactants, which may, to some extent, regenerate the endogenous energy of the reservoir and facilitate movement of the crude oil to the well. The composition of the crude oil may become altered by biodegradation of asphaltic, napthenic and/or paraffinic components of the oil. The fraction being biodegraded varies according to the microbiological population present. In general terms mixed populations are more effective in biodegradative processes and production of surfactants. A combination of sewage microorganisms and reservoir microorganisms adapted to 95°C and high pressure, was found satisfactory. Molasses is a suitable supplementary substrate for the growth of such a mixed population. A decrease of viscosity of oil, resulting from biological degradation, may be a composite effect of degradation of highly polymerised hydrocarbons, precipitation of asphaltenes and solution of biologically produced gases in the oil. Such biogenic gases dissolved in the reservoir water may, in combination with biologically produced acids, contribute to the slow solution of the sedimentary rock, thus increasing the rock's permeability and facilitating migration of the oil through the reservoir.

The biological activity in the reservoir is influenced by a number of parameters (pH, Eh, temperature, pressure, oil-water dispersion, mineralisation). The permeability of the reservoir rock is of primary importance. Rocks of permeability less than 150 md are not suitable for biologically enhanced recovery. Field tests indicate that biological activity in a reservoir may result in a drop of 50 per cent in the oil viscosity, a three-fold increase of oil production over several months, increase in water acidity and additional production of gas with a recorded pressure increase from 2 atm to 27 atm. The area affected by the biological activity depends on the mineralogy and permeability of the reservoir rock, sandstone and limestone of permeability higher than 600 md being most suitable.

Further properly controlled and documented laboratory and field experiments are urgently required before the feasibility of microbiologically enhanced oil recovery can be firmly established.