Estimation of the Potential of an Oil-Viscosity-Reducing Bacteria, Petrotoga Isolated from an Oilfield for MEOR

Abstract

Abstract One thermophilic anaerobic oil-degrading bacterium was successfully isolated from reservoir brine of Yabase oilfield, INPEX Corp., Akita, Japan. The potential of the isolated bacterium as a candidate for Microbial EOR (MEOR) was estimated in this study. This bacterium was identified as Petrotoga sp. by DNA sequencing analysis. The isolated bacterium can degrade long chain hydrocarbons in crude oil into shorter chain hydrocarbons in reservoir brine containing 2 (v/v) % of crude oil as a carbon source and 0.1 g/l of yeast extract as a nitrogen source. As a result, the viscosity of crude oil decreased. The isolated bacterium can form their bacterial colonies on a solid medium exclusively under the presence of CO2 in gas phase. In addition, the results of GC-MS analyses of crude oil showed that the isolated bacterium can degrade longer chain of n-alkanes more selectively under 10 % CO2 atmospheric condition. These results show that CO2 stimulate the growth of the isolated bacterium and selective degradation of longer chain of n-alkanes by the bacterium. The suitable nitrogen source for the isolated bacterium was evaluated by reduction of viscosity of crude oil. Ammonium nitrate, urea, and yeast extract were evaluated for the nitrogen source and added into the medium containing crude oil with 2 (v/v) %. The highest growth rate of the bacterium was observed in the medium containing yeast extract. Viscosity of crude oil reduced by 40 % of its original viscosity in this case while ammonium nitrate, urea and non-nitrogen source addition gave 38 %, 35 % and 12 % oil viscosity reduction after 3 weeks incubation. 0.05 g/l of yeast extract is enough for the growth of the isolated bacterium. The isolated bacterium can grow under high salinity condition such as 90 g/l of NaCl and it can grow under the temperature between 50 to 80 oC. These results show that the isolated bacterium can be applied to wide range of reservoirs. Introduction Microbial enhanced oil recovery (MEOR) is a tertiary oil recovery technique based on the application of biotechnology or biological activities of microorganisms. In general, effective microorganisms is injected into reservoir with its nutrient sources and it is incubated in reservoir. Viscosity reduction of crude oil by bio-degradation of crude oil and/or effective bio-products such as surfactants and polymers lead to the improvement of oil recovery1). Molasses which is produced in sugar refinery process as a byproduct has been used for a nutrient of microorganisms in many MEOR field trials2). Molasses is carbon source which digested by the bacteria to generate gases such as hydrogen, carbon dioxide and other chemicals, which in turn increase the mobility of oil in the reservoir and as a result the flow of oil increases. However, the cost of molasses has been progressively increased over the past decades due to extensively usage as raw material for bio-ethanol production. For example, Chinese molasses rose in its price from 40 US dollars at 2000 to 200 US dollars at 20073). In the near future, it is predicted that the use of molasses in MEOR will be dissatisfied economically.