One of the major contributing factors leading to rapid exhaustion of fossil fuel resources is inefficient extraction methods. Currently oil extraction techniques only recover 40% of an available stock; in this ‘proof of concept’ study a researchers from the University of Oklahoma, have found that “oil residing in marginal reservoir samples [stranded oils] can be converted into methane by using a methanogenic microbial consortium as an inoculant.” This recovery of methane gas from underutilized hydrocarbon bearing resources via microbial biodegradation could provide a more efficient, cleaner burning, energy fuel.
The researchers exposed crushed residual oil bearing sandstone core material (from 200m depth with ≅0.013g oil per gram of core ≅ 30-40% oil core saturation) with an inoculant containing a methanogenic consortium of Archaea (already known for their methanogenic properties) enriched from gas-condensate-contaminated subsurface sediments along with a sterile fluid medium. Methane levels were monitored by gas chromatography (GC) of the incubation vessels headspace contents. DNA analysis was used to construct a phylogenic tree of the inoculation consortium.
Methane production varied between different core and inoculant combinations, but on average “0.15 to 0.40 µmol/day/g core (or 11 to 31 µmol/day/g oil), with yields of up to 3 mmol CH4/g residual oil” were observed (See Fig.1). Rates of methanogenesis were greatly reduced in incubations containing only formation oil without core material and production was totally absent in uninoculated incubations. These results shows that the microbial consortium is directly responsible for the methanogenesis and that an inoculation could potentially be used to recover methane from residual oil reservoirs. It should be noted that the core material play an, as yet, unidentified role in the process but is nonetheless vital for an effective methane yield. The DNA analysis run by this team identified many fermentative bacteria (e.g. Clostridia) but it is believed their role was as a syntrophic organism, exploiting byproducts of the methanogens themselves and were not directly involved in the breakdown of hydrocarbons. This information is useful as it allows the creation of more efficient inoculants in the future.
Methanogenic biodegradation of hydrocarbon sources already occurs naturally but the use here of an artificially magnified inoculation provides the bases for accelerating the process and making commercial use of methanogen consortiums. An application of an oil-to-methane recovery process in marginal reservoirs could significantly increase the energy yield from current global oil stocks and while in its infancy, the use of this technology could be of great interest to “help nations reduce reliance on foreign imports and increase the value of domestic reserve[s]” and perhaps reduce the pressure to locate and exploit new oil reserves in sensitive areas. Unfortunately a large scale lack of global oil collected via traditional methods may be the impetus that finally pushes this recovery process to the forefront of energy production, and not environmental concerns.
Gieg, L.M, Duncan, K.E, Suflita, J.M, (2008). Bioenergy Production via Microbial Conversion of Residual Oil to Natural Gas. Applied and Environmental Microbiology. 74 (10), pp.3022-3029