Realization of ULTRG (Unrecoverable Liquid fuels To Recoverable Gaseous fuels) as an Innovative Microbial EOR
Authors: Maeda H. , Ikarashi M. , Wakayama T. , Yonebayashi H. , Mayumi D. , Tamazawa S. , Tamaki H. , Sakata S. , Kamagata Y. , Iida T. , Osaka N. 
Published: June 2017

Research study has been carried out to investigate the possibility of conversion of residual oil to methane using microbes inhabiting subsurface depleted oil reservoirs. We call the above concept "Unrecoverable Liquid Fuels to Recoverable Gaseous Fuels (ULTRG)" technique.

This technique supposes microbial conversion process of crude oil into methane, and the mechanism is estimated as follows. First, hydrocarbon degradation microbes (so called bacteria) are utilized to produce hydrogen and organic acid such as acetate from residual oil that could not be recovered by primary and/or secondary recovery process in matured oil field. Next, methane-producing microbes (so-called methanogens) are used to generate methane from the produced acetate, or hydrogen and carbon dioxide.

The reactions of microbial conversion of crude oil (assuming as alkanes) to methane are estimated as follows (Maeda et al., 2010).

Acetic acid producing reaction from alkanes CnH2n+2+nH2O→n/2CH3COOH+(n+1) H2

Hydrogen producing reaction from acetate CH3COOH+2 H2O→4 H2+2CO2

Methane producing reaction from acetate CH3COOH→CH4+CO2

Methane producing reaction from hydrogen and carbon dioxide CO2+4 H2→CH4+2 H2O

Fluid samples oil and formation water produced from oil and gas fields in Japan were collected and analyzed in order to clarify the existence and survivability of indigenous hydrogen- and methane-producing anaerobes under reservoir conditions.

It has not been clarified that crude oil degradation can occur in subsurface oil reservoirs. However, our recent incubation experiment has successfully observed methanogenic crude oil degradation by using subsurface indigenous microbes mimicking the certain reservoir condition as a target of field application. Various culture incubation experiments revealed that methane production occurred by using indigenous microbes inhabiting the reservoir brine, and some nutrient additives accelerated the methane production rate and volume.

Further, culture incubation experiments were conducted using artifact-free homogeneous sand grains mimicking porous environments as subsurface oil reservoirs. The porous environment significantly increased microbial activity of methane production, and the subsequent subcultures also continued to produce methane. Stable carbon isotope tracer method was used to clarify the degradation of hydrocarbon compositions. The degradation was judged as variation between the original and post-incubated oils. After the incubation, Scanning Electron Microscope (SEM) observations was observed for the microbes attached on the sand grain, in order to estimate how such porous environment accelerates microbial activities of hydrocarbon degradation and conversion to methane. Furthermore, metagenome analysis will be planned to conduct to investigate the microbial reaction and related microorganisms.

In this paper, we are focusing on crude oil biodegradation degree to evaluate the existence of microbes which are involved in the reaction of crude oil degradation and the necessity of nutrients to enhance such reaction.

It will be a worth to establishment the ULTRG because this technique will be a big step toward building an innovative microbial methane conversion system from remaining unrecoverable oil.