High-Pressure Microscopic Investigation on the Oil Recovery Mechanism by in Situ Biogases in Petroleum Reservoirs

Biogases, including CO2 and CH4, are an essential category of the complex metabolites in microbial enhanced oil recovery (MEOR). However, the recovery mechanisms and contributions of in situ biogases have not been individually studied. A microbial consortium from oil fields, which produced gases but few metabolites, was inoculated in microscopic flooding experiments mimicking reservoir pressure. Additional oil [14.8% of the original oil-in-place (OOIP)] was recovered. In terms of gas yields, porous media were favorable culturing substrates compared to bottles (up to 1.2 and 0.42 mL of biogas per mL of medium, respectively). Under pressurized reservoir conditions (8 MPa), contributions from typical mechanisms, including oil displacement and pressurization by gas bubbles, can be excluded as a result of the effective biogas dissolution in crude oil. The dissolved gases promoted the spontaneous expansion of microbial activities, so that 34.1% of the additional oil was recovered from the margin regions, area of which only occupied 19.76% of the model. Therefore, the significant sweep efficiency enhancement is suggested to be the dominant recovery mechanism exhibited by in situ biogases. The findings also provided a new perspective on the effects of CO2 and CH4 (natural gas) flooding on hydrocarbon migrations in reservoirs.