Microbial-enhanced oil recovery at simulated subsurface reservoir conditions

Core runs were conducted to elucidate the principal mechanism(s) of microbial enhanced oil recovery (MEOR) at simulated subsurface reservoir conditions up to 1500 meters depth. Two species of bacteria were used: Bacillus licheniformis JF-2 and Clostridium acetobutylicum.; Sandstone cores (70 cm{dollar}sp3{dollar} PV) were equilibrated to the desired simulated reservoir conditions, saturated with oil and brine, and flooded to residual oil saturation. The waterflood brine was displaced with a nutrient solution (3% cattle-feed-molasses in brine). The cores were then inoculated with bacteria (10{dollar}sp8{dollar} cells/ml) and shut-in. The pore pressure was monitored until there was no further increase in pore pressure with respect to time. Finally, a pressure maintenance waterflood was conducted and the additional oil recovery was recorded (17-19% of original oil in place). Pore pressure increases up to 23 MPa (from initial pressure of 17 MPa) due to biogenic gas generations were observed. The gas dissolved in the oil and brine establishing a solution gas drive mechanism. The MEOR recovery efficiency was found to be directly related to the dissolved gas-oil ratio. The principal MEOR mechanism observed in this work was solution gas drive. All other possible mechanisms (surface tension reduction, pH reduction, selection pore plugging, etc.) were shown to be minor mechanisms compared to solution gas drive. These core runs suggested a procedure for field application of MEOR using a gas-generating bacteria.