Mini-coreflood set up
These studies used high-permeability (>400 mD), 1” diameter by 6” long Berea SanstoneTM cores (Cleveland Quarries, Vermilion, OH, USA) that were dried to completeness in an oven at 110 °C overnight. Once dried, the cores were saturated with a synthetic brine comprising a salinity (9400 mg/L and a hardness of 250 mg/L) closely resembling fluids from the representative wells to determine pore volumes (PV) by the mass of brine that was retained in the core. Following brine saturation, mini-coreflood reactors were assembled in the anaerobic chamber and they consisted of a PVC sleeve (1” inner diameter, McMaster-Carr, Cleveland, OH, USA) that surrounded the core and a plastic canister to hold the core in place (built by Purdue University’s Chemistry Precision Machine Shop, West Lafayette, IN, USA). The plastic canisters were then filled with DI water that was pressurized to 80 PSI to apply pressure on the sleeve and keep it tightly fit to the core. Once mini-coreflood reactors were assembled, dead oil from the representative wells was processed and injected into the core at a rate of 0.5 ml/min until the core was saturated with oil; the amount of brine displaced represented the volume of original oil in place (OOIP) before the primary water flood. The primary flood was carried out by injecting of 2 PV effluent oil well water at rate of a 1 ml/min to displace the oil for primary recovery and simultaneously seed microbes into the core. The secondary flood was carried out with 1 PV of a mixture of nine parts well water to one part of the molasses solution with supplements as indicated in the “Cultivation” methods section above. The flow rate of the secondary flood was 1 ml/min, and no additional oil was recovered during this nutrient injection step. Mini-corefloods were shut-in for 10 days inside the anaerobic chamber (27 °C) replicating ground conditions. Pressure generated over the 10-day period was measure with an in-line pressure gauge. Oil generated by the microbial pressure was collected and then an additional 2 PV of brine was used as a final water flood to recover oil displaced by the microbial activity. The collected brine flood volumes were centrifuged, and DNA was extracted from the pellet; V4-V5 16S rRNA gene regions were amplified and sequenced as described above (see “DNA extraction, PCR, Sequencing & analysis ”).