Microbial enhanced oil recovery by Bacillus subtilis strains under simulated reservoir conditions

Abstract

Microbial Enhanced Oil Recovery (MEOR) is a tertiary oil recovery process in which microorganisms and their metabolites are used to retrieve unrecoverable oil from mature reservoirs. Stimulation of microorganisms that produce biosurfactants and degrade heavy oil fractions in situ reduces the capillary forces that retain the oil into the reservoir and decreases oil viscosity, thus promoting its flow. As a result, oil production can be increased. In previous work, Bacillus subtilis strains that are useful for application in MEOR were isolated from crude oil samples. Those isolates were able to grow and produce extracellular biosurfactants at 40ºC under anaerobic conditions in mineral medium supplemented with hydrocarbons. In addition, some isolates degraded the long-chain n-alkanes of paraffinic mixtures. Mobilization of residual oil by those isolates was evaluated using sand-pack columns at 40ºC. Additional oil recoveries obtained with the different isolates using paraffin ranged from 16 to 31%. The recovered paraffin showed a decrease in the percentage of large alkyl chains and its viscosity was reduced. In the present work, the degradation of long-chain n-alkanes and the mobilization of residual oil by the same isolates were studied using heavy oil. The results obtained confirmed that the isolates were able of degrading the long chain n-alkanes of crude oil, and also additional oil recoveries between 8 and 10% were obtained. Furthermore, for a better simulation of the oil reservoir conditions, a core flooding equipment that can work at the oil reservoir pressure was designed and will be used to confirm the applicability of selected isolates to increase oil recovery. The sampled reservoir consists of sands with an average porosity of 25% and a permeability of 50mD. The oil is paraffinic, with low viscosity, high pour point and a gravity of 25º API, with very low solution gas. The reservoir pressure and temperature are respectively 398-440psi and 40ºC.