Numerical Modeling on the Influence of Reservoir Porosity and Microbial Kinetics on Enhanced Oil Recovery by Microbial Flooding

Abstract

Abstract During the implementation of microbial enhanced oil recovery (MEOR) technique in reservoirs, various reservoir and microbial kinetic parameters play major roles in governing the efficiency of crude oil recovery from hydrocarbon reservoirs. The present study numerically investigates the sensitivity of reservoir porosity, injected microbial species at different temperatures, maximum microbial specific growth rate, Monod saturation constant and yield coefficient on biomass and biosurfactant production and their impacts on microscopic oil displacement efficiency within the reservoir. A black-oil biochemical multi-species reactive transport model in porous media is developed by coupling the kinetic model with the corresponding transport model. The governing equations involve coupled transport of nutrients and microbes by dispersion and convection, growth and decay rates of microbes, chemotaxis, nutrient consumption, and deposition of microbes and nutrients on rock-grain surfaces. Coupled empirical equations are used to estimate biosurfactant production, oil-water interfacial tension reduction, change in viscosity of injection fluid and their impacts on oil mobility and decrease in residual oil saturation within reservoir. Finite difference discretization technique is adopted to solve the governing equations. Results of the present model are found to be numerically stable and match very well, when verified, with the previously published analytical and experimental results. The model results suggest that at very low reservoir porosity (less than 20%), an early breakthrough of nutrients, microbe and biosurfactant leave insignificant concentrations in their respective fronts which are insufficient for the recovery of the trapped oil. Also, increase in porosity beyond 20% causes loss of nutrients, microbes and biosurfactant because they undergo higher dispersion during their transport within reservoir. Further it is observed that the nature of microbes and nutrients used for MEOR application affect biosurfactant production and in turn oil recovery to a large extent. Those microbial species having very less Monod saturation constant values have high affinity towards their substrates. This phenomenon drastically increases the rates of nutrient consumption and production of biomass and biosurfactant within reservoir when suitable substrate compounds are used, irrespective of differences in the yield coefficients of the microbes. The optimized reservoir and microbial kinetic properties increase capillary number above 10−3 which further increases oil mobility towards production well and there is a significant decline in the effective residual oil saturation (less than 5%) within the reservoir. The present study provides an improved understanding of the combined effects of reservoir porosity and microbial kinetic parameters on fundamental MEOR processes which will better characterize the suitability of a MEOR technique in a typical petroleum reservoir. Moreover, the developed numerical model is easier to implement and produces faster results with relatively lower computational cost which helps in making quick decision before applying MEOR processes in the field.