Polyelectrolyte Complex Nanoparticle Nutrient delivery system for Microbial Enhanced Oil Recovery

Abstract

Microbial enhanced oil recovery (MEOR) techniques stimulate naturally occurring reservoir microbes or inject specially selected consortia of natural bacteria into the reservoir. The stimulated microbes produce specific metabolites such as surfactants, polymers, acid or gases that lead to the extraction of oil trapped in capillary pores of the formation rock or in areas not swept by the classical or modern enhanced oil recovery (EOR) methods. Although the potential benefits of MEOR applications are considerable, improvement of oil recovery via the manipulation of microbial metabolism in the reservoir remains an unproven concept. One of the major issues of MEOR application is that biomass and extracellular polymeric substances produced in response to high injected nutrient concentrations can cause plugging close to the injection point (near wellbore plugging). In this study, a novel approach to MEOR is discussed. Polyelectrolyte complex nanoparticles (PECs) were used to encapsulate and propagate nutrients, and release the nutrient substrate slowly, to enable stimulation of the microbes to occur over an extended distance and prevent near wellbore plugging. In the present study, polyethylenimine-dextran sulfate (PEI-DS) polyelectrolyte complexes were used to entrap nutrients commonly used in microbial stimulation. Stability of PEC nanoparticles were tested over time. The toxicity of PEI and DS to a bacterial consortium isolated from Wellington oil field, Wellington, KS was also monitored by aerobic batch culture. The microbes survived at a DS concentration up to 6000 ppm and a PEI concentration up to 3000 ppm. Bacteria growth, as revealed by plate counting, was delayed, compared to equivalent systems where the nutrient mixture was not entrapped (positive control group). This is consistent with the hypothesis that PEC nanoparticles delay the growth of microbes by entrapping their nutrient source. Compared to the positive control group, the entrapped nutrient group has a 102 magnitude decrease in microbial number for the first 72 hours; also the entrapped nutrient group requires at least 96 hours more to reach the stationary phase.