Laboratory Study On Activating Indigenous Microorganisms to Enhance Oil Recovery
Authors: Abdel-Waly A.A. 
Published: March 2010

Abstract
A heavy crude oil (19 ° API gravity) and connate water (from the Mauddud formation in El-Bahrain) were used in the present study, to investigate the effect of microbial growth on water viscosity, pH and oil/water interfacial tension. Displacement tests were carried out to study the effect of nutrient concentration, incubation temperature and carbonate content on the oil recovery from waterflooded residual oil.
The study showed that the indigenous bacterial population in the formation water and the crude oil could be activated by a molasses based nutrient to produce biosurfactants, acids, polymers, and gases. It was found that a nutrient slug of 4% by weight molasses concentration was the optimum at which the indigenous microorganisms have been given maximum growth and resulted in higher oil recovery than that obtained at other tested molasses concentrations. For the tested incubation temperatures, the results showed that the indigenous microbial growth was better at 25 ° C which in turn was reflected positively upon oil recovery. It was also found that the presence of carbonates in a reservoir can provide a buffering capacity for complete fermentation to occur where it neutralizes the acids produced by bacteria and increases the carbon dioxide production. The oil recovery was increased by increasing the carbonate content. However, with further increase in carbonate content above 20% no more oil was recovered.

Introduction
Microbial methods for increasing oil recovery are potentially cost-effective even at relatively low crude oil prices. Microbial formulations can be applied in a variety of methods including permeability modification treatments and microbial-enhanced waterflooding. A number of mechanisms have been considered to be important for the recovery of residual oil through the application of microbial enhanced oil recovery (MEOR). Such mechanisms include: production of biosurfactants to reduce the oil/water interfacial tension and improve the displacement efficiency, production of biopolymers to improve mobility control and increase sweep efficiency, production of solvents that reduce the oil viscosity and increase its mobility, and production of biogenic acids that dissolve carbonate rock, increase pore space and produce more carbon dioxide. Carbon dioxide in the gaseous form would expand producing a driving mechanism, or if dissolved would reduce the crude viscosity thus increasing its mobility(1–5). The in situ application of the process used to accomplish these mechanisms requires the growth and/or metabolism of microorganisms in the rock matrix. Whether the microorganisms used to carry out these functions are indigenous or added exogenously, they must function effectively at reservoir conditions of salinity and temperature with the aid of suitable nutrients.
The existence of indigenous bacteria in many oil reservoirs was practically proved. These indigenous microorganisms nearly always belong to undescribed species, or even to new genera(6). In nearly all the cases studied up to now, the indigenous bacteria consist solely of strict anaerobes. The species developing in the oil reservoirs seem to have undergone a natural selection in situ as a function of their physiological properties. They may also be related to the geological period during which they were put into place.