# A Novel Approach to Enhance Crude Oil Recovery Ratio Using Selected Bacterial Species
## 4 Discussions  
Experiments were conducted on the growth of bacteria, adsorption of bacterial cells and metabolites on dolomite rocks, reduction in viscosity, and biotransformation of crude oil. In this study, three strains, _Bacillus_ species, _Pseudomonas putida_, and _Nocardiia cyriacgeorgica_, with potential for biotransforming heavy crude oil were isolated from heavy crude oil-contaminated soil samples collected from Libyan oil fields. The potential of three identified microbial genera for EOR was reported for the first time in Libya.  
The reservoir factors such as temperature, salinity, pH, and pore size affect the effectiveness of microbes in the reservoir [19]. The isolated strains were grown in pH between 6-8 and a crude oil range of 1.0-3.0%. The growth of the bacterial species was increased with increased crude oil concentration to an optimum at 2.0% concentration and only limited growth occurred when the concentration was further increased to 3.0%. Higher concentrations of hydrocarbons might inhibit biodegradation by limiting access to nutrient or oxygen supply or by its toxic effects [20].  
Experience has shown that, with careful planning and repeated sub-culturing, it is possible that these strains can grow at higher crude oil concentrations normally encountered in the reservoirs; however, this was not investigated further in this study.  
Based on optimum temperatures for growth and crude oil concentration requirements, the optimum temperature of the three facultative anaerbes, _Bacillus_ species, _Pseudomonas putida_ and _Nocardiia cyriacgeorgica_, was 55 \({}^{\circ}\)C, while they did not grow at temperatures above 60 \({}^{\circ}\)C. This was similar to the optimum temperature range reported by [PERSON] et al. [21]. _Methanobacterium_ showed optimum growth temperatures of 63 to 66 \({}^{\circ}\)C with a specific growth rate of 0.62 h\({}^{-1}\)[22]. Many oil reservoirs have an initial temperature greater than 75 \({}^{\circ}\)C but due to continuous injection of water for oil recovery over time, the temperature becomes much less than the initial reservoir temperature as the field matures [23]. The modeling work presented in the previous sections showed that the potential strains used can withstand both high crude oil concentration (2.0%) and high temperature conditions (55 \({}^{\circ}\)C). Lower growth rates were recorded at 37 \({}^{\circ}\)C with different concentrations of Bouri crude oil (1.0, 2.0, and 3.0%). Only the _Bacillus_ species gave good growth with crude oil concentrations of 2.0%.  
Many of the bacteria strains used in MEOR usually experienced low efficiency when maintained at higher temperatures, but _Bacillus_ species and _Pseudomonas putida_ were able toexhibit good potential with crude oil concentrations of 2.0% at high temperature. However, what makes these strains better lies in their ability to grow at a high temperature of 55 \({}^{\circ}\)C, in comparison to many bacteria isolated for microbial enhanced oil recovery purposes. The microorganisms that are most used for MEOR field processes are species of _Bacillus_ and _Clostridium_. These species produce spores, which are resistant stages, allowing them to survive in petroleum reservoirs and more stressful environmental conditions [24].