Microbial enhanced oil recovery process (MEOR) involve the use of reservoir microorganisms or specifically selected natural bacteria to produce specific metabolic events that leads to enhanced oil recovery. The processes that facilitate oil production are complex and may involve multiple biochemical processes. Microbial biomass or biopolymers may plug high permeability zones and lead to increased mobilization of residual oil, may increase gas pressure by the production of carbon dioxide, or may reduce the oil viscosity due to digestion of large molecules. (Banat, 1995)
From the microbiologistâ€™s perspective, microbial enhanced oil recovery processes are somewhat akin to in situ bioremediation process. Injected nutrients, together with indigenous or added microbes, promote in situ microbial growth and/or generation of products that mobilize additional oil and move it to producing well through reservoir repressurization, interfacial tension/oil viscosity reduction and selective plugging of the most permeable zones (Bryant and Lindsey, 1996). Alternatively, the oil-mobilizing microbial products may be produced by fermentation and injection into the reservoir.
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This technology requires consideration of the physiochemical properties of the reservoir in terms of salinity, pH, temperature, pressure, and nutrient availability (Khire and khan, 1994). Only bacteria are considered promising candidates for microbial enhanced oil recovery. Molds, yeasts, algae, and protozoa are not suitable due to their size and inability to grow under the conditions present in the reservoir. Many petroleum reservoirs have high concentration of sodium chloride (Jenneman, 1989) and require the use of bacteria that can tolerate these conditions is necessary. Bacteria producing biosurfactants and biopolymers can grow at sodium concentration up to 8 percent and selectively plug sandstone to create a biowall to recover additional oil (Raiders et al., 1989).
Organisms that participate in oil recovery produce a variety of fermentation products such as carbon dioxide, methane, hydrogen, biosurfactants, and hydrocarbons. Organic acids produced through fermentation readily dissolve carbonate and can greatly enhance permeability in limestone reservoir. Attempts have been made to promote anaerobic production.
The microbial enhanced oil recovery process may modify the immediate reservoir environment in a number of ways that could also damage the production hardware or the formation itself. Certain fate reducers can produce hydrogen sulphide, which can corrode pipeline and other component of the recovery equipments.
Despite numerous microbial enhanced oil recovery test, considerable uncertainty remains regarding process performance. Ensuring success requires an ability to manipulate environmental conditions to promote growth and/or product formation by the participating microorganisms. Exerting such control over the microbial system in the subsurface is itself a serious challenge. In addition, conditions vary from reservoir to reservoir, which calls for reservoir-specific customization of the microbial enhanced oil recovery process. This alone has the potential to undermine the economic viability of the microbial process.
Microbial enhanced oil recovery differs from chemical enhanced oil recovery in the method by which the enhancing products are introduced into the reservoir. In oil recovery by the cyclic microbial method, a solution of nutrient and microorganism is introduced into the reservoir during injection. The injection well is shut for an incubation period to allow the microorganism to produce carbon dioxide gas and surfactants that assist in mobilization of the oil. The well is then opened, and oil and oil products resulting from the treatment are produced. The process is repeated as often as oil can produce from the well. In the oil recovery by microorganism flooding, the reservoir is usually conditioned by the water flush, and a solution of microorganism and nutrient is injected into the formation. As this solution is pushed through the reservoir by water drive, gases and surfactants are formed, and oil is mobilized and pumped through the well; however, even though microbes produce the necessary chemical reactions in situ whereas surface injected chemical may tend to follow areas of higher permeability, resulting in decrease sweep efficiency (increase in volume of rock contacted by injected fluid), there is need for caution and astute observation of the effects of the microorganisms on the reservoir chemistry.
In a microbial enhanced oil recovery process, conditions for microbial metabolism are supported via injection of nutrient. In some processes, this involves injection of fermentable carbohydrate into the reservoir. Some reservoir also require inorganic nutrient to serve as substrate for cellular growth or to serve as alternative electron acceptor in place of oxygen or carbohydrate.
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The stimulation of oil production by in situ bacterial fermentation is thought to proceed by one or a combination of the following mechanisms:
Improvement of the relative mobility of oil to water by biosurfactants and biopolymers.
Partial repressurization of the reservoir by methane and carbon dioxide.
Reduction of oil viscosity through the dissolution of organic solvent in the oil phase.
Increase of the reservoir permeability and widening of the fissures and channels through the etching of carbonaceous rock in limestone reservoir by organic acids produced by anaerobic bacteria.
Cleaning of the well bore region through the acid and gas from in situ fermentation. The gas pushes oil from dead space and dislodges debris that plugs the pores. The average pore size is increased, and as a result, the capillary pressure near the well bore is made more favourable for the flow of oil.
Selective plugging of high permeable zones by injecting slime-forming bacteria followed by sucrose solution, which initiates the production of extracellular slimes and improves aerial sweep efficiency.
One of the major attributes of microbial enhanced oil recovery is its low cost, but there must be recognition that it is a single process. Furthermore, reports on the deleterious activities of microorganisms in the oil field contribute to the scepticism of employing technologies using microorganisms. It is also clear that scientific knowledge of the fundamentals of microbiology must be coupled with an understanding of the geological and engineering aspects of oil production in order to develop microbial enhanced oil recovery technology.