Microbial enhanced oil recovery (MEOR) flooding involves the injection of microorganisms that react with reservoir fluids to assist in the production of residual oil. The US National Institute for Petroleum and Energy Research (NIPER) maintains a database of field projects that have used microbial technology. There has been significant research conducted on MEOR, but few pilot projects have been conducted. The Oil and Gas Journal’s 2012 survey reported no ongoing projects in the United States related to this technology.24 However, researchers in China have reported mild success with MEOR.25
There are two general types of MEOR processes—those in which microorganisms react with reservoir fluids to generate surfactants or those in which microorganisms react with reservoir fluids to generate polymers. Both processes are discussed here, along with a few concluding comments regarding the problems with applying them. The success of MEOR processes will be highly dependent on reservoir characteristics. MEOR systems can be designed for reservoirs that have either a high or low degree of channeling. Therefore, MEOR applications require a thorough knowledge of the reservoir. Mineral content of the reservoir brine will also affect the growth of microorganisms.
Microorganisms can be reacted with reservoir fluids to generate either surfactants or polymers in the reservoir. Once either the surfactant or polymer has been produced, the processes of mobilizing and recovering residual oil become similar to those discussed with regard to chemical flooding.
Most pilot projects have involved an application of the huff and puff or thermal-cycling process discussed with regard to thermal flooding. A solution of microorganisms is injected along with a nutrient—usually molasses. When the solution of microorganisms has been designed to react with the oil to form polymers, the injected solution will enter high-permeability zones and react to form the polymers that will then act as a permeability reducing agent. When oil is produced during the huff stage, oil from lower permeability zones will be produced. Conversely, the solution of microorganisms can be designed to react with the residual crude oil to form a surfactant. The surfactant lowers the interfacial tension of the brine-water system, which thereby mobilizes the residual oil. The oil is then produced in the huff part of the process.
The reaction of the microorganisms with the reservoir fluids may also produce gases, such as CO2, N2, H2, and CH4. The production of these gases will result in an increase in reservoir pressure, which will thereby enhance the reservoir energy.
Since microorganisms can be reacted to form either polymers or surfactants, a knowledge of the reservoir characteristics is critical. If the reservoir is fairly heterogeneous, then it is desirable to generate polymers in situ, which could be used to divert fluid flow from high- to low-permeability channels. If the reservoir has low injectivity, then using microorganisms that produced polymers could be very damaging to the flow of fluids near the wellbore. Hence a thorough knowledge of the reservoir characteristics, particularly those immediately around the wellbore, is extremely important.
Reservoir brines could inhibit the growth of the microorganisms. Therefore, some simple compatibility tests could result in useful information as to the viability of the process. These can be simple test-tube experiments in which reservoir fluids and/or rock are placed in microorganism-nutrient solutions and growth and metabolite production of the microorganisms are monitored.
MEOR processes have been applied in reservoir brines up to less than 100,000 ppm, rock permeabilities greater than 75 md, and depths less than 6800 ft. This depth corresponds to a temperature of about 75°C. Most MEOR projects have been performed with light crude oils having API gravities between 30 and 40. These should be considered “rule of thumb” criteria. The most important consideration in selecting a microorganism-reservoir system is to conduct compatibility tests to make sure that microorganism growth can be achieved.
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