l. (35) used a mixture of anaerobic thermophilic bacteria that fermented molasses in an MEOR field test to recover naphthenic crude. They also used raw sewage as an inoculum (100 L, along with 20 to 40,000 kg molasses) in later trials (1969-70). The deepest reservoir was 8,200 ft, where the pressure was 228 atm and the temperature was 97 °C. In most reservoirs tested (7 out of 10), the introduced microbial populations produced a positive effect on cumulative oil recovery.
Karaskiewicz conducted 18 field trials in Poland between 1961 and 1969 (36). Microbial cultures were obtained from soil and water samples taken in the vicinity of the oil fields and from sugar factory waters. The mixed culture includes the genera Arthrobacter, Clostridium, Mycobactetium, Peptococcus, and Pseudomonas grown in 10-L bottles with formation water plus 4% molasses, and incubated at 32 °C. The wells ranged in depth from 1,650 to 5,000 ft. The rate of additional oil recovery ranged from 20 to 200% of the original production rate. A second injection of nutrients was considered a major factor in the increased oil recovery.
Lazar (37) recently published an extensive review of MEOR work done in Romania during the last decade. Three areas of research are emphasized: (a) examination of the bacterial population in the formation water of the reservoir; (b) adaptation of these microorganisms in the laboratory for oil release; and (c) field testing of such adapted cultures. Seven wells were treated with microbial formulations and two conclusions were drawn: the bacterial population caused an increase of oil flow up to 200% for 1 to 5 years in 2 out of 7 reservoirs (the other five were unaffected), and much information about the ecology of the reservoir is needed before initiating any MEOR activity.
Overall, MEOR field trials in eastern Europe have been promising. Field testing in Romania is still in progress, and is supported in part by the Romanian government. There are obvious difficulties in obtaining key information regarding European MEOR field tests; a large amount of information may have been accumulated, but at present it is unavailable to the general public.
In the early 1940s, it was observed that bacteria could clog or plug a reservoir. Early studies relating to bacterial plugging of a reservoir were mainly concerned with the prevention of this process.
Kalish et al. (38) studied the effects of bacterial plugging in sandstone cores and determined that the slime-forming and larger bacteria have a much greater effect on permeability reduction than the smaller and non-slime-forming ones. Jang et al. (39) and Jenneman et al. (40) examined the effect of rock permeability on bacterial transport. Both concluded that nutrients such as dissolved sucrose, inorganic salts, and yeast extract move easily through porous media. Jang et al. demonstrated that bacterial spores, because of their size and cell surface properties, travel faster and with less adsorption to rock through sandstone cores than bacterial cells. Jenneman et al. reported that nutrients and bacterial cells are selectively transported to zones of high permeability.
It is clear that microbial movement should receive more attention. Other microbial technological processes used in the environment may help to further transport knowledge and research. Virtually no studies have been done to examine microbial migration under actual reservoir conditions of pressure, temperature, and field brine and oil. These studies are vital to our understanding of microbial transport for MEOR processes.
Because of the decreasing supply of petroleum and the increasing production costs, a potentially inexpensive method such as MEOR may prove useful and economical. However, MEOR has only recently undergone an overall examination of the available technology. This review indicates that many reservoir characteristics must be determined before applying MEOR. Some of these characteristics are porosity, permeability, salinity, temperature, and pressure. It is unlikely that a single MEOR method can be applied to all types