In recent years, solid-free, hight-density brines such as CaBr2-CaCl2 or NaBr-NaCl conc. solution, became to be used as completion fluid. Generally, the advantages using the brines are recognized, i.e, they are chemically stable, and their corrosivity are not high. However, as is usual at pure system, it was concerned about that the circumstanes should change if some contaminant come into those brines, owing to lack of their buffer action. Several laboratory corrosion tests were carried out in those brines using L-80 test coupons. In case of oxidizing agent coexisted, the corrosivity of the brines increased drastically. For example, 1% of NaClO3 added into NaBr-NaCl brine, pushed up the corrosion rate more over 80 times as compared with solution of, 1% NaClO3 only. Organic and inorganic inhibitors were tried to control the corrosion, but the performance of both inhibitors were unsatisfactory in the very severe conditions. This phenomenon is considered to be induced from hydration of high concentrated electrolyte, which restricts free water molecules. The interpretation was supported by analytical research of 1H-NMR measurements of thebrines.
Enrichment culture has been carried out to develop microorganisms for MEOR. In the 1st report, it was reported that 49 samples were collected and 41 samples developed cultures with kerosene as substrates at 30°C, It was also reported that the growth rate, pH and IFT as well as the salt tolerance were used as representatives of the function of bacteria for selection, and 8 cultures were finally selected. In the 2nd report, 4 cultures out of 8 reported in the 1st report were chosen for isolation and 4 isolated bacteria were used for investigating the influence of concentration of metal ions generally present in the water of oil field. In these studies, bacteria obtained show functional characteristics for EOR at some degree but not outstandingly. This may be attributed to the unsatisfactory nature of kerosene as substrates. In the present study, the yeast extract was added to the liquid medium as well as kerosene and the effect was investigated for 4 bacteria used in the previous study, from the viewpoint of functional characteristics for MEOR. Results are summarised as follows; 1. Growth became more active for all bacteria and the concentration reached 108-109 cells/ml in 1 to 2 days. The growth rate increased 3.8 to 9.1 times to that in culture with only kerosene as substrate. 2. The maximum concentration was 7×108 to 5×109 cells/ml, which was insignificantly higher than that in case of kerosene only. 3. The pH of the liquid mediums at the end of the culture was 7 to 8.5. 4. IFT between kerosene and liquid medium after 2 week culture decreased from several 10 mN/m to 1mN/m. Particularly 2-F shows IFT less than 0.1mN/m, which implies the useful character for MEOR. From results summarised above, it may be concluded that the adding the yeast extract to the liquid medium with kerosene is effectve for MEOR.
Our previous studies1-3) showed a bacterium 2F-NW 8901 among several ten bacteria could be useful for MEOR. In the present study, conditions for enrichment culture of 2F-NW 8901 in an oil bed were investigated. Results may be summarized as follows. 1. Supplying oxygen by any manner of means is required for active growth, but supply of sugar is not required. 2. Since 2F-NW 8901 grows relatively fast and lives at the anaerobic condition, a short-time supply or cyclic supply of air may be sufficient for growth. 3. Surface active agents produced may be only the metabolites in hydrocabon oxidation pathways. 4. Adding the yeast extract or equivalents to the liquid medium is effective for MEOR. 5. Because the concentration of Fe2+ ions in a formation water is generally less than that required for the growth of bacteria, adding Fe2+ may be effective for the growth of 2F-NW 8901. 6. 2F-NW 8901 is Al3+ tolerant to some extent, so that the enrichment culture may be effectively done with the presence of low concentration Al3+ ions.