Microbial mineral precipitation occurs constantly over the geological time. This process induces natural cementation or plugging in sediments or rock formations. Petroleum microbiologists employed the process as a reservoir selective plugging method to enhance the production of hydrocarbon resources.
Mineral precipitation is induced as a result of microbial activities. Bacteria can deposit minerals directly from the medium through their metabolic activities. They can also precipitate minerals indirectly from the medium by changing regional geological environmental conditions. Mineral precipitations and the dead bacteria bodies can persist as a part of the environment and result in plugging or cementing in pores in that environment. The process is optimized with bacteria Bacillus Pasteurii to precipitate CaCO3 so that the bacteriogenic cementation occurs in hours rather than in years. It is suggested that the process be used to plug fractures in water-producing zones to prevent excessive water production during oil recovery. The same technique can be used to consolidate sands in an unconsolidated fractures.
A series of experiments was conducted to investigate the possibility of using microbial plugging process to remediate fractures and to test factors affecting that process. The effects of pH, temperature and medium on mineral precipitation and bacteria growth are studied in detail. Also, the effect of fracture width and fracture fillings is studied. It is found that the microbial mineral plugging technique is effective in plugging fractures.
Microbes (procaryotes and eucaryotes) distribute widely in geological environments. Natural surface rocks have been observed to have 103 bacteria or fungal cells per gram of stone (Eckhardt, 1985). Microbial metabolic activities play an important role in deposition and diagenesis process in a geological environment (Ferris et al., 1988).
Microbio-mineral-precipitation is not an unusual process in nature. Minerals such as calcite, silicon, oxidized manganese and oxidized iron usually do not precipitate naturally because of the low ionic concentration.