Microbial mineral precipitation occurs constantly over the geological time. Recently, a parented technology has been introduced to expedite this bacteriogenic mineral precipitation process. Petroleum geologists have employed the process to selectively plug an unwanted zone in the reservoir to enhance oil recovery. The process induces natural cementation or plugging in sediments or rock formations. The 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 bacterial cells 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 Paiteuni 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 formation to prevent sand production.
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, as evidenced through measurement o f local permeabilities and compressive strength.
Finally, a mathematical model is presented The model incorporates both bacterial transport and growth within a porous medium. By introducing a phenomenological relationship between bacterial concentration and porosity, the permeability of the system can be accurately predicted. This permeability, then can be linked to the compressive strength and fracture mechanics of the system. The numerical model allows one to optimize operating parameters of a bacterial consolidation scheme.
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. But when bacteria interact with these ions such as Ca2+, Si4+, Fe3- and Mn4+, precipitation takes place (Beveriidge et al., 1985) and plugging or cementing occurs as a result.
Bacteria are found to absorb and precipitate metal in the tresh water of the Amazonian River system (Konhauser et al., 1993). The most abundant mineral phase associated with bacteria is a complex (Fe, Al) silicate with a variable composition. The amount of metal sorption and biomineralization largely reflect the availability of dissolved metals in the water.
