A Study of Formation Damage of Selective Clay and Other Minerals Caused By Bacterial Plugging

Abstract

Summary In this study, we investigate the nature of formation damage caused by growth of bacteria on formation minerals. The bacteria growth results in plugging the pores with consequent reduction in permeability. We performed experiments by injecting live bacteria cells through pipettes packed separately with different clay and nonclay minerals. We tested a total of eight minerals: nonclay (quartz, feldspar, calcite, and olivine) and clay (illite, kaolinite, calcium montmorillonite, and sodium montmorillonite). Bacteria used for the experiment was Bacillus Cereus. A 10% trypticase soy broth (TSB) solution as a brine was passed through packed minerals, and the flow rates were recorded periodically before and after the bacteria injection. The flow pressure, temperature, and the concentration of bacteria cells were held constant during these experiments. The absolute permeability of packed porous media was determined with Darcy's equation. The pore plugging effects were analyzed with the percentage ratio of reduced absolute permeability to the initial absolute permeability vs. the injection PV. We measured the zeta potential for each mineral under test before and after its interaction with bacteria. This was another way of testing the affinity of bacteria with a negative charge toward any particular mineral and their subsequent accumulation around that mineral in the pore space. For this purpose, we used a particle microelectrophoresis instrument to measure the zeta potential of the bacteria, the minerals under test, as well as the cumulative zeta potential of mineral-bacteria assemblage. Permeability measurements and zeta potential readings indicate that the type of mineral present has a significant effect on plugging characteristics of the porous media. We determined that olivine as compared with other minerals had the highest tendency to attract bacteria. The reason for this appears to be the abundance of magnesium and iron as nutrient sources for bacteria. These studies also showed that both clays (calcium and sodium montmorillinite) carry a larger net negative charge than other clay minerals after interaction with bacteria. To be certain about the accuracy of the results of mineral-bacterial interaction, we measured the optical density of solutions as well with a third testing before and after injection of bacteria into porous column. Furthermore, we verified the growth and plugging effect of bacteria in the minerals porous column through the use of a scanning electron microscope (SEM). As remedial measures, although not a part of this work, we have recommended a few techniques.