Selectivity and depth of microbial plugging in Berea sandstone cores

Abstract

The depth of plugging by the in situ growth of either injected or indigenous microorganisms was investigated using Berea sandstone cores with pressure taps located along the length of the core. The continuous injection of aerobically prepared sucrose-mineral salts medium with 5% NaCl and 0.1% NaNO₃ resulted in large permeability reductions (70–98%). The plugging was localized at the inlet and outlet faces of the cores, and was attributed to microbial biomass production at the inlet face and biogas accumulation at the outlet face. Batch addition of aerobic medium resulted in more uniform permeability reduction along the core's length, but the magnitude of the permeability reduction was not as large (about 65%). The semi-continuous injection of oxygen-free medium resulted in a slower but a more uniform permeability reduction throughout the core compared to cores which received aerobically prepared medium. The selectivity of the process was investigated in a dual core system where two cores of 240 and 760 mdarcy permeability were connected parallel to each other without crossflow. Initially, about 85% of the total fluid flow passed through the high permeability core. After the addition ofBacillus species and medium, the flow pattern changed and about 85% of the total fluid passed through the low permeability core. These results show that the in situ growth of microorganisms can selectively plug high permeability zones and that control of the process may be achieved by alterations in the method of nutrient injection.