Biomass plug development and propagation in porous media.

Abstract

Understanding the plugging of porous media with bacteria and their exopolymers (i.e., biomass) is important for application of bacterial profile modification. A model bacterium, <italic>Leuconostoc mesenteroides</italic>, was used in packed bed and micromodel experiments to elucidate (1) biomass plug development and propagation mechanisms and (2) pore-scale physics. Biomass plugging of porous media occurred in three phases. The first phase was exopolymer induction, characterized by a low, constant pressure drop across the porous media. During this phase, colonies grew but remained noncontiguous and did not fill the pore spaces. The second phase was plugging, characterized by a steady increase in pressure drop across the porous media concurrent with the filling of pore space as the bacterial colonies grew. The third phase was plug propagation, characterized by pressure oscillations across the porous media. The oscillations corresponded to sequential plug development in the direction of nutrient flow followed by formation of breakthrough channels as the biomass, which acted similar to a Bingham plastic, yielded. Pore-scale investigations revealed that the plugs were developed as biofilm grew along dominant flow paths created by interconnected pore throats with the larger radii. Just before channel breakthrough, the plug contained isolated interconnected groupings and individual empty pore throats. Many of these empty throats were incorporated into the subsequent breakthrough channel that developed along a path of lowest wall surface area. The breakthrough initiated at the back boundary of the plug and propagated forward, because the biomass exhibiting lower shear stress, i.e., weaker biomass, was found here and nutrient was funneled to this region as plugging proceeded, causing a higher pore throat velocity and wall shear stress. The weaker biomass was due to production of less exopolymer because of lower sucrose concentrations in this region. Subsequent plugs developed as the breakthrough channel refilled with biomass and a new area of the porous media just downstream of the previous plug also filled with biomass. The breakthrough channel location shifted with nutrient injection time due to <italic>in situ</italic> growth under high shear stress. Two-dimensional network modeling confirmed that the combination of <italic>in situ</italic> growth and Bingham plastic behavior of the biofilm accounted for plug development and propagation. The key implication of these findings to bacterial profile modification is that biomass growth and exopolymer production must be delayed to encourage plug development away from the well bore. This approach is needed because of the high pressure drop per unit distance associated with normal biomass plug development and propagation in porous media.