The Effect of Bacteria Penetration on Chalk Permeability

Bacteria selective plugging is one of the mechanisms through which microorganisms can be applied for enhanced oil recovery. Bacteria can plug the water-bearing zones of a reservoir, thus altering the flow paths and improving sweep efficiency. It is known that the bacteria can penetrate deeply into reservoirs, however, a complete understanding of the penetration behavior of bacteria is lacking, especially in chalk formations where the pore throat sizes are almost comparable with the sizes of bacteria vegetative cells. This study investigates the penetration of bacteria into chalk. Two bacteria types, the spore forming Bacillus licheniformis 421 and the non-spore forming Pseudomonas putida K12, were used. The core plugs were Stevns Klint outcrop with initial permeability at 2-4 mD. The results revealed that bacteria were able to penetrate and to be transported through the chalk. Furthermore, a higher number of B. licheniformis was detected on the effluent compared with P. putida. However, in the experiment with B. licheniformis mainly spores were detected in the effluent. The core permeability decreased rapidly during injection of bacteria and a starvation period of 12 days did not allow the permeability to return to initial condition.