Permeability profile modification using bacterially formed calcium carbonate: comparison with enzymic option

Porous media permeability profile modification was studied using biomass and bacterially formed CaCO3 as plugging agents. The candidate bacterium, an isolate from a Canadian oil field, was able to decompose urea at concentrations as high as 20 g/L. Bacterial production of CaCO3 was achieved in the cultures containing 5, 10 and 15 g/L of urea and equimolar concentrations of CaCl2·2H2O (12.5, 25 and 37.5 g/L, respectively). Combination of higher concentrations of urea and CaCl2·2H2O (20, 50 and 30 and 75 g/L) prevented microbial activity and the production of CaCO3. The maximum concentration of bacterially produced CaCO3, 21.5 g/L, was substantially lower than 58 g/L of CaCO3 obtained in the presence of urease enzyme. In contrast to the enzymic reaction, bacterial production of CaCO3 was not sensitive to temperature. Injection of inoculated medium containing only urea (plugging by biomass) resulted in a 52-56% decrease in the permeability of the porous media, with the extent of plugging being independent of urea concentration. Bacterially formed CaCO3 contributed to plugging of porous media when 10 g/L of urea and 25 g/L CaCl2·2H2O were used in the injected medium, resulting in a further 13% decrease in permeability. The repeated injection of inoculated medium containing urea or a combination of urea and calcium chloride increased the extent of plugging in porous media. A comparison of the permeability ratio following the plugging by biomass (52%), a combination of biomass and bacterially formed CaCO3 (65%) and enzymically formed CaCO3 (62%) indicates that bacterial and enzymic options are both efficient in plugging of porous media. However, with the bacterial option, the decrease in permeability is mainly due to formation of biomass, which is not a durable plugging agent and contribution of bacterially formed CaCO3 is small.