Process Biochemistry 40 (2005) 925–933 Permeability profile modification using bacterially formed calcium carbonate: comparison with enzymic option M. Nemati a,∗ , E.A. Greene b , G. Voordouw b a Department of Chemical Engineering, University of Saskatchewan, Saskatoon, Sask., Canada S7N 5C5 b Department of Biological Sciences, University of Calgary, Calgary, Atla., Canada T2N 1N4 Received 28 January 2004; accepted 25 February 2004 Abstract Porous media permeability profile modification was studied using biomass and bacterially formed CaCO 3 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 CaCO 3 was achieved in the cultures containing 5, 10 and 15 g/L of urea and equimolar concentrations of CaCl 2 ·2H 2 O (12.5, 25 and 37.5 g/L, respectively). Combination of higher concentrations of urea and CaCl 2 ·2H 2 O (20, 50 and 30 and 75 g/L) prevented microbial activity and the production of CaCO 3 . The maximum concentration of bacterially produced CaCO 3 , 21.5 g/L, was substantially lower than 58 g/L of CaCO 3 obtained in the presence of urease enzyme. In contrast to the enzymic reaction, bacterial production of CaCO 3 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 CaCO 3 contributed to plugging of porous media when 10 g/L of urea and 25 g/L CaCl 2 ·2H 2 O 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 CaCO 3 (65%) and enzymically formed CaCO 3 (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 CaCO 3 is small. © 2004 Elsevier Ltd. All rights reserved. Keywords: Microbially enhanced oil recovery; Porous media; Permeability; Selective plugging; Biomineralization; Calcium carbonate 1. Introduction Microbially enhanced oil recovery (MEOR) is a poten- tially important tertiary technology, employing microor- ganisms and their metabolites for the recovery of the oil remaining in the reservoir after primary and secondary productions. Microbial cells contribute to enhanced oil re- covery in a number of ways. Microbial acid production has generally been considered one of the most important mech- anisms for MEOR in carbonate reservoirs which increases the permeability and the porosity of the oil-bearing strata [1,2]. Biogenic production of gas is another contributing factor in the recovery of oil. The produced gas increases the reservoir pressure, enhances the mobility of the oil and ∗ Corresponding author. Tel.: +1-306-966-4769; fax: +1-306-966-4777. E-mail address: Mehdi Nemati@engr.usask.ca (M. Nemati). changes the flow patterns by introducing another mobile phase. Microbial production of solvents, such as butanol and acetone, which are known to decrease the viscosity of oil and production of biosurfactants which reduce the in- terfacial tension between water and oil are other important mechanisms for the enhancement of oil recovery. One of the major problems in secondary oil recovery by water flooding is the variation of reservoir rock permeabil- ity. Injected water preferentially flows through the areas of high permeability, thus the residual oil in the region of low permeability is by-passed and remains unrecovered [3,4]. Selective plugging of reservoir rock by microbial biomass and extracellular biopolymers produced by micro- bial cells has been suggested as an efficient way for profile modification and improving the sweeping efficiency of the injected fluids [1,3,5,6]. In addition, some species of bac- teria have the ability to accumulate or induce the formation and precipitation of various inorganic compounds, such as 0032-9592/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2004.02.019