Modification of porous media permeability, using calcium carbonate produced enzymatically in situ

Abstract

Modification of porous media permeability, using enzymatic formation of CaCO 3, was studied in unconsolidated porous media and Berea sandstone cores. The enzymatic formation of CaCO 3 in a batch system was dependent on the concentrations of enzyme (urease) and reactants (urea and calcium chloride), as well as temperature. An increase in enzyme concentration (from 0.01 to 0.1 g l −1) enhanced the rate of CaCO 3 production, while increases in reactants concentrations up to a certain level (urea and CaCl 2·2H 2O concentrations: 36, 90 g l −1, respectively) increased the quantity of produced CaCO 3 under the experimental conditions examined. With a low concentration of enzyme (0.02 g l −1), an increase in temperature (from 20 to 50 °C) enhanced both the production rate of CaCO 3 and the extent of conversion in the batch system. Experiments in unconsolidated porous media indicated that an increase in urease concentration enhances the extent of CaCO 3 precipitation and leads to a significant decrease in permeability of porous media. The proportional increase of reactants and enzyme concentrations, as well as multiple injections of reaction medium had a marked effect on the extent of plugging, observed as a decrease in permeability of porous media. With 0.03 g l −1 urease, an increase of temperature from 22 to 30 °C enhanced the extent of plugging and the permeability ratio calculated at 30 °C was 26% lower than that achieved at 22 °C. In the presence of a higher concentration of enzyme (0.1 g l −1) temperature did not have a significant effect and the permeability ratios were identical at 22 and 30 °C. Plugging studies in a core-flooding system with Berea sandstone confirmed the effectiveness of CaCO 3 in reducing permeability. Following the injection of reaction mixture the core permeability decreased from 394.4 to 31.3 md (permeability ratio: 0.08; 92% decrease in permeability). A second injection of reaction mixture further decreased the permeability (73%). The overall decrease in permeability after these two injections was 98%. Enzymatic formation of CaCO 3 in situ appears to be a very efficient method for reducing the permeability of porous media. This may have beneficial applications in enhanced oil recovery and consolidation of sand in oil reservoirs, as well as capture of radionuclides and trace element contaminants in ground water systems, through the co-precipitation of these contaminants with enzymatically formed calcium carbonate.