Summary Natural or induced fractures in a chalk reservoir can reduce the recovery of an oil field significantly. Therefore, the plugging of fractures with a wide range of materials has been investigated over the years. Calcium carbonate is an obvious candidate, being the main constituent of the reservoir itself. However, to apply calcium carbonate as a plugging fluid, a mechanism is required for delaying the precipitation until the chemical reaches the fracture. An enzymatically induced plugging mechanism has been suggested, in which the urease enzyme converts urea into ammonia and carbonate. This carbonate will then precipitate with calcium as calcium carbonate. However, the amount of calcium carbonate produced was relatively low and the cost of the stabilizer and high-purity-enzyme source was prohibitively high for practical use. Furthermore, the calcium carbonate precipitated as a slurry of small particles, which is deemed less efficient for fracture plugging when compared to larger crystals or aggregates. In this paper, work is presented on design of an improved plugging fluid based on enzymatic calcium-carbonate precipitation and optimization toward a field-applicable solution. The relatively expensive stabilizer and enzyme source are replaced with low-cost ingredients, and the rate of precipitation is improved. By optimizing the concentrations of the reactants, we have improved the yield of calcium carbonate from 20 to more than 200 g/L. Furthermore, the crystallization can be controlled to obtain much larger calcium carbonate crystals. Laboratory plugging experiments have shown that larger crystal sizes improve the durability of the formed plugs significantly. Introduction Different authors have proposed the use of active, urease-producing bacteria for precipitation of calcium carbonate (Ferris et al. 1996; Stocks-Fischer et al. 1999). The concept of using the urease enzyme directly without in-situ microbiological production has been proposed by Nemati and Voordouw (2003), who demonstrated delayed precipitation and plugging of packed limestone columns. The results presented in this paper illustrate how the reaction rate and reaction yield depend on the reactants. Furthermore, we present improvements to the calcium carbonate crystal size and plugging performance by stoichiometric variations and addition of various chemicals. The perspectives for field trial are discussed in the penultimate section of the paper.

Natural or induced fractures in a chalk reservoir can significantly reduce the recovery from a field. Therefore, plugging of fractures with a wide range of materials has been investigated over the years. Calcium carbonate, being the main constituent of the reservoir itself, is an obvious candidate. In order to apply calcium carbonate as a plugging fluid, a mechanism for delaying the precipitation has previously been suggested in the literature. This mechanism involves enzymatically induced plugging based on the mechanism of the enzyme urease converting urea into ammonia and carbonate. When carbonate is formed, it will precipitate with calcium in solution forming calcium carbonate. However, the amount of produced calcium carbonate was relatively low and the costs of the used stabilizer and high purity enzyme source was prohibitively high for bulk use. Furthermore, the calcium carbonate was produced as a slurry of small particles, which is deemed less efficient for fracture plugging compared to larger crystals or aggregates. In this paper work on design of an improved plugging fluid based on enzymatic calcium carbonate precipitation and optimization towards a field applicable solution is presented. The relatively expensive stabilizer and enzyme source is replaced with a low cost ingredient, which also improves the rate of precipitation. By optimizing the concentrations of the reactants, we have improved the yield of calcium carbonate from the initial 20 g/L to more than 200 g/L. Furthermore, we have been able to control the crystallization and obtain much larger calcium carbonate crystals. Laboratory plugging experiments have shown that larger crystal sizes significantly improves the durability of the formed plugs. Introduction Different authors have proposed the use of active, urease-producing bacteria for precipitation of calcium carbonate (ref. 1–2). The concept of using the urease enzyme directly without in situ microbiological production has been proposed by Nemati & Voordouw in 2003 (ref. 3), who demonstrated delayed precipitation and plugging of packed limestone columns. The results presented in this paper illustrate how the reaction dynamic and reaction yield depend on the reactants. Furthermore, the effects of stoichiometric variations and various additives on calcium carbonate crystal size and plugging performance are presented. The perspectives for field trial are discussed towards the end of the paper.