Plugging of Fractures in Chalk Reservoirs by Enzyme-Induced Calcium Carbonate Precipitation

Natural or induced fractures in a chalk reservoir call 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 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 its 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 oil design Of all improved plugging fluid based Oil 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 front 20 to more than 200 g/L. Furthermore, the crystallization can he 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