Plugging of Fractures in Chalk Reservoirs by Enzyme-Induced Calcium Carbonate Precipitation
- Jan Larsen (Maersk Olie og Gas AS) | Mikael Poulsen (Danish Technological Institute) | Thomas Lundgaard (Danish Technological Institute) | Mikkal Agerbaek (Danish Technological Institute)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- November 2008
- Document Type
- Journal Paper
- 478 - 483
- 2008. Society of Petroleum Engineers
- 4.2.3 Materials and Corrosion, 4.2 Pipelines, Flowlines and Risers, 5.8.7 Carbonate Reservoir, 4.1.2 Separation and Treating, 4.3.4 Scale, 1.6 Drilling Operations, 4.1.5 Processing Equipment, 6.5.2 Water use, produced water discharge and disposal, 4.5 Offshore Facilities and Subsea Systems, 3 Production and Well Operations, 1.2.3 Rock properties, 5.5 Reservoir Simulation
- natural fractures, chalk reservoirs, enzymatic calcium carbonate
- 1 in the last 30 days
- 499 since 2007
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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.
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.
|File Size||1 MB||Number of Pages||6|
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