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.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
23 May 2007
- Meeting paper published:
4 September 2007
- Revised manuscript received:
26 October 2007
- Manuscript approved:
8 January 2008
- Version of record:
15 November 2008
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