Summary
Cleanup of drilling-fluid filter cake in long horizontal and multilateral
wells is a difficult task. Both mechanical and chemical means (acids,
oxidizers, chelating agents, and enzymes) have been used in the field. However,
these methods have serious limitations and can affect well performance
adversely. Acids and oxidizers are very reactive, but are nonspecific. Enzymes
can be used to degrade starch and xanthan polymers. The major limitation of
enzymes is their inability to stay active at temperatures greater than
200°F.
The activity of enzymes at 207°F was less than 10% of their activity at
200°F. Previous studies to increase the stability enzymes were aimed at: (1)
changing salinity, (2) changing the source of the enzymes, (3) using additives
that alter the conformation of enzymes, or (4) modifying the enzymes
chemically. These techniques did not maintain the activity of the enzymes at
200°F. For filter-cake removal, enzymes have been used globally at higher
temperatures, though their activity is known to be much less than 2%.
This paper examines the use of chemicals that function as a stabilizer for
enzymes up to 250°F. Optimized concentrations of this stabilizer not only
inhibit the coagulation of enzymes at these high temperatures but also maintain
the activity of enzymes at the high temperatures that are encountered in the
oil field. Laboratory studies on the stability, compatibility, and
filter-cake-damage reversal were conducted with stabilized-enzyme
formulation.
Experimental results indicated that the new proprietary enzyme stabilizer
was very effective in eliminating the deactivation tendency of enzymes at high
temperatures. Conventionally, enzymes coagulated at temperatures greater than
140°F and charred when exposed to temperatures greater than 200°F, but in the
presence of the stabilizer, precipitation and burning were not observed. When
using an enzyme at high temperatures without the stabilizer, the burned residue
accumulated on the surface, resulting in a thin film that reduced the
permeability of the cake even further. High-pressure/high temperature (HP/HT)
filtration studies using the enzyme stabilizer showed that this film was not
formed, which resulted in a cake with much higher retained permeability.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
15 July 2009
- Meeting paper published:
5 October 2009
- Revised manuscript received:
31 December 2009
- Manuscript approved:
9 March 2010
- Published online:
8 July 2010
- Version of record:
11 August 2010
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