Summary
Acid-fracturing treatments are used commonly to enhance the productivity of
carbonate formations with low-permeability zones. Various forms of hydrochloric
acid (HCL) are used to create deep etched fractures. However, regular HCl
reacts very fast with limestone and high-temperature dolomite formations and,
unless retarded, will produce a fracture with low conductivity. In addition,
concentrated HCl-based acids are very corrosive to well tubulars, especially at
high temperatures. To address problems associated with concentrated acids,
various retarded acids were introduced. Organic acids were used also in some
cases. These organic acid systems were used successfully to acid fracture
several wells in a deep gas reservoir in Saudi Arabia. Field data, however,
indicated that there is a need to create deeper and more-conductive fractures.
To achieve this goal, it was decided to conduct a field trial with a newly
developed acid system.
The new acid system is an ester of an organic acid in the form of solid
beads. The ester reacts with water (hydrolyzes) at bottomhole temperature and
produces lactic acid, which reacts with carbonate minerals and etches the
surface of the fracture. The system was examined thoroughly in the laboratory
and showed promising results.
The treatment was conducted in the field without encountering operational
problems. After successful placement of the solid beads in the fracture, the
well was shut in for 24 hours to give ample time for the ester to hydrolyze and
for the generated acid to react with the formation rock. The well was allowed
to flow, and samples of the fluids produced were collected to understand
chemical reactions that occurred during the treatment. The treatment has
resulted in a slight increase in gas production, and no significant improvement
was noted over a 9-month period. Consequently, the well was matrix acidized
with 28 wt% HCl and responded positively to the treatment.
This paper will discuss major reactions that occurred during these
treatments and how they impacted well response. Lessons learned and
recommendations to improve the results of this new acid system will be
given.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
15 August 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
16 September 2008
- Manuscript approved:
13 October 2008
- Published online:
1 May 2009
- Version of record:
1 May 2009
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