Summary
Long horizontal and multilateral oil and gas wells provide an attractive
option for maximizing reservoir contact. Formate-based brines have been used in
drilling and completion operations for more than 10 years. Advantages of these
fluids include high density, solid-free brines, better compatibility with XC
polymer, starches, and less potential for formation damage. These advantages
were reported in several previous publications.
Filter cakes that are generated by drilling fluids can act as a barrier to
the fluid flow in both cased and openhole wells. Calcium carbonate particles
are frequently used as weighting material to maintain the pressure that is
required for well control and minimizing leakoff rate. These solid particles
become consolidated and trapped in a polymeric material, and this makes the
filter cake a very effective permeability barrier.
The conventional method for cleaning filter cake is by using solids-free
formate brines, either by soaking or circulating for many hours at high flow
rates. This mechanical technique removes only external filter cake. Chemical
means like acids, oxidizers, chelating agents, enzymes, or combinations of
these chemicals are usually used as an alternative method for dissolving both
the external and internal filter cake. Most of these fluids cannot give a full
coverage to the wellbore because of the formation heterogeneity.
A new precursor (ester of an organic acid) can generate an acid downhole at
a low release rate, which results in uniform fluid distribution through the
wellbore. Compatibility and thermal stability tests between the precursor
solution and formation brine were studied in detail. Return permeability
experiments were conducted by using high-pressure/high-temperature (HP/HT)
dynamic fluid loss cells. The ester solution was effective in cleaning the
filter cake in comparison to the formate brines.
A field application, where formate brine was used to drill and complete a
gas producer in a sandstone reservoir, is included in this paper. The gas well
was drilled in a weakly consolidated reservoir and was completed with
expandable sand screens (ESSs). The bottomhole temperature was 285°F. The
produced gas contained 2–3 vol% carbon dioxide and no hydrogen sulfide. Full
analysis of flowback samples indicated that most of the returned solids were
calcium carbonate. Laboratory tests indicated that the ester solution can be
used to restore well productivity by removing damage induced by the drilling
mud filter cake.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
22 August 2007
- Meeting paper published:
4 December 2007
- Revised manuscript received:
21 November 2008
- Manuscript approved:
8 December 2008
- Published online:
6 August 2009
- Version of record:
28 September 2009
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