Summary
Cased cemented completions have not been the preferred horizontal-well
completion method in offshore Brazil. Lower-cost solutions such as uncemented
preperforated liners were often used in completing horizontal wells offshore
and are usually very effective. Often, however, low production rates mean that
stimulation treatments become necessary for many wells. The use of conventional
stimulation technology has generally been ineffective for these completions,
which posed a challenge for the operator to find an effective solution for
continuing developments in some fields. These challenges included reevaluating
the more expensive cased cemented completions to allow more effective options
for future stimulation, as well as trying to find newer stimulation techniques
that can be effective with lower-cost completions (noncemented liners).
In the attempt to find an economical yet effective stimulation solution, the
operator chose to implement a unique and relatively new hydrajet stimulation
technique that has a proven success rate in onshore applications. The technique
can be applied in either sandstone or carbonate formations, which are
commonplace in this field; therefore, stimulation plans in this area will
include fracture acidizing as well as propped fracture stimulations that use a
high concentration of proppants or curable resin-coated proppants (RCPs).
This paper discusses the early results of this investigation. Wells that
were completed and evaluated using different completion schemes are
reviewed.
Background
As with vertical wells, underachieving wells are commonplace in horizontal
completions. This situation can be caused by many things, such as unexpectedly
low permeabilities in the area. However, among horizontal wells, underachieving
wells are probably most often attributable to one or more of the following
circumstances (McDaniel et al. 2002a; Surjaatmadja et al. 1994; Surjaatmadja et
al. 2002a; Surjaatmadja et al. 2003; Abass et al. 1994, 1996):
-
Permeability anisotropy (especially
vertical permeability limitations).
-
Skin damage or near-wellbore plugging of
a natural fracture network.
-
Ineffective stimulation
techniques.
The third circumstance is especially common in openhole horizontal wells,
and even more so in wells that are completed using slotted or preperforated
liners. Preperforated liners could even be deemed “unstimulatable” when
conventional fracture-type stimulation techniques are being considered.
Matrix-type stimulation or wellbore wall-cleaning techniques would probably be
the only viable solution for production enhancement in this type of well. For
moderate- to high-permeability reservoir applications, hydrajetting, when
coupled with “squeeze” techniques (below fracturing pressures), has proved to
be a very beneficial production-enhancement process (Rees et al. 2001;
Surjaatmadja et al. 2002b). Unfortunately, because formation layers tend to be
horizontal, horizontal wells tend to stay within one layer over a long
distance, and permeability anisotropy becomes a serious problem.
Another characteristic of horizontal wells is that they are often used in
formations with very limited thickness; hence, their natural production
performance has been artificially disadvantaged. Fracture stimulation
techniques are believed to offer the best opportunity to achieve adequate
stimulation under these conditions.
One possible solution that has been attempted is the use of very high-rate
water-frac treatments. These treatments have often produced a disappointing
production response, and fracture-mapping techniques applied during some of
these treatments have demonstrated the inefficiency of this treatment method
when the operator attempts to create multiple fractures along the entire length
of the wellbore (McDaniel and Willett 2002; Duttlinger 2001; Fisher et al.
2002; East et al. 2004). A logical continuation of this line of thought might
lead to the use of sealing devices. Unfortunately, sealing devices such as
inflatable packers tend to be ineffective when used in horizontal openhole
completions, because fractures tend to jump over them, creating a passageway.
For preperforated-liner applications, a passageway obviously has been
established by the annulus region behind the liner.
Based on these stipulations, the operator of an offshore Brazil field
decided to implement a relatively new hydrajet fracturing process. This would
be the first hydrajet fracturing stimulation performed offshore.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
7 June 2004
- Revised manuscript received:
4 October 2006
- Manuscript approved:
27 January 2007
- Version of record:
20 June 2007
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