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Rate (MBPD) Presure (psi)
Target 10 2,200
Pre-job 1.7 2,200
Post-job 20.5 570
Table 4. Results of injection test in Well 1
Other Wells
In the other six wells, TD was reached without any problems
and the acid fluids were placed as per the plan. Only two
wells did not need the assistance of the CT tractor. Two
misruns were due to the CT tractor malfunctioning, which
were fixed promptly. Similar injection results to those in
previous wells were observed. A pre-treatment injection test
was not recorded in these wells to save operational time.
In Well #8, where the CT tractor was not needed, the CT
could not pass after 9,210 ft, presumably due to an
obstruction or washout. After the inclusion of 16 ft of straight
bars, the CT was able to reach TD at 12,587 ft. Table 3
provides a summary of the main statistics in these wells.
RESULTS
The first eight ERWs in the Manifa field were stimulated with
CT. The CT reached TD in all wells except one. More than
14,000 bbls of acid fluids were injected during the campaign.
Only one well did not need production logging data. The
operational experience with the hydraulic tractor included
servicing six injector wells and “walking” over 39,500 ft, the
majority of which was open hole. Four times lock-up occurred
while the BHA was still inside the casing, resulting in the
tractor having to operate through the shoe, all of which were
successful. Average speeds for each run ranged from 480 ft per
hour - 1,000 ft per hour. Other accomplishments include
setting a world open hole “walking” record of 5,986 ft in a
6 1 ⁄8” open hole.
MPLT was recorded in a total of six wells. The average
injection performance after the treatment was 7 bpm at 700
psi; however, there were wells with injection pressures of 250
psi for the same rates. This represents a significant
improvement compared to injection conditions before the
treatment at 3.5 bpm and 2,200 psi.
Based on the multiple RIHs, the friction coefficients in open
hole and tubular wells observed during the interventions in
the Manifa field are on average 30% lower than the standard
figures in other fields in the region, therefore, eliminating the
need for use of friction reducers.
LESSONS LEARNED AND RECOMMENDATIONS
The lessons learned concerning CT tractor operation include
the following:
1. The operational experience gained has led to several new
product developments and procedural improvements to
ensure more reliable operations. Tractor capability was
increased, resulting in shorter time downhole.
2. Improvements to the gripping elements were incorporated
to further reduce stresses when tractoring in open hole and
alignment features were added to further assure complete
collapse. Refurbishment and maintenance procedures were
adjusted according to data gathered from the many runs.
3. The tractor on/off valve was moved to an external
repeating circulation sub (RCS) to better diagnose the
condition of the tractor. This new external sub now enables
the operator through the presence or lack of a 1,000 psi
pressure reading to determine if the tractor is in the correct
condition. The RCS also allows the tractor to be utilized
post-treatment. This is needed if multiple treatments are
planned in one tractor run, again minimizing time
downhole.
4. The use of the friction reducer is only needed as a
contingency.
Recommendations for improvement include:
1. Additional indications of tractor performance while in hole
would help in improving operation efficiency. Downhole
measurements of operational parameters, normally
monitored at surface would be ideal.
2. Use of at least 16 ft long straight bars is recommended to
overcome open hole irregularities.
3. A smaller diameter tractor may be necessary for smaller
completion wells.
CONCLUSIONS
This experience demonstrates the feasibility of performing
stimulation operations with CT in the Manifa field to
achieve full zone coverage, yielding optimum water
injection rates at the lowest development cost. With the
average injection rate increased more than tenfold, the
requirements to drill water injection wells were reduced
from the original estimations.
In spite of a limited prior field track record in open hole
environments, the CT tractor performance was overall a
success. Moreover, the CT tractor technology has become a
critical enabler in the Manifa field development plan. As with
all new technologies, the exposure to jobs has accelerated the
learning curve facilitating the necessary improvements.
New developments are necessary to address the challenges
ahead and continue improving interventions in the Manifa
field development.
ACKNOWLEDGMENTS
The authors thank the management of Saudi Aramco, WWT
and Schlumberger for permission to publish and present this
article.
8 SUMMER 2010 SAUDI ARAMCO JOURNAL OF TECHNOLOGY