Following successful implementation of underbalanced drilling (UBD) in
Petroleum Development Oman (PDO), an evaluation of the value of UBD in a
carbonate field was commissioned. The study showed that there was a production
upside to developing the reservoir with UBD. A four-well trial campaign was
conducted between June and October 2004 to assess the potential.
The trial campaign was implemented following a structured program built from
PDO’s previous experience. During this campaign, a reservoir-characterization
(RC) system allowing identification of multiple high-permeability features was
developed. Water-shutoff devices were installed in two wells on the basis of
the results of the RC system. Production from the four wells was equivalent to
or greater than the field average, and the campaign scope has been
increased. The campaign was also a stepping stone toward the goal
of integrating UBD, RC, and water-shutoff techniques.
When UBD operations commenced in this carbonate field, 46 wells had been
drilled underbalanced for PDO in the preceding 2 years (Francis et al. 2003a,
2003b; Ramalho et al. 2003; Eissa and Al-Harthi 2003; Culen et al. 2003).
The field is a heavily faulted and fractured carbonate reservoir. The main
reservoir is the Aptian Shuaiba formation, consisting of rudist-bearing
reefal-shelf margin deposits with a complex depositional architecture.
Porosities range from 14 to 26%. Matrix permeability is highly variable,
ranging from 1 md to 1 darcy, with an average of approximately 20 md. The
fracture system plays a substantial role, both in early high-rate oil
production and in the increase in water production. In some cases, the high oil
rates are believed to be a “flush production” of oil that has accumulated in
the fractures. Previous reservoir studies also concluded that the fractures
were implicated in the movement of water. Post-drilling fracture systems were
identified by the interpretation of formation-image logs; the conductivity of
the fracture could not be determined.
A reservoir study to assess the value of UBD in this field indicated a
production upside associated with drilling underbalanced. Four wells were
drilled underbalanced during the campaign trial (Fig. 1).
The primary objectives of the campaign were to realize an increase in
production and to evaluate the use of real-time RC to identify features,
including fractures, that were believed to be responsible for water production.
RC, as applied here, is the ability of UBD to statically and dynamically
characterize the reservoir while drilling by collecting and monitoring surface
and bottomhole (by use of a pressure-while-drilling sub) flowing parameters in
© 2006. Society of Petroleum Engineers
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- Original manuscript received:
5 January 2005
- Revised manuscript received:
6 November 2005
- Manuscript approved:
11 January 2006
- Version of record:
20 September 2006