Casing while drilling (CwD) has proven to be an effective method of reducing
drilling costs and solving drilling problems. Most of the current CwD
activity focuses on drilling vertical wells, but interest in directional wells
is increasing as CwD benefits in straight holes are demonstrated.
A directional CwD system has been run sufficiently to prove that directional
drilling with casing is practical with casing sizes from 7 to 13⅜ in. The
system uses a wireline-retrievable-directional-drilling assembly (positioned at
the lower end of the casing) to replace the conventional directional tools used
when drilling with drillpipe. These tools have been used to drill to
inclinations greater than 90° and have been retrieved and rerun at inclinations
ranging from vertical to horizontal.
Directional CwD can be used for a broad range of directional applications to
capture proven advantages demonstrated in vertical wells.
Growing commercial activity shows that drilling with casing is increasingly
accepted as a practical method of reducing drilling costs and solving drilling
problems.1–5 This activity includes both onshore applications in which the
entire well is drilled with casing and offshore applications in the Gulf of
Mexico and Gulf of Thailand, in which only the first hole section or two are
drilled with casing.
Most CwD activity has been focused on drilling vertical intervals, but
interest in drilling with casing in directional wells is increasing as the
processes for drilling straight holes become proven, CwD benefits are
demonstrated, and more versatile tools become available.
Vertical wells may be drilled with casing, using a simple system consisting
primarily of a special bit attached to the casing that can be drilled out to
run subsequent casing strings. But when there is a need to drill with a
motor without rotating the casing, or the section cannot confidently be drilled
with a single bit, then a retrievable drilling assembly that can be recovered
and rerun is required. Even some sections that can be drilled with a
drillout bit may be more cost-effectively drilled with a retrievable
Retrievable CwD equipment is required for directional wells because of the
need to recover the expensive directional drilling and guidance tools, the need
to have the capability to replace failed equipment before reaching the casing
point, and the need for quick and cost-effective access to the formations below
the casing shoe.
Versatile CwD tools required for successful directional-drilling operations
have become available recently. A wireline-retrievable
directional-drilling assembly, positioned in the lower end of the casing,
replaces the directional tools used in a conventional bottomhole assembly
This directional CwD system has been used with 5½-, 7-, 9⅝-, and 13⅜-in.
casing to drill deviated wells with relatively low inclinations. The
system also has been used with 7- and 5½-in. casings to drill several test
wells with inclinations that approached or exceeded horizontal.
Successful directional operations require more than simply having
directional tools available that can be run below the casing. BHA response
may be quite different when drilling with casing, compared to drilling with
conventional systems. Torque and drag must be managed through selecting
the casing connections, stabilization, and operational practices at the
wellsite. Special surface-handling equipment is often required to make
the CwD process efficient.
The following paragraphs explain the processes that are used to
directionally drill with casing, highlight some of the issues that must be
addressed when planning these operations, and discuss some of the testing and
field applications in which the system has been used.
© 2005. Society of Petroleum Engineers
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- Original manuscript received:
8 May 2003
- Revised manuscript received:
10 January 2005
- Manuscript approved:
29 January 2005
- Version of record:
15 March 2005