Page 65 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 41

1.4.2
Casing Design [ ]

drawdown while producing.
The top of a liner hanger generally incorporates a PBR. This can be
used for tieback liners (to convert the liner into a full string to surface),
for scab liners (a length of liner with a packer on top in case the liner
lap does not seal), or for setting retrievable completions without using
a packer in the casing. These tools stab into the PBR and incorporate
sealing elements.

1.4.2. Hole and Casing Sizes: Considerations

“Slimhole,” according to the current industry definition, is a well
which is completed in 4 4 in or smaller hole at TD. Monobore comple-
3/
tions allow the well to be drilled in smaller diameters and also have
other advantages and some drawbacks. These were discussed in the pre-
vious section on completion design.
There are substantial savings possible by drilling smaller hole sizes
than would be traditionally used. While a well may not necessarily be
“slimhole” according to the definition above, hole sizes can still be
reduced in many cases with no loss of objectives. The current state of
development at the time of writing is summarized within this section, fol-
lowing by the procedure for choosing the final hole size (Section 1.4.3).
As the advantages of slimhole drilling become more apparent, new
tools are being developed for small holes. Standard logging tools are
now available for hole sizes below 6 in and coiled tubing drilling tech-
niques are advancing rapidly for drilling small diameter holes.
The optimum radial clearance outside casing for running and
cementing is 1 /2 in. When planning hole and casing sizes, try to main-
1
tain a hole diameter at least 3 in larger than the casing diameter. Also,
check that the drift size of the casing is greater than the bit diameter for
the next hole section.
Planning a slimmer well. The starting point in planning is where
you want to end up. What final hole or liner size is required to meet all
of the well objectives? Is a contingency hole size needed (exploration)
or not (development)? Can a monobore completion be used (and if not,
why not)? What are the required logging tools and what is the mini-
mum hole size in which they can be run? Are sand control measures
needed and would they affect the final hole or liner size?
If company experience of drilling slimhole is limited, a dedicated
team could be formed that would include drilling engineering (opera-

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