Page 77 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 53

Casing Design [ ]
1.4.5

wellbore stability problems should be avoided for the buildup section
since hole enlargement will make directional control more difficult.
Builds should be finished some distance before setting the next casing.
Note those formations that should not be used for the buildup part of
the well.
It is good practice to run casing to cover the kickoff section of a
directional well. This consolidates the well and reduces the danger of
keyseating in the dogleg section. However, the casing point should not
be inside or just at the end of the kickoff section or else a keyseat could
be worn in the casing shoe, which may be impossible to get free from
if you get stuck in it.
In a horizontal well it is common to build to horizontal in the
reservoir, run the production casing, and then drill ahead horizontally
for a slotted liner or prepacked screen completion. This protects the
buildup section and (assuming a good cement job) isolates production
fluids from the formations above.
Now decide where suitable formations will fit in with the potential
problems and the tentative directional plan. This will give the preferred
casing setting points.
Starting at the bottom section, define the required hole size for
each section. Once the preferred casing setting points and hole sizes
are established, kick tolerances should be checked to ensure that these
can be safely reached. A good target to set casing is in a transition zone
giving you the shoe strength to drill ahead. Starting at the surface, cal-
culate kick tolerances for each hole section so that, based on the
assumed mud densities required and the likely fracture gradients, each
section can be drilled to the preferred casing setting points with
acceptable kick tolerance limits. If this is not the case, the casing shoe
depths should be revised and it may be that an extra casing string is
needed. (See Appendix 1 for calculating kick tolerances.)
Once the casing points allow a safe well, isolate problem areas, and
with acceptable kick tolerances, revisit the directional plan. Adjust
kickoff points and build rates as necessary to fit in with the casing
points.
Having finalized the casing and directional program, note the TVD
and measured depth (MD) of each shoe and the maximum anticipated
pore pressure in the subsequent hole section.
We looked previously at summarizing offset data by using hole
section summaries. Even in exploration areas these should be done
with any offset wells that may be relevant even if they are quite far

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