Page 79 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 55

1.4.6
Casing Design [ ]

Surface casing. Surface casing performs quite a few functions. It
also allows a BOP to be nippled up. It is set deeper to allow the pro-
duction or intermediate casing point to be reached. Surface casing pro-
tects the wellbore from shallow gas, unconsolidated sands, sloughing
shales, lost circulation zones, and key seats (if the well is kicked off in
surface hole). It isolates fresh water sources.
If high-risk gas indications are present (shallow seismic/field expe-
rience), surface casing should be set not far above the danger zone and
a BOP nippled up. Do not deliberately drill into high-risk gas anom-
alies with a diverter. A diverter should not be thought of as a means of
well control; the purpose of a diverter is to allow time for the rig to be
evacuated by diverting the flow away from the well for a short period
of time. If you are lucky, the flow will stop by depletion or bridging;
once flowing the chances of a dynamic kill are low and the risk of
equipment failure during the flow is high.
In many areas, lost circulation zones (mostly unconsolidated for-
mations) will be present during top hole drilling for surface casing.
Once a competent formation is reached, casing should be set to protect
the well from further losses prior to drilling ahead.
Generally surface hole is drilled as a small pilot hole, which is then
opened up for the casing to be run. There are advantages and disad-
vantages to drilling a pilot hole first; usually the safety aspect will be
paramount and this will outweigh the apparent disadvantages.

Advantages.

1. If shallow gas is encountered, the flow rate will be much lower
with smaller diameter hole. This increases the (low) chance of a
successful dynamic kill and it will give you a bit more escape time
while diverting. It is more likely to bridge off quickly but with an
extended flow the hole will erode and enlarge.
2. If the well will be kicked off in surface hole, the smaller hole will
be easier to control directionally.

Disadvantages.

1. Time—two drilling runs to reach the objective.
2. Hole opening must be done carefully (especially in a deviated
bore) since an inadvertent sidetrack could be drilled.
3. Hole cleaning could be a problem where a small pilot hole is
drilled through a large ID conductor pipe.

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