Page 36 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:55 PM Page 12

[ ] Well Design
1.1.2

With the hole section summaries completed, you now have a
detailed overview of all the relevant wells. Now look at each formation
and list all the problems seen within that formation: tight hole,
enlarged hole, kicks, stuck pipe, etc. For each problem, do a complete
analysis and establish:

What were the contributing factors that can be seen from the
data?
What other factors may have been relevant but were not noted
in the records?
How can this problem be eliminated, or at least reduced?
What actions can be taken if the problem is seen on the next
well to mitigate the effects of the problem?

If possible, avoid relying on the conclusions of other people
who have reported on the problem. It is better to look at the source
data yourself and make your own conclusions. Let me give a real
case example.
An offshore well was being planned in the Mediterranean.
According to prognosis, the pore pressure was to increase from normal
pressure (hydrostatic) only 500 m below the seabed. Offset wells, even
using oil-based mud, had reported very unstable wellbore conditions in
shallow Pliocene shales with large quantities of cavings. A report by the
previous concession holder had looked at seismic and sonic data, con-
cluding that increased sonic transit times were due to undercompact-
ed/overpressured shales. On the surface, this was consistent with the
drilling problems that were experienced .
A closer look at the hole section summary revealed some interest-
ing facts. Using oil-based mud in the first offset well, bottoms up after
wipertrips had brought up large quantities of cavings. Mud density was
increased, but the problem got worse, not better. However, they con-
tinued to increase the mud density and still the cavings level increased
every time they wipertripped, which was often. The third offset well
was drilled with a pilot hole through these shales and opened up with
seawater with no flow from the well.
Unfortunately, samples from the shakers were not available from
this interval on any of the offset wells. However, all the evidence
seemed to be consistent with fractured shales, not overpressured
shales. The mud used on the new well included additives to plug off

12