Page 176 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 152

[ ] Well Programming
2.2.2

not break down the formation. This is a leakoff test. The test may be
stopped at a predetermined limit, which is a procedure known as a
limit test. Both of these tests are known as formation integrity tests. If
weak or fractured formation is exposed, a limit test is usually per-
formed. The recommended procedure is described in Appendix 2,
“Formation Integrity Test Recommended Procedure”.
BOP equipment rating. The rating of the BOP equipment has to
take into account the degree of exposure to risk and the degree of pro-
tection needed.
IADC classifications require that the minimum BOP pressure rat-
ing shall be defined by the least of the following:

the burst pressure of the casing to which the BOP stack is connected
the formation breakdown pressure at the shoe minus the hydrosta-
tic pressure of the casing filled with gas. (You can assume gas gra-
dient to be 0.1 psi/ft.)
the maximum anticipated surface pressure to which the equipment
may be exposed
Sometimes a section of the well will be drilled with a certain mud
weight when field experience indicates that a kick is possible in one of
the formations in that section; one example would be potentially
drilling into an overpressured raft within massive salt. A casing seat
will be chosen so that the shoe is close to the potential kick zone. Some
operators under these circumstances prefer to keep a tank of heavy kill
mud standing by, ready to pump quickly. There are some reasons
against doing this:

1. If the surface to bit volume is more than the bit to shoe volume,
then heavy kill mud will enter the wellbore after the influx has
risen into the casing. Therefore, pressures on the shoe and open
hole will not be reduced. If this is the case then having kill mud
ready serves no advantage at all. It may even have to be dumped
afterwards if not used (water-based mud), wasting money.
2. It is unlikely that the gradient of the kill mud will be exactly what
is needed and so would not be pumped straight down the well any-
way; if it were pumped down this would complicate the kill and the
annulus pressures would be more difficult to calculate. Rather than
pumping it “neat,” it could save some time if it is mixed into the
active system to reduce the time needed to add barite.

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