Page 331 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 307

2.9.3
Drilling Problems--Avoidance Planning [ ]

Stabilizers or other full gauge tools should not be run above the jar,
because if the string became mechanically stuck the jar could not be
used. Keyseat wipers may be run above but should be sized halfway
between DC OD and hole diameter.
Sometimes sufficient strain cannot be taken in the pipe. At shallow
depths not enough DP stretch is available, in deep-deviated holes drag
may restrict stretch available at the jar. In these circumstances, a drilling
jar intensifier (accelerator) can be run in the string. In effect, it is a pow-
erful spring that sits above the collars and provides the strain energy
that the drillpipe, due to hole drags or string length, cannot. Therefore,
for correct jar and intensifier placement, run Bit - BHA to Jar - Jar - 2 or
3 DCs - Intensifier - HWDP - DP. If the intensifier is run right above the
jar, this eliminates the jarring force because the intensifier will absorb
the jar stroke. The drilling supervisor and drillers must be familiar with
the operation of the jar in use—its settings and limitations.

2.9.3. Lost Circulation

Mud losses are a major cost in drilling operations worldwide. Field
experience will give good guidance as to what types and severities of
losses may be expected. This will allow the well to be planned as far
ahead as possible to avoid recurrence of losses and to be ready to react
in the best way possible if losses do occur.
Some losses cannot be avoided. In this case, the choices are to drill
ahead without returns, to use a lighter fluid column or an air assisted
medium (such as foam) to case off the loss zone (that may need spe-
cial cementing techniques), or to abandon the well.
Top hole, to surface or seabed outside conductor. Where this
occurs, jack-up rigs may lose leg support. Offshore, this will not hap-
pen if returns are to seabed; however, where a riser is used the extra
hydrostatic can fracture to seabed.
To avoid such losses do everything possible to reduce hydrostatic
pressure on the formation:

1. Set the conductor as deep as is practical.
2. Drill with returns to seabed. Use subsea diverters if available.
When drilling from a floating rig, drill riserless until surface casing
is set and a BOP can be installed.

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