Page 447 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 423

3.7.2
Directional Drilling [ ]

For vertical wells, locked BHAs should also be used to prevent bit
walk and crooked hole. Consider running the Anderdrift sub in the
BHA with a Totco ring on top. This tool will give inclination surveys
up to 5° when a connection is made by transmitting mud pulses to sur-
face mechanically (the driller can see the pulses on the gauges). This
will allow close monitoring of your verticality with virtually no loss of
time and eliminates the risk of running slickline tools inside the drill-
string to get a Totco survey.
Design of rotary BHAs for building or dropping angle. For a build
assembly, run the second stabilizer undergauge or remove it altogether,
depending on the severity of build required. (Bit - NB Stab [Full
Gauge] - DC - Stab [ /4 in- /2 in undergauge] - DC - Stab [Full Gauge]
1
1
- rest of BHA.)
For a drop assembly, run the NB undergauge, remove it, or remove
both the NB and next stabilizer depending on the severity of drop
required. (Bit - NB Stab [1/4 in-1/2 in in undergauge] or bit sub - DC -
Stab [optional] - DC - Stab - rest of BHA.)
Note that the more flexible (severe) a build or drop assembly is, the
more likely it will walk sideways. This may cause an unwanted change
in azimuth. Soft formation mill tooth bits also tend to walk more on a
flexible assembly than most other bit types.
The closer a stabilizer is to the bit, the more it will influence the
forces at the bit. A near bit stabilizer undergauge by /16 in will have
1
much more effect on the deviation performance than if the next stab at
10 m goes undergauge by the same amount. Ensure that the bottom
three stabilizers are gauged on every trip out of the hole and changed
out if necessary.
Choose stabilizers compatible with the formations. The stabilizer
blades should be long enough to avoid them digging into the wall; a
short blade opposite soft formation would be a bad choice. If longer
bladed stabs are not available and this causes concern, run two stabs in
tandem to spread the load.

3.7.2. Preventing Keyseating

A keyseat could be cut on the inside of a dogleg section by the
drillpipe rotating on the wall while being pulled against it. A keyseat
wiper run on top of the DCs will help to backream through the keyseat

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