Page 198 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 174

[ ] Well Programming
2.3.3

dips below 40°. A couple of degrees below the line at the end of the
kickoff before locking up would compensate for this. The more weight
run on the bit, the stronger the tendency to build. If the assembly is
fully locked up, full bit parameters can be used for maximum rate of
penetration and the build tendency can be quite small.
One other thing to remember when planning to finish the build a
degree or two low is that if you build too much up to the direct line to
the target (or above it) and later have to drop angle, you will compro-
mise ROP with the low weights needed with drop-off assemblies. If,
however, you need to build, a short-build assembly needs weight to
work so your ROP will not be compromised. Pendulum assemblies lose
lateral control and may wander off azimuth. Pendulums are also prone
to drilling a spiral hole as angle decreases towards the vertical and this
is detrimental to wellbore stability, hole drags, sticking tendency, and
casing wear. Therefore, you should err on the side of being slightly too
low rather than too high.
If offset wells indicate that certain formations have definite ten-
dencies to deviate the wellpath (such as giving a slow build with a
rotary locked assembly), then the plan can incorporate these measured
tendencies to modify the kickoff slightly. If you expect a long slow
build in the tangent section then use this information to improve your
directional planning.
Kicking off by jetting or badgering. The fastest way to deviate a
well in suitable (soft) sandstone formations is by jetting. One large noz-
zle is run with two small nozzles so that most of the flow goes to one
side of the bit. By setting down weight on the bit and pumping fast, the
formation is washed on one side and deviates the wellbore. Only the
first few feet of each kelly are jetted and the rest is rotary drilled to the
next connection. Ream once or twice before making the connection.
The BHA will normally be set up as a build assembly with about 60 ft
between the NB and first string stabilizer. If the formation can be drilled
fast with a tricone bit (> 50 ft/hour) then jetting should be possible.
Smith Tool produces a special bit for jetting. This is like a tricone
bit with one cone missing and a large circulation slot where the third
cone would normally be. This bit has a right-hand walk tendency.
MWD tools are not normally used when jetting due to the large
flow rates necessary. Single shot surveys are quite adequate and much
cheaper than MWDs; string orientation is accurate from the table as

174