Page 108 - Drilling Technology in Nontechnical Language

Chapter 4 – PLANNING AND DRILLING A DEVELOPMENT WELL OFFSHORE 99

If the well is kicked off high up and the displacement of the bottom of
the well is low, the inclination of the well will be low. (Displacement is the
horizontal distance between the target and the location directly below the

wellhead.) Below about 15° inclination, it is difficult to control the azimuth
of the well. In that case, a deeper kickoff would be preferable because

that will give sufficient inclination (which is more controllable) to reach
the target.
On the other hand, the lower the well is kicked off, the more hole has
to be drilled to reach the target. Higher inclinations start to cause a variety
of problems, such as more severe wellbore instability, and it gets harder
to circulate cuttings out of the well (especially at inclinations above 40°).
On this well, it is better to avoid the complications of drilling a kickoff

without fluid returns to the rig. The kickoff will be done in the first
intermediate hole section.
There are quite a few considerations for selecting a kickoff point.
In addition to the kickoff depth, a decision is necessary for how quickly
to build up inclination. The severity of a change in wellbore direction
is measured in degrees per hundred feet of hole (or per 30 m on metric
operations). The buildup rate should not be too high, because this will
increase wear on downhole components while drilling. Too low a buildup
rate will take more time to complete the kickoff. A build up rate of around
2½° to 3° per 100 ft is usually planned. At 3°/100 ft buildup rate, about
1,000 ft of hole have to be drilled to attain an inclination of 30°.
The surface casing will be drilled out once the BOP is latched and
tested. This will be with a normal rotary drilling assembly to the kickoff
point because the well is vertical above the kickoff point. Once the kickoff
point is reached, the rotary drilling assembly is pulled out of the hole.
From the bit up, the directional assembly will consist of the drill bit,
a downhole motor, bent sub, MWD tool (which measures and transmits
directional information to the surface), drill collars, and drillpipe. The
MWD tool is set up so that the azimuth of the inside of the bend on the
bent sub is known. This is important because the drilling assembly will
drill in the direction that the inside of the bend points. The inside of the
bent sub bend is called the tool face, and the direction it points at any
particular time is the tool face azimuth (TFA). The MWD tool transmits
three pieces of information to the surface—the wellbore inclination and

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