Page 185 - Drilling Technology in Nontechnical Language

176 Drilling Technology in Nontechnical Language Second Edition

It is now the job of the drilling engineer to look at the possible surface
locations (there may or may not be a choice about this) and then design a

well path to meet the target requirements at the lowest cost. Some flexibility
as to the surface location (such as a desert or offshore when drilling straight
into the seabed) can be a great help in minimizing cost.

As an example, figure 8–2 shows a planned well profile, from a
plan view (looking down from above) and a side view. This is a simple
directional well. There are two targets to hit, shown as rectangles on the
plan view and lines on the side view. The simplest (cheapest) directional

well profile is a build and hold to the target, called a J-shaped profile. Note

that the well does not try to hit the centers of the targets; a target is an area,

not a point. By using the available area for the plan, the cheapest profile to
achieve can be designed. While in practice very small targets can be hit, it
costs more money to get within a small target than to hit a big one. In the

well shown in figure 8–2, the lower target will be hit on its edge closest to
the surface location. There are advantages compared to planning to hit the
target center:
1. Going for the closest edge means that the well can be built to a
lower inclination (angle from vertical), and less hole needs to
be drilled.

2. If the well does not build angle fast enough, and the well might

miss the target, getting a higher build rate does not sacrifice
drilling speed. The opposite case—having to reduce inclination to
get into the target—does require sacrificing drilling rate. This is

because making the bit build angle generally requires more weight
on the bit, whereas dropping angle requires less weight on the bit.
There are tools that this does not apply to; rotary steerable tools
can deal with this. Rotary steerable tools are very expensive, so
unless the drilling operation has a very high daily cost, these types
of tools would not be considered for the purpose of correcting a
directional problem. By aiming at the low edge, any directional

correction work needed does not sacrifice drilling speed.
There are some other considerations in planning the well path. When
the well changes direction, the drillpipe has to bend around that curve.
If the well is curved close to the surface, as the well gets deeper and the
weight of drillpipe below the curve increases, the drillpipe tension in the
curve increases. This causes a greater side force between the wellbore

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