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Chapter 8 – DIRECTIONAL AND HORIZONTAL DRILLING 175
▪ Optimum orientation in the reservoir. Reservoirs are not nice
and even, the same in all directions. Permeability will be better
in one direction than another. In a fractured limestone reservoir,
the well should intercept as many fractures as possible to get the
best production. These factors will determine which direction the
well should be drilled in the reservoir. Some wells follow very
complex well paths, turning corners, to get in the best position in
the reservoir.
▪ Remedial work (sidetracks). A well might be cemented back
to a shallower depth, and a new wellbore drilled away from the
original bore, for a number of reasons. One reason might be due
to drilling problems, such as stuck pipe that cannot be freed and
must be cut. Or an old producing well might be sidetracked to
another location.
▪ Relief wells. The worst reason to have to drill a well is to create a
relief well. For example, a well has suffered a blowout and is still
blowing hydrocarbons into the environment. For some reason, the
well cannot be killed at the surface. Another well, the relief well,
is drilled to intercept the blowing well. Dense mud is circulated
down the relief well and into the blowing well, killing it from the
bottom. The Deepwater Horizon disaster in the Gulf of Mexico
in April 2010 was one of these situations. The BOPs on the
seabed would not operate to shut in the well at the seabed. Two
rigs moved in to drill relief wells to depths below 18,000 ft to shut
down the blowout.
Directional Well Planning
It is the job of the geologists and reservoir engineers to decide where
the wellbore should be placed. This could be as simple as determining
one single target, often defined as a tolerance of say 100 m around a target
point (so that would be a round target). It does not matter at what angle the
well enters the target. At the other extreme, the well may have to penetrate
multiple targets, and the final target could be defined as a cylinder that the
well must stay within, or perhaps the well must be drilled to follow the
“roof” of the reservoir and stay within a certain distance of the cap rock
above. This is called geosteering and will be covered later in this chapter.
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