Definition of Directional Drilling
Directional drilling can generally be defined as the science of directing a
wellbore along a predetermined trajectory to intersect a designated subsurface
target.
Applications
Multiple wells from offshore structures
The most common application of directional drilling techniques is in
offshore drilling. Many oil and gas deposits are situated well beyond the
reach of land based rigs. Drilling a large number of vertical wells from
individual platforms is both impractical and uneconomical. The obvious
approach for a large oilfield is to install a fixed platform on the seabed,
from which many directional boreholes can be drilled. The bottomhole
locations of these wells are carefully spaced for optimum recovery.
In conventional development, wells cannot be drilled until the platform has
been constructed and installed. This can mean a delay of several years
before production begins. Such delay scan be considerably reduced by predrilling
some of the wells through a subsea template while the platform is
being constructed. These wells are directionally drilled from a semisubmersible
rig and tied back to the platform once it has been installed.
Relief Wells
Directional techniques are used to drill relief wells in order to “kill”
blowouts. Relief wells are deviated to pass as close as possible to the
uncontrolled well. Heavy mud is pumped into the reservoir to overcome
the pressure and bring the wild well under control.
Controlling Vertical Wells
Directional techniques are used to “straighten crooked holes”. When
deviation occurs in a well which is supposed to be vertical, various
techniques can be used to bring the well back to vertical. This was one of
the earliest applications of directional drilling.
Sidetracking
Sidetracking out of an existing wellbore is another application of
directional drilling. This is done to bypass an obstruction (“fish”) in the
original wellbore, to explore the extent of a producing zone in a certain
sector of a field, or to sidetrack a dry hole to a more promising target.
Wells are also sidetracked to access more reservoir by drilling a horizontal
hole section from the existing well bore.
Inaccessible locations
Directional wells are often drilled because the surface location directly
above the reservoir is inaccessible, either because of natural or man-made
obstacles.
Fault Drilling
Directional wells are also drilled to avoid drilling a vertical well through a
steeply inclined fault plane which could slip and shear the casing.
Salt Dome Drilling
Directional drilling programs are sometimes used to overcome the
problems of drilling near salt domes. Instead of drilling through the salt,
the well is drilled at one side of the dome and is then deviated around and
underneath the overhanging cap.
Shoreline Drilling
In the case where a reservoir lies offshore but quite close to land, the most
economical way to exploit the reservoir may be to drill directional wells
from a land rig on the coast.
These are only some of the many applications of directional drilling.
Although it is not a new concept, one type of directional drilling, horizontal
drilling, is the fastest growing branch of drilling, with major advances
occurring in tools and techniques. As with directional drilling, there are
numerous specific applications for horizontal drilling.
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