Page 80 - Drilling Technology in Nontechnical Language

Chapter 3 – DRILLING A LAND EXPLORATION WELL 71

are no special considerations for making the well path follow a particular
course to the target. It only needs to be reasonably vertical and straight.
A well can be nearly vertical but might have a spiral path downwards.
Even a slightly spiral well can cause problems, and this situation is best
avoided. A drillstring under high tension (from the weight of the steel
below it) in a spiral hole will touch the hole in many places, with a high
force pressing into the wall. This will cause a lot of friction that will wear

the pipe and make running in and pulling out the drillstring more difficult.
It will also make the wellbore less stable, so that it enlarges, with all the
problems related to an overgauge hole.
One tool in the driller’s armory to keep the well straight is the
stabilizer.

A stabilizer is run within the bottomhole assembly (fig. 3–19). Imagine
a short section of drill collar, about 5 ft (1.5 m) long, with drill collar
connections on top and bottom. In the middle of this collar are fastened
blades that stick out, sized so that they touch the wall of the hole. If a
stabilizer is run in the hole and if the hole is in gauge, the effect is to
centralize the drill collars above and below the stabilizer in the wellbore.
Now a drill collar, being very thick steel, is quite stiff—that is, it resists
bending forces. If a bottomhole assembly is configured with a stabilizer

below each of the lowest three drill collars, and as long as the hole is in

gauge, the drill bit requires a very large force to deflect from drilling a
straight line. Three stabilizers, close to each other and connected by very
stiff drill collars, with three points of contact with the wellbore, keep the
drill bit drilling in line with the existing wellbore. This type of BHA is
called a tangent or locked BHA.

Fig. 3–19. Drawing of a stabilizer

_Devereux_Book.indb 71 1/16/12 2:07 PM