BAKER HUGHES

PR I NCIPLES OF DR ILLING TECH NOLOGY
Chap te r 2 . 4
– 5
Well Path
V ertica l Dr illing
So. We have now learnt what a drill string looks like. But
what does a well look like? Is it true that we sometimes
drill curves? And if so, why do we do that?
The technique of rotary drilling, i.e. drilling with a
rotating drill string is in itself not very old. It did not
become popular until the beginning of the last century.
At that time the rotary drill string only consisted in
principle of the bit and drilling assembly above it
comprising the drill collars and the drillpipes which
were rotated from on the surface. The drilling mud
is pumped downwards through the drillpipes to the
bottom of the hole. Once there it cools the bit and
carries the cuttings out of the borehole back to the
surface through the annular space between the borehole
wall and the drill pipes.
The path of a well drilled using the traditional rotary
principle was usually designed to be vertical. In fact, a
genuinely vertical well only succeeded in the rarest of
cases. The path of the well tended to be deflected by
the position of the rock strata in respect of each other,
their angle of inclination and the varying hardness of
the rock in the strata. It was possible to influence the
direction of the well slightly via skilled arrangement of
the stabilizers within the drilling assembly. So-called
building assemblies are used to increase the deflection
of the well from the vertical and drop assemblies to
reduce the inclination.
If a well produced a positive result, additional wells
were sunk in the area to maximize the delivery of the
field. Photographs from the period around 1900, for
example, including those of oilfields near Celle at
Wietze, are a reminder of what was state of the art at
that time. The rigs stood side by side as far as the eye
could see.
There was already a suspicion: drilling straight
down into a reservoir containing oil or gas somehow
did not represent the most effective method of opening
it up. It is true that normal reservoirs are often
widely spread out in the horizontal plane (length and
width) but at the same time only have a shallow thickness.
You should not imagine oil and gas reservoirs to
be like underground seas. No, the hydrocarbons (oil
and gas molecules are consisting of carbon and hydrogen
atoms) are located in the fine pores of the rock and
cannot flow freely in any desired direction. A vertical
bore, which runs from top to bottom through the thin
reservoir therefore only reaches a relatively small
amount of oil or gas.
Usually better results are achieved with a “directional”
borehole, as is described in the next chapter.
0
5
10
15
26
27
20