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Drilling of non-vertical oil wells is
done by controlled directional drilling. It is the technique of directing a bore
well along a previously determined path to a target located at a given distance
from the vertical.

It is useful when the area above the
target reservoir is inaccessible or where the rig cannot be set up due to
various obvious reasons like reservoir found below mountains or river beds,
etc,.

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Most common applications are

Ø 
Off
shore drilling, where an optimum number of wells can be drilled from a single
platform, thereby minimizing the cost of operation.

Ø 
Fault
Drilling where it eliminates the hazard of drilling vertical thru a inclined
fault plane

Ø 
Reaching
inaccessible target reservoirs

Ø 
Avoiding
Salt dome during drilling

Ø 
Drilling
of Relief wells, where the mud and water can be pumped into the uncontrolled
well and thereby kill the blow out.

Ø 
Sidetracking
and Straightening etc.

Most directional wells begin as
vertical wellbores. At a designated depth, known as the kickoff point, the
directional driller deviates the well path by increasing the inclination to
begin the build section. The direction of the bit and the tool face or the
orientation of the measurement sensors in the well are decided based on the
surveys taken during the drilling process. The directional driller constantly observers
these measurements and fine-tune the trajectory of the wellbore as needed to
intercept the next target along the well path.

The deviation in directional drilling is achieved by the
following techniques:

 I) Downhole Hydraulic Motors (Mud Motors)
with a “Bent sub”:

In this technique, the wellbore
direction is controlled by using a bent motor housing, which is oriented to
point the drill bit in the desired direction. Downhole hydraulic motors, also
called as Mud motors, are powered by drilling mud flowing through the motor to
rotate the bit without rotating the drill string from the surface. These allow
for the drill bit to continue rotating at the cutting face at the bottom of the
hole, while most of the drill pipe was held immobile.

A piece of ‘bent sub’ between the top
of the motor and the stationary drill pipe allow the direction of the wellbore
to be changed without needing to pull all the drill pipe out. The bit and the bend
are first oriented in the desired direction, then the downhole mud motor alone
drives the bit leaving the drill string above the bit stationary.

By alternating intervals of rotating
mode, where the complete drill string rotates from the surface) and sliding
mode, where the drill string is stationary, the directional driller can govern
the wellbore trajectory and navigate it in the desired direction.

 

A whipstock  is a wedge shaped,
long inverted concave steel tool with heavy steel collars (through  which the drillstem fits) deployed
downhole to mechanically alter the path of the well.
The whipstock is  positioned to
deflect the bit from the original borehole at a slight angle and in the
desired direction for the sidetrack
This tool deflect  the rotating
drillstem to the side of the hole and is then removed once proper deviation
is achieved. This technique can be used in cased or open holes.

II) Using Whipstock :

This technique is used where subsurface formations are relatively
soft and the borehole can be deflected using the hydraulic energy of the drilling
mud through a jet bit.
Here bits are conventional tricone bits with one of their three
nozzles opened up and the other two openings are closed or reduced in size.
In soft formations, drilling mud can be circulated at high velocity to wash
out the side of the hole.
The washed out section is a path of least resistance and hence
becomes the path which the bit and drill string will follow.
 

III) Using Jet Bits:

 

IV) RSS
(Rotary Steering System):

Directional drilling with a downhole motor needs occasionally stopping the
drill pipe rotation and sliding the pipe through the channel as the motor cuts
a curved path. Sliding can be difficult at times in some hard formations, and
it is always a slow process almost and hence more expensive than drilling while
the pipe is rotating. So the ability to steer the bit while the drill pipe is
rotating is most desired to speed up the process. Several companies have
developed various tools that allow  directional
control while rotating. These tools are referred to as Rotary Steering Systems (RSS).

There are two steering concepts in the RSS—”point the bit” and “push the
bit”. The “point the bit” system uses the
same principle employed in the bent-housing motor systems. In RSSs, the bent
housing is contained inside the collar, so it can be oriented to the desired
direction during drill string rotation. “Point the bit” systems  allow the use of a long-gauge bit to reduce
hole spiraling and drill a straighter wellbore. The “push the bit” system uses
the principle of applying side force to the bit, pushing it against the
borehole wall to achieve the desired trajectory. The force can be either
hydraulic pressure or by way of mechanical forces.

RSS designs
characterized by their steady-state behavior:
In “point the
bit” systems (left side in the picture), the bit is tilted relative to the
rest of the tool to achieve the desired trajectory.
“Push the bit”
systems (right side in the picture) apply force against the borehole to
achieve the desired trajectory.

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