Directional drilling is a broad term used to describe any boring that doesn’t go in a straight line vertically down.
In fact, even in a vertical well, it might be necessary to deviate to avoid a geological formation or a previous stuck pipe, then return to the original path.
In this instance, the driller uses sidetracking techniques.
Directional drilling techniques have been employed for almost 100 years now.
Over the past few decades, technological improvements have meant that angles, turns and underground distances covered are amazing feats of engineering.
Why is the technique so valuable?
Multiple down holes can be drilled from the same rig, minimising surface disturbance and environmental impact.
Also, these boreholes can extend up to a mile down, and for more than five miles at shallower angles.
The end of the drill is?
It’s not possible to see hundreds of metres underground, in fact, the drillers and engineers rely entirely on technology to ‘see’ where they are going.
A directional driller has a guide that has been created by the engineers and geologists.
Every 10-150 metres, (with 30-40 being typical), survey data is sent back to make sure that the original ‘blue line’ well path is being followed.
Can the drill make a turn?
If you were to imagine the mechanics of directional drilling without seeing the technology, you might wonder how the drill could suddenly change direction.
Since the motor that turns the drill is at the surface, how can the drill string continue to rotate at 360 degrees while going around a corner?
Drill bit sensors can tell the driller about external weight, and rotary speed that can also be used to influence the trajectory.
Mud motors can also be used to change direction.
With a steerable drill pipe, there’s a bend near the bit.
The drillstring stops turning, and then there is plenty of time to use chosen directional techniques to reposition the bit to the desired trajectory.
When it starts spinning again, it’ll start going in the direction that it’s now pointing towards.
Wellbore stability considerations
Well integrity is perhaps the most crucial aspect of directional drilling.
Drilling at deeper, or extended distances, and especially changing direction causes a number of additional engineering challenges and stresses on the equipment.
Directional well planning
2D and more recently 3D modelling programmes give the geoscientists and engineers a visualisation of the planned path.
Computer simulation programmes are used to simulate the well plan.
Horizontal Directional Drilling
There are a few different types of directional drilling.
Multilateral drilling is where a downhole bore has multiple lateral (90 degrees) offshoots. For example, a well might be 1000 metres in depth but have numerous lateral wells connected to it.
Extended reach drilling (ERD) is categorised by ever longer wellbores drilled from the rig.
Perhaps the most interesting type is horizontal directional drilling because it was the first type, and perhaps the most controversial.