The process of drilling a pilot hole along a present path between the predetermined entrance and exit points is known as directional drilling. Over the past 50 years, it has undergone modifications to allow for the drilling of multiple holes from a single rig; in the past, vertical drilling was the only method available for drilling a wellbore.

It is a method or ability to direct the drill stem and bit to a certain downhole in order to divert a wellbore to a predetermined destination. Directional wells are bored straight down to a predetermined depth and then gradually bent at one or more places to penetrate one or more target reservoirs. Directional drilling is often achieved by using a fluid-driven downhole motor to turn the drill bit.

The limitations of directional drilling are primarily dependent upon maximum hole angle, rate of angle change, and torque or friction considerations.

Below are the various Directional Drilling techniques used in Oil & Gas field:

1. Multiple wells from a single location: Directional drilling also allows multiple production and injection wells to be drilled from a single surface location such as a gravel pad or offshore production platform, thus minimizing cost and the surface impact of oil and gas drilling, production, and transportation facilities.This greatly simplifies gathering systems and production techniques.

2. Side Tracking & Straightening: It is used as a remedial operation to side track obstruction by deviating the wellbore away from obstruction by deviating the well bore back to vertical by straightening out crooked holes.

3. Controlling Straight Holes:Multiple wells can be drilled using this drilling technique from a man-made structure, such as offshore platforms, drilling pads, or man-made islands.

4. ERD Well: The aim of ERD is to reach a larger area from one surface drilling location and to keep a well in a reservoir for a longer distance in order to maximize its productivity and drainage capability.

Instead of drilling wells vertically, Extended Reach Drilling uses longer horizontal wells with a high inclination angle to reach subterranean oil/natural gas resources further away from the drilling site. A more advanced variant of directional drilling is ERD.

ERD combines both directional and horizontal drilling methods, and its overall vertical depth-to-horizontal distance ratios are significantly greater than those of traditional directional drilling.

More advanced steerable drilling equipment is used, and the wellbore's conditions are continuously monitored in real time. Through careful consideration of drilling mud properties and flowrates as well as rotation of the drill string during drilling, more care must also be taken to ensure the wellbore remains clean.

5. Drilling to avoid geological problems:

Fault Drilling: This eliminates the hazard of drilling a vertical well through a steeply inclined fault plane which could sip and shear the casing.
Salt Dome Drilling: To reach the producing formation which often lie underneath the over-hanging cap of the dome, the well is first drilled at one side of the dome and is then deviated to producing zone to avoid drilling problems such as large washouts, lost circulation and corrosion.

6. Horizontal Well:A horizontal well is a sort of multi-directional drilling technique that drills with an inclination of at least 80 degrees, in order to improve reservoir performance. When drilling for oil and gas is not possible with vertical wells or is problematic due to the reservoir's geometry, the horizontal approach is employed as an alternative.

7. Inaccessible locations: A reservoir may be located beneath a city or riverbeds, mountains, cities, parks etc, where drilling is either impossible or prohibited. This reservoir could still be tapped by drilling the well at an angle that intersects the reservoir.

8. Multilateral Drilling: It is defined as a well that has more than one horizontal or near horizontal laterals drilled from single site and connected back to a single well bore. The geometry of an ML well is determined by:

the production targets
their relative depths
lateral extents.

The laterals may bein the:

same horizontal plane at the same true vertical depth, or
same vertical plane and direction but at different depths.

Within these two broad categories an almost infinite number of combinations can be created, each of which can cover multiple zones or spread out in different directions within a single reservoir.

9. Relief Well Drilling: If a well is out of control, a "relief well" can be drilled to intersect it to seal the original well or to relieve pressure in the out-of-control well. This technique is applied to the drilling of relief wells so that mud may be pumped into the reservoir of the uncontrolled well.