Directional drilling the science of drilling non vertical wells. Directional drilling can be broken down into three main groups; Oilfield Directional Drilling, Utility Installation Directional Drilling (commonly known as H.D.D./Horizontal Directional Drilling) and in-seam directional drilling (Coal-Bed methane).
History
A number of prerequisites were necessary before this suite of technologies could become productive. Probably the first requirement was the realisation that oil wells (or water wells, but since their depths are normally trivial, the development was essentially done in the oil industry) are not necessarily vertical. This realisation was quite slow, and didn't really grasp the attention of the oil industry until the late 1920s when there were several cases of lawsuits alleging that a well drilled from a rig on one person's property had actally crossed the boundary and was penetrating a reservoir on an adjacent property. Initially proxy evidence such as changes in production from pre-existing wells was accepted, but such cases fuelled the development of small diameter tools capable of surveying wells as (or during) their drilling.
Measuring the inclination of a wellbore (its deviation from the vertical) is comparatively simple - one needs a pendulum of some sort. But measuring the azimuth (direction with respect to the geographic grid in which the wellbore is running from the vertical) was a lot more difficult. Magnetic fields could sometimes be used, but were open to the influence of the metalwork used to line wellbores, as well as the metalwork used in drilling equipment itself. The big step forward was in the modification of small gyroscopic compasses by the Sperry company, who initially were making these compasses for aeronautical navigation. Sperry did this work under contract to Sun Oil (who were involved in a lawsuit as described above), and a spin-off company was formed under the name "Sperry Sun ", which brand continues to this day, absorbed into the Halliburton oil services conglomerate.
(The author used to work for Sperry Sun - it is plausible that people with a different history have heard different corporate histories.)
Once viable surveying tools were available it was necessary to develop techniques for directing a drilling bit in a particular direction. The first common tool to do this was the whipstock , a sort of wedge that could be anchored in a wellbore and oriented in a particular direction. When drilling resumed, the drilling bit would be forced to deviate in the desired direction. This initial deviation acquired the name "kick-off point", which is still used in modern terminology.
Prior experience with rotary drilling had established a number of principles for the configuration of drilling equipment down hole ("Bottom Hole Assembly" or "BHA")that would be prone to "drilling crooked hole" (initial accidential deviations would be increased away from the vertical). Counter-experience had also given these early directional drillers ("DD's", on many whiteboards on many rigs around the world to this day) principles of BHA design and drilling practice which would help bring a crooked hole back towards the vertical.
Combined, these survey tools and BHA designs made directional drilling possible, but it was decidedly arcane. Many DDs took a perverse delight in making it sound more arcane than it actually was - using Ouija boards to perform calculations instead of slide rules for example.
The next major advance was in the 1970s, when downhole drilling motors became commonplace. These allowed the bit to be rotated on the bottom of the hole, while most of the drill pipe was held stationary (power to the motor is supplied by the hydraulic effect of the drilling fluid pumped down the inside of the drill pipe). Including a piece of bent pipe (a "bent sub") between the stationary drill pipe and the top of the motor allowed the direction of the wellbore to be changed without needing to pull all the drill pipe out and place another whipstock. Coupled with the development of mud pulse telemetry (which allows tools down hole to send data back to the surface without disturbing routine drilling operations), directional drilling got hugely easier.
The most recent major advance in directional drilling has been the development of a range of tools from various companies which combine whipstock, survey tool and computer into one downhole package and produce a BHA which actively steers itself in the direction it is instructed to go. These tools (PowerDrive from Schlumberger, AutoTrak from Baker Hughes and GeoPilot from Sperry-Sun / Halliburton) have almost automated the process of drilling highly deviated holes in the ground. But they are not cheap, so more traditional directional drilling will continue for the foreseeable future.