Directional drilling is an oil well drilling technique used to drill any well that does not have a vertical trajectory as a trajectory, not being vertical then it is considered a directional trajectory, which can range from horizontal wells, S-type wells, among other trajectories considered as directional.
Historically, the acceptance of directional well drilling evolved gradually until in the mid 1920's it was already accepted the idea that the drilling of oil wells did not have to be purely vertical.
In the same evolutionary syntony, the use of tools used downhole in the drill pipe string to improve the directional trajectory of the well was improved until reaching the technologies that we have today to drill in a deviated way.
What are the benefits of drilling a well directionally?
It is impossible to talk about a single benefit that can be achieved with directional drilling, as they are a set of benefits that can be explained below:
[1] Increase the length of the exposed section through the reservoir by drilling the reservoir at an angle.
This means that once we start to deviate from the vertical section you can drill that trajectory in a controlled way by keeping track of the angle of deviation, for this there are tools such as the RSS downhole motor that can help control drilling at a desired angle.
[2] Drilling in reservoirs where vertical access is difficult or impossible. For example, an oilfield under a city, a lake or a difficult to drill formation.
This means that if an oilfield is in a location where vertical drilling is impossible, the well can be drilled in a location that allows drilling and then start sidetracking the well until the producing reservoir below the inaccessible geographic zone can be reached.
[3] Drilling along the lower part of a fault that restricts the reservoir allows multiple productive sands to be completed at the highest stratigraphic points.
When the geological conditions of the subsurface through a fault, makes it necessary to drill many times horizontally, it is there where horizontal drilling takes special prominence.
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[4] Drilling a relief well to relieve pressure from an unrestricted producing well (a blowout). In this case, another well could be drilled starting at a safe distance from the blowout, but intersecting the problem well.
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This, I believe, is one of the most important benefits because it has to do with the safety of when a blowout occurs, once a blowout occurs, the fire will not stop until the pressure in the producing reservoir does not drop, then a well is drilled adjacent to the fire and sidetracked until it reaches the producing reservoir, then heavy fluid (kill fluid) is pumped into the relief well to suppress the high pressure in the original well that caused the blowout, all with the intention of lowering the reservoir pressure and may end the fire in the well where the blowout occurred.
References
"Development tendency of horizontal directional drilling". DC Solid control. 6 June 2013.
Horizontal Drilling System (HDS) Field Test Report - FY 91