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What is horizonal drilling, and how does it differ from vertical drilling?

02/11/14 at 02:32 AM

Horizontal drilling is a drilling process in which the well is turned horizontally at depth. It is normally used to extract energy from a source that itself runs horizontally, such as a layer of shale rock. Horizontal drilling is a common was of extracting gas from the Marcellus Shale Formation. 

Since the horizontal section of a well is at great depth, it must include a vertical part as well. Thus, a horizontal well resembles and exaggerated letter “J.” When examining the differences between vertical wells and horizontal wells, it is easy to see that a horizontal well is able to reach a much wider area of rock and the natural gas that is trapped within the rock. Thus, a drilling company using the horizontal technique can reach more energy with fewer wells.


In this figure, well B represents a vertically drilled well and well A represents a horizontally drilled well. Vertically drilled wells are only able to access the natural gas that immediately surrounds the end of the well. Horizontal wells are able to access the natural gas surrounding the entire portion of the horizontally drilled section.

As you can imagine, drilling a horizontal well is a more complicated process that drilling a conventional vertical well. The driller must first determine the depth of the energy-rich layer. That is done by drilling a conventional vertical well, and analyzing the rock fragments that appear at the surface from each depth. 

Once the depth of the shale is determined, the driller withdraws the drilling assembly, and then inserts a special bit assembly into the ground that allows the driller to keep track of its vertical and horizontal location.

The driller calculates an appropriate spot above the shale in which the drill must start to turn horizontally. That spot is known as the ‘kickoff point.” From there, the drill bit is progressively angles so that it creates a borehole that curves horizontally. If done properly, the well reaches the ‘entry point’ and makes its way into the rock where the natural gas is trapped. The horizontal portion of the well is drilled, and provides much more contact with the rock than a vertical well.

Historical records suggest that horizontal drilling dates back to as early as 1929. It was first used in Pennsylvania in 1944.  It became an especially common practice during the 1980s when improved equipment, motors, and other technology were developed. In recent years, horizontal drilling has been shown in many cases to be more productive than vertical drilling, and a corresponding increase in the use of horizontal drilling has occurred.

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