To deviate a well from a vertical path, and get it to follow an intended well trajectory , it is necessary to put some side force onto the bit. The amount of this, as well as its direction, are vital in order to keep the bit to its intended path. Other factors will also have an influence, including the hardness of the rock which is being drilled, as well as bedding plane angles. There are numerous different ways of developing a controlled side force on the bit. Two of the earliest developed methods are whipstock and jetting which will be discussed in this article.
Jetting as the Directional Drilling Tool
A tricone drill bit possesses three drilling cones, with a nozzle in between each one. Should a large nozzle be set into a single nozzle pocket, and two smaller nozzles used alongside it, then the majority of the mud flow would pass through the larger nozzle. Drilling fluid will be ejected from the drill bit with a significant amount of force, and so long as the formation is not overly hard, will erode the rock in its path. As the large nozzle directs the majority of the flow to a single point, a pocket will be carved into the rock in this direction. The well may be deviated simply by aligning the bit in the necessary direction, and then circulating without rotation.
Once between 5-6 feet have been washed away, the bit is then rotated, and drilling continues as normal. This process can be repeated continuously until an angle of around 12° is produced, or until rock is reached which is too solid to jet through. Figure 1 to 3 illustrate a jetting operation by a rotary drilling assembly, which is used to allow the well to keep building an angle while drilling and rotating take place.
Whipstock as the Directional Drilling Tool
After drilling the well to the kickoff point, the bit is pulled out of the hole. A whipstock is a wedge that is set in the hole. The fat edge of the wedge is on the bottom of the hole. The drillstring is rotated and the bit drills formation to the planned direction. Forced against the side of the hole, it starts to cut out of the original hole. After drilling below the whipstock, the bit is pulled out of the hole again. The drilling assembly is run back in. The deviation at the bottom of the hole will be worn and cut away as operations continue (Figure 4).
Whipstocks are often used to sidetrack the well out of casing (Figure 5). Instead of a drill bit, a mill is used that can cut metal. In this application, the whipstock is left in place to guide tools through the cut in the casing. The drillstring will bend to allow the bottomhole assembly to go around the curved hole.
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