Directional Drilling by Rotary Drilling Assembly

Rotary drilling assemblies can typically control a directional of a well by having proper stabilizer placement. With this kind of drilling assembly, only inclination can be controlled and a well cannot be directionally oriented to required direction. In this article, it briefly describe how stabilizer placement can affect the well direction.

Rotary Build Assembly (Fulcrum Assembly)

When a drill collar is supported at both ends but not held vertically, its own weight causes it to sag in the middle. This phenomenon is used in rotary drilling assemblies to create the necessary side force at the bit to alter the angle (Figure 1).

Figure 1 – A build rotary assembly

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Why Directional Wells Are Drilled?

Even outside the drilling industry, the concept of directional drilling, whereby a drill is precisely guided through a particular target, is a fascinating one. This article will describe about applications of directional drilling in oil and gas industry. Later on, we will discuss in several aspects of directional drilling such as directional drilling tools, well path design, wellbore navigation tools, etc. Let’s get started.

Why Drill Directional Wells?

It is a fact that it is always more expensive to drill a deviated well to a target not directly below the rig location, as opposed to simply drilling down vertically to the target.

However, there is good reason why a directional well might be used: in some circumstances, it can actually lower the total cost of the project. Some potential reasons for this include:

Multiple exploration wells from a single wellbore

It is possible to drill a well to evaluate it, and then cement it back up. This well may then be deviated from its original path to an additional target. This may be done in order to evaluate multiple compartments in a single reservoir, if it is naturally split into several sections, or to extend the knowledge of the structure using a single well.

Figure 1 – Example of Multiple Exploration Wells from a Single Wellbore

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Scale Factor in UTM Mapping System and its Application in Directional Drilling

Scale factor represents distortion from a mapping system since the Earth is mapped into a flat surface, but the actual surface is in curvature. Figure 1 illustrates a scale factor with a reference location of the Earth’s surface. If the location is above the map projection plan, the scale factor will be less than 1.0. However, if the location is below the mapping projection, the scale factor will be more than 1.0. A scale factor less than 1 means that the actual distance on the Earth’s surface is longer than the actual distance on the map. Whereas, the scale factor of more than 1 demonstrates that the actual distance on the Earth is shorter than the map distance.

Figure 1 - Scale Factor

Figure 1 – Scale Factor

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Universal Transverse Mercator Application in Directional Drilling

Universal Transverse Mercator (UTM) is one of the commonly used map projection methods in directional drilling. In UTM, the world is divided up into 60 zones between 84° North and 80° South and East-West from 180 degrees longitude. Then the Earth is flattened with the Zone 1 starting at 180 E-W longitude. Since the earth is divided into 60 zone so each zone has 6 degree wide. Figure 1 illustrates the concept of UTM and Figure 2 shows the UTM grid. This method will not cover the polar region.

Figure 1 - Universal Transverse Mercator Diagram

Figure 1 – Universal Transverse Mercator Diagram

Figure 2 - The Universal Transverse Mercator (UTM) grid

Figure 2 – The Universal Transverse Mercator (UTM) grid

Zones are numbers from 1 to 60 and the zone 31 has 0 degree meridian (Greenwich, England) on the left and 6 degree East on the right (Figure 3). Each one in UTM is divided into grid sections which cover 8 degrees of latitude and the system uses letters from C to X, excluding I and O.

Figure 3 - UTM zone 31

Figure 3 – UTM zone 31

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