The well planning process starts from geologists and reservoir engineers who decide the best place for the wellbore. They may only need to determine a single target, which will often be a tolerance of about 330 ft (100 m) around a certain target point. In this case, the angle at which the well enters the target can have various degree of deviation from the plan since a plan requires to hit only one target. On the other hand, it might be necessary for the well to penetrate multiple targets, with the final target being increasingly complex. This requires what is known as “geosteering”, a process which will be discussed later in the directional drilling series. The drilling engineer therefore needs to examine potential surface locations (if more than one is available) and design a well path which meets all necessary target requirements at the lowest possible cost. Cost can be minimized most effectively when there is a certain degree of flexibility when it comes to the surface location.
Directional Well Profiles
Well profiles can be simply divided into two groups which are 2-D design and 3-D design.
2-D Well Profile Design
The 2-D design well plan is a profile which inclinations are changed in order to hit required target but it does not have or slightly change in azimuth. The example of 2-D well profiles are shown in figure 1-4.
3-D Well Profile Design
The 3-D well design have changes in both inclination and azimuth. The example of 3-D well profiles is illustrated in figure 5.
Example of Well Planning
Figure 6 (Jones et al, 2008) provides an example of a planned well profile, from a plane view and a vertical view. This is a relatively simple directional well, which is designed to hit two targets, as shown by the boxes on the plan view. The easiest type of directional well profile is a so-called “J-shaped profile”, which is a build and hold to the target. The target in this case is an area, rather than a single point, and the well need not therefore hit the center of the target. Although it is possible to hit small targets, this increase in accuracy comes with a higher financial cost. In Figure 6, the lower target will be hit on the edge nearest to the surface location.
This method has several advantages:
- Opting for the nearest edge allows the well to be built to a lower inclination, and therefore not as much hole needs to be drilled.
- Should the well fail to build angle at a fast enough speed, then it could end up missing its target. However, a higher build rate does not have a negative effect on the drilling ROP. On the other hand, reducing the angle to reach the target will mean compromising the drilling rate. This is caused by the fact that decreasing the angle usually required removing weight from the bit. However, this does not apply to all tools: some, such as rotary steerable tools, are exempt from this problem, although come at a higher financial cost. Unless the drilling operation already has a high daily cost, rotary steerable tools would not normally be used to correct a directional issue. If the low edge is aimed at, then directional correction work will not have a negative impact on drilling speed.
Other factors need to be taken into consideration when planning a well path. Whenever the well changes direction, the drillpipe needs to bend around that curve, and if the well is curved when still near the surface, this curve will cause additional drillpipe tension the deeper the well gets and the more weight is put on the drillpipe. This additional side force can cause numerous problems, including metal fatigue or wear on the pipe, and may even cause the pipe to become completely stuck.
Rate of Change in Direction
Rate of change in direction is measured in angle of directional change for every 100 ft (or 30 m) of hole drilled. This is known as the dogleg severity, as a dog’s leg bending is a signifier of a bent hole. High dogleg severity needs to be avoided while at a shallow point in the hole, since it causes high forces between the pipe and hole wall.
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Jones, S., & Sugiura, J. (2008, January 1). Concurrent Rotary-Steerable Directional Drilling and Hole Enlargement Applied Successfully: Case Studies in North Sea, Mediterranean Sea, and Nile Delta. Society of Petroleum Engineers.
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Mitchell, R.F., Miska, S.Z. and Aadnoy, B.S. (2012) Fundamentals of drilling engineering. Richardson, TX: Society of Petroleum Engineers.
VisCo (2011) Oil and gas – 3D animation – Shale drilling 02. Available at: https://www.youtube.com/watch?v=RZgAVjCw3OI (Accessed: 18 February 2017).