P LANNING A D IRECTIONAL W ELL
The geometry of a directional well can be defined with three parameters: • • • Build rate Hold inclination, (drop inclination), and Kickoff point (KOP)
The directional well configuration can be determined by assuming any two of the above three parameters and then, calculating the third. The assumption of a particular parameterrequires good understanding for an intelligent selection. Hold inclination and kickoff point are easier to calculate than the build rate. The build-rate can be chosen to minimize fatigue in drill pipe, minimize keyseat possibility, or help to minimize torque and drag. If drilling a horizontal well, the build rate may be selected based on steerability of the bottomhole assembly. The hold inclination canbe chosen based on any number of concerns. At low inclinations, it may be difficult to maintain the direction of the wellbore. Bit walk is greater at low inclinations because the direction can change significantly with small changes in dogleg severity. Above 30 degrees, it is more difficult to clean the hole with 45o to 60o being the hardest to clean. Above 60o, open hole logs may no longer fall.If the hole is not very clean, open hole logs may not fall at inclinations above 50o. In cased hole, wireline tools will not fall at inclinations greater than 70o. Tubing conveyed perforating or coiled tubing conveyed perforating will be required. The kickoff point may be selected based on hole conditions and target constraints. Many times it is desirable to case the build curve to minimize thepossibility of a keyseat; therefore, the kickoff point may be based on casing seats. It may be desirable to drill some troublesome formations such as lost circulation or sloughing before kicking the well off. MWD tools do not tolerate large quantities of LCM for extended periods of time. In sloughing formations, stuck pipe may lead to loss of very expensive directional tools. If the troublesomeformations are too deep, it may be desirable to be drilling a hold section in these formations. Generally, the build rate is chosen trying to keep below the endurance limit of the drill string in order to minimize the possibility of fatigue damage. The higher in the hole the kickoff point, the lower the dogleg severity needs to be in order to minimize fatigue in the drill string through the buildsection. It may not always be possible to drill a directional well and not cause some fatigue in the drill string or to keep the inclination below 30o. It depends upon the target departure. With high departure targets, high inclinations will be required. After all, the objective of the directional well is to hit the target or to hit multiple targets.
Copyright © 2007 OGCI/PetroSkills. All rightsreserved.
Horizontal and Directional Drilling Chapter 4
DETERMINING DIRECTIONAL WELL PLAN
The majority of today's directional well planning is performed on computers. Computers are fast and can incorporate both changes in build and drop rates and changes in direction. All directional drilling service companies offer this service; therefore, a final well plan should be generated by acomputer. However, there are times when a directional driller or engineer may need to estimate the inclination needed to achieve a specified departure, or he may need to change the well plan while drilling the well. Sidetracking around a fish is an example. At these times a computer may not be available. Presented here is a simple method of planning a directional well which can be used in theoffice or field. With little practice, this method can be used to plan and plot a directional well. If possible, the final plan should be processed by the directional drilling contractor on a computer. In this chapter, we will look at two dimensional planning only. Three dimensional planning (incorporating direction changes) is beyond the scope of this manual. It should be remembered that if you can...
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