![]() Figure 1 illustrates how a severe dogleg can cause fatigue failures. Point “A” on the drill pipe is in maximum tension while point “B” is in minimum tension due to bending. Most failures in drill pipes are fatigue failures resulting from gradual progressive growth of minor irregularities into major cracks even when the stresses are less than the yield strength of the metal. Drill Pipe Fatigueĭrill pipe fatigue is also associated with doglegs. Logging tools and drill collars can become stuck ( check also free point calculation for stuck pipe) in a keyseat drillingthat is considered one of the Mechanical Sticking problems. Other problems associated with severe doglegs are wearing of tool joints and worn spots in the casing which can lead to collapse or a hole in the casing. Soft formations require a lower force than hard formations to form a keyseat. A keyseat is formed if the lateral force is large enough to cut into the wall. The lateral force causes the drill string to cut into the wellbore wall at the dogleg. KeyseatsĪs drilling continues, the drill string tension in the dogleg increases which increases the lateral force. The heavy-weight drill pipe is more flexible and reduces the overall string weight while maintaining the same available bit weight. ![]() This can be accomplished by removing excess collars or replacing the collar with a heavy weight drill pipe. Oil and other commercially available lubricants reduce the coefficient of friction between the drill string and borehole wall thereby, reducing the torque and drag. Another method is to reduce the tension in the drill string. Torque and drag can be reduced using lubricants in the mud system. ![]() Once a severe dogleg exists in the wellbore, its effect can be decreased somewhat by reaming but only by a small amount. The torque and drag can be reduced by several different means. One method is to keep the dogleg severity low. As tension on the drill string increases (depth below dogleg increases) the lateral force increases therefore, the torque and drag increase. The drill string exerts a force on the formation as shown in Figure 1. When the drill string is in tension, it tries to straighten while going around a dogleg. Torque and drag are caused by the friction between the drill string (check drill string components) and the borehole wall. An assembly should never be forced to the bottom it should be reamed to the bottom. The bottom hole assembly may go to the bottom, but it might not come back up through the dogleg. In a horizontal or high inclination well, the torque may be lower while rotating off the bottom. Care should be taken when tripping after a significant change in hole inclination and/or direction. However, while tripping or reaming in drilling, the torque will be greater because the collars are in tension and increase the overall tension in the drill string. ![]() The drill string will experience less torque from a dogleg while drilling because the Drill collars are in compression except in the case of a horizontal well or high inclination well. One of the immediate problems associated with doglegs is torque and drag. More severe doglegs will cause higher torque and drag. When a dogleg becomes a problem, then it is considered severe. Traveling Block Application & Inspection ĭoglegs are not necessarily a problem in directional wells.Wireline Retrievable Subsurface Safety Valve.Tubing Retrievable Safety Valve (TRSV) Guide.
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