One of the most significant problems associated with extended reach or horizontal drilling is torque and drag which is caused by the friction between the drill string and the hole. The magnitude of the torque and drag is determined by the magnitude with which the pipe contacts the hole wall and the friction coefficient between the wall and pipe. Figure 8-1 shows the forces associated with an object on an incline. The weight component along the axis of the incline (w SinO) would be the force required to move the object in a frictionless environment.
Forces on an inclined plane
Unfortunately, friction is always present and will contribute to the force required to move the object. The friction force is equal to the normal force times the friction coefficient. Therefore, the force required to pull the pipe from the hole is:
Where: T = Axial Tension
W = Buoyed Weight of Pipe ^ = Friction coefficient ® = Angle of incline
The force required to push the pipe from the hole is: T= -W Sin® - ^ WCos®
The friction coefficient depends upon the type of drilling fluid in the wellbore and the roughness of the wellbore walls. Cased hole should have a lower friction coefficient than open hole. Untreated water based muds will have a higher friction coefficient than oil based muds. Friction coefficients have been reported to range from 0.1 to 0.3 for oil based muds and 0.2 to 0.4 for water based muds. When hole curvature is considered, an additional force is added to the normal force. Pipe placed in a curved wellbore under tension will exert a force proportional to the tension and rate of curvature change.
Buckling of the drill string while tripping into the wellbore causes an additional drag force. The critical buckling load is a function of the inclination, pipe size and radial clearance. Once the compressive forces in the drill string exceed the critical buckling load, an additional normal force is imposed on the drill string increasing the drag force in sections of the wellbore.
The torque in the drill string is determined by the normal force times the friction coefficient and is the force resisting rotation of the drill string. The torque and drag will increase as the tension and dogleg severity increases. In normal directional wells, the drag is the main concern but as depth, inclination, build rate and length of hold section increase the torque can become a major concern. Torque will also limit the tension capability of drill pipe when combined with tensile loads.
There are three main ways to reduce the drag in the well; 1) change friction coefficient by changing mud system, 2) change the directional profile or 3) change the string weight or tension. Since the drag is proportional to the coefficient of friction, finding a way to reduce this value by half will halve the drag.
Changing the directional profile can have significant benefits but if you've already drilled a good portion of the profile, wiper/reamer trips to smooth out any ledges or doglegs in the build section can have significant benefit.
Replacing drill collars with hevi-weight or regular drill pipe can have a significant effect on reducing the tension and normal forces thus drag.
There are excellent torque and drag models in the market that very accurately predict values for a chosen wellpath. It must be remembered this is just a model and one of its better design uses is for comparison of different profiles with all other factors the same. Another very helpful place to utilize this tool, is while drilling horizontal wells. Large changes between predicted and actual drag values can indicate the hole is not cleaning. These models are also used to effectively design the drill string from the bottom of the well to surface.
Spring 2002 Calgary
Positive and Negative Pulse Valves
This page intentionally left blank.
Was this article helpful?