Advantages of Heavy Metal Collars in Directional Drilling and Deviation Control
Authors: Bradley W.B. , Murphey C.E. , McLamore R.T. , Dickson L.L. 
Published: December 2007

A heavy, stiff-bottom drill collar can substantially improve deviation performance, theoretically increasing penetration rates by 50 to 100 percent in deviation-prone areas. The theory, performance characteristics, and experience in fabricating and field testing two depleted-uranium alloy heavy metal collars are detailed.

Introduction
Reducing the weight on bit provides one of the major techniques for controlling or reducing hole angle. This results in lower penetration rates and increased drilling costs. In both straight holes in deviation-prone areas and in directional wells, the cost of deviation control can become a major portion of the total drilling cost, particularly in areas of high rig cost. particularly in areas of high rig cost. This paper presents a method for theoretically increasing penetration rates by 50 to 100 percent when drilling in deviation-prone areas where the weight response is reasonably linear. By using a heavy, stiff bottom collar, the deviation performance of the drillstring can be improved substantially. As is illustrated in Fig. 1, the addition of a heavy metal collar to the drill-collar string allows more weight to be applied to the bit while maintaining the same hole angle. The paper develops the underlying theory that shows how a heavy metal collar can be used to advantage in deviation control. The theory is used to develop practical charts on the performance characteristics of heavy metal drill-collar performance characteristics of heavy metal drill-collar strings for use in selecting suitable hookups. The paper concludes by detailing Shell Development Co.'s experience in fabricating and field testing two heavy metal collars made from a depleted-uranium alloy.

Hole Deviation Mechanics
The combination of drill-bit/rock interaction and drillstring mechanics that controls hole deviation has been discussed in detail. A principal concern of hole-deviation mechanics is to develop techniques to maximize weight on bit for faster drilling while controlling hole deviation. The tendency for increasing hole deviation with increased weight on bit must be resisted by increased lateral force on the bit. This lateral force is supplied by the pendulum behavior of the drill collars. The longer the pendulum length and the heavier the pendulum, the higher the lateral force will be to resist deviation.
Pendulum length and weight increase with drill-collar stiffness and density. The effect of drill-collar density and stiffness on deviation performance can be illustrated by the three cases shown in Fig. 2. The first case, shown for comparison, is an unstabilized 8-in.-diameter steel drill-collar string in a 5 degrees, 12 1/4-in. -diameter hole. The second case is identical to the first (in size and weight per foot), except the collar stiffness is reduced to the stiffness of a 6 1/4-in.-diameter collar string. The final case shows the effect of replacing the bottom steel drill collar with a drill collar made of a heavier material, such as depleted uranium. Notice that, to maintain the same drilling conditions with the more flexible drillstring, the weight on bit must be reduced by 20 percent for the condition shown. Conversely, by using the heavier drillstring. the weight on bit can be increased 60 percent in this example, resulting in a corresponding increase in the penetration rate.
Heavy metals structurally suitable for drill-collar service are much more expensive per unit volume than steel.
JPT
P. 521