Using Large Drill Collars Successfully
Authors: Brinegar Doyle W. , Crews Sam T. 
Published: December 2007

As holes are drilled to greater depths, the use of larger drill collars increases; but an understanding of the problems involved in using these large collars, and of how the problems can be solved, lags far behind drilling capability. Here is information on design criteria and on the selection of an adequate connection.

Introduction
During recent years there has been a marked increase, worldwide, in the number of deep holes drilled. It is easy to recall when a 17 1/2 -in. hole to 1,000 ft, or a 12 1/4 -in. hole to 3,000 ft, was considered a deep, big hole. Today, drilling programs often call for 8,000 to 9,000 ft of 17 1/2-in. hole, and 13,000 to 15,000 ft of 12 1/4-in. hole. With all certainty, these hole depths will increase in the years ahead. Large-hole drilling at shallow depths generally does not require high bit weights because the formations near the surface are soft and drill easily. But at increased depths, the denser formations and higher mud weights require maximum bit weights if these large-diameter holes are to be drilled economically. Today it is not uncommon to find 9-, 10- and even 11-in. OD drill collars in holes where 7 3/4- and 8-in. OD collars were once considered sufficient. During the past 10 years the use of drill collars above 8 in. in diameter has increased and will likely continue to do so. Fig. 1 reflects this increase in the use of big collars over a 9-year period. In the last 9 years there has been an 11-fold increase in bar sales in the 9- to 11-in. size range.

Advantages of Large Collars
This increased use of large drill collars can be attributed to an industry-wide awareness that performance improves in large holes when small collars are replaced by maximum OD collars. This improved performance is a result of
weight concentration, performance is a result of weight concentration, deviation control and fewer connection problems.

Weight Concentration
Drill collars are used primarily to supply usable weight to the drill bit. This objective is best achieved when the weight is concentrated as near the bit as possible. Long strings of small collars lose much of their apparent weight through friction and buckling in the hole. Advantages in weight concentration should be obvious, considering that 14 collars 10 in. OD × 3 in. ID weigh 102,000 lb, or exactly the same as 25 collars 7 3/4 in. OD × 3 in. ID. The number of large collars that can actually be run in specific areas is often tempered by down-hole conditions and drilling practices.

Hole Deviation
Theoretical work done in the 1950's by Lubinski and Woods, and later verified under field conditions by many drilling contractors. stated that three of the prime factors affecting hole deviation are
prime factors affecting hole deviation are clearance between the drill collars and the wall of the hole, stiffness and weight of the bottom drill collars, and weight on the bit.
The hole-straightening force is the pendulum effect created by the vertical component of the weight of the drill collars between the bit and the first point of contact with the hole wall above the bit.
JPT
P. 999