Roller Cone/Diamond Drag Core Bit
Authors: Herrick G. Richard 
Published: December 2007

Abstract
The use of tungsten carbide inserted roller cones in core bits has been developed by the Deep Sea Drilling Program. During this extensive coring operation, all types of core bits were tried. Most of the types were very limited in the range of formations that they could core effectively. Formations that were made up of chert stringers or largely chert proved to be the most difficult to obtain core recovery during the earl portions of the ocean bottom coring. portions of the ocean bottom coring. They reasoned that a standard TCI three cutter could successfully core these formations and additionally it would have the advantage of being able to have the newest technological advances in the industry. This type of bit soon became their standard.
Determination of when the life bit was completed was made by measuring the core diameter and when it became much less than design, the bit was pulled. However, at the Hot Rock Project at Los Alamos, New Mexico, bits made to core the granitic formation ran into the problem of oversize core because the abrasive wear on the core trimming inserts was more rapid than bearing wear.
With the advent of G.E.'s monocrystalline diamond stratapax, a highly abrasive resistant element used to trim the core and TCI cones to produce stabilization and control the penetration, a most promising new kind of core bit is being prepared to core these very difficult formations.

Introduction
Our experience with core bits began in 1969 when representatives of the Deep Sea Drilling Program inquired about the possibility of manufacturing a core bit made from standard 7 7/8 cones with tungsten carbide inserts for evaluation.
Hard formation insert bits had evolved to the point where a small core was allowed to form. The point where a small core was allowed to form. The subsequent core would be so small that it would easily break off. It was reasoned that if the cones for a smaller bit were moved outward, then a large core would be allowed to form and this core could then be retrieved in a conventional core barrel.
The first core bits were then made using standard 7 7/8 hard formation cones and adapting them for a 2 1/2" core. This functioned very successfully except in soft formations and so shortly thereafter cones were designed with chisel crest inserts with greater extension in order to cover a wider range of formation types. The advantage of using basically standard 3-cone bit cutters was to take advantage of the development of bits, first sealed roller and later sealed journal, without requiring an expensive R and D project.
Because of the success of these bits, the Los Alamos Scientific Laboratory requested a 9 7/8 core bit to cut pink granite in the drilling of their Hot Dry Rock hole at Fenton Hill near Los Alamos. This hole was projected to core all the way using air as a circulation medium. A special bit was designed for this project but the bit did not perform as expected as project but the bit did not perform as expected as core recovery was very poor and the footage that could be drilled per bit fell much below the projected. An on-the-site observation showed that the poor core recovery was caused by abrasive wear on the inserts that trimmed the core. This resulted in the core rapidly becoming oversize and therefore not able to enter the core barrel. As a result, the core was fragmented and recovery was poor. This was unexpected for on the Deep Sea Drilling operation oversize cores had not been a problem and in fact the decision as to when to pull the bit was determined by how much the core was undersize. The abrasive wear was somewhat relieved by using a harder grade insert and although the bit was still not capable of drilling continuously, at least reasonably good core recovery resulted.