New concepts in drilling

Thermally stable polycrystalline diamond has been available to the drilling industry for a number of years now and cutters manufactured from this material have been found to perform extremely well in certain formations and are regularly used, for example, for drilling abrasive sandstones and hard limestones, say P.N. Tomlinson and I.E. Clark of De Beers Industrial Diamond Division in the United Kingdom.

“Initially, it was thought they might also broaden the use of diamond in the harder rock categories and offer substantial improvements in performance over natural diamond bits currently employed to drill these strata. Despite some reported successes in hard rock formations, thermally stable products (TSPs) have not, as yet, proved to be a universal drilling material,” they say in the introduction of a paper delivered at the annual convention of the Canadian Diamond Drilling Association in Ottawa.

“One of the limitations of TSP cutters has been their apparent inability to cope with rapidly changing rock conditions underground, leading to cutter damage or wear-flat development. TSP bits generally require a different set of drilling parameters to those of conventional surface set or impregnated bits and tend to demand frequent adjustment of bit loads to maintain a fast penetration rate and to minimize the possibility of wear-flat development” Tomlinson and Clark say thermally stable cutters have tended to consist mainly of small cubes, triangular shapes and, to a lesser extent, hexagon and rounds. In their paper they introduce a new TSP format in the form of long, thin rectangles or “pins” in conjunction with two new concepts in bit design. “Currently, there is still a lack of information regarding the preferred operating conditions for a particular formation. Consequently, end-users, rather than risk an expensive failure, have often tended to rely upon the more established impregnated bits which, although possibly much slower than TSP bits, do give a known reliability, particularly in deep hole operations involving hard and varied conditions.

“A further factor impeding the wider use of TSP bits has been price. TSP bits generally tend to be significantly more expensive than equivalent sized impregnated or surface set bits and unless there exists a distinct and visible offsetting gain in penetration rate, coupled with reasonable bit life, there will naturally remain a reluctance to use such bits under practical field conditions.”

SYNDAX pins, they say, have been developed to overcome some of the earlier weaknesses encountered with the use of the more traditional TSP shapes and to enable bits to be run successfully in a wider range of medium-to-hard formations.