A new thermally stable diamond cutter has proven effective in drilling medium to hard limestone and moderately abrasive sand. This new type of diamond is called "Ballas" which is q synthetic polycrystalline diamond. Ballas can be thought of polycrystalline diamond. Ballas can be thought of as a more refined, or pure, polycrystalline diamond compact cutter which is free of foreign material within or attached to the diamond mass. The Ballas cutter is thermally stable up to 1200 degrees C compared to a conventional polycrystalline diamond compact which is only thermally stable up to 750 degrees C. This additional temperature resistance allows the use of self sharpening polycrystalline diamond to be used in harder, more abrasive formations which cannot be cut with the present diamond compact material.
The use of the Ballas material has shown significant rate of penetration advantage in drilling the carbonate formations of East Texas and the depleted sands of So. Louisiana. Along with the success, present bit designs have experienced some problems drilling some of the clean plastic shales problems drilling some of the clean plastic shales common on the Gulf Coast.
The term "Ballas" was originally applied to a type of naturally occurring industrial diamond with a polycrystalline structure. The same term will be used in this paper to describe a new type of synthetic polycrystalline diamond cutter. The ballas cutting element, like the diamond compact, consists of thousands of synthetic diamond crystals which are grown together under extreme temperature and pressure forming a polycrystalline mass. In this respect they differ from the natural diamonds used in drill bits which are mono-crystalline structures. The advantage of a polycrystalline structure is that the cutting element is self sharpening, unlike that of a natural diamond which will dull when worn. The polycrystalline structure continuously exposes new sharp diamonds throughout the cutters life. Because the ballas cutters are polycrystalline they resemble the black polycrystalline they resemble the black polycrystalline wafer that is found on the diamond polycrystalline wafer that is found on the diamond compact cutters, but the ballas cutter differs from the diamond compact in two ways. First it is not bonded to a backing post, and secondly it's internal makeup differs from the diamond compact significantly. A ballas cutter is currently triangular shaped and can be furnaced directly into the tungsten carbide matrix which forms the body of the bit. However, many new variations in size, shape and composition are continually being designed and tested for abrasion and impact resistance. A diamond compact is 1/2" in diameter and is bonded to a backing post which is brazed on to the bit after furnacing. The manufacturing and bit design of ballas bits is more like that of a natural diamond bit than that of a diamond compact.
In the diamond compact cutter, the diamond layer contains impurities (metals like cobalt and nickel) which are used as a catalyst in the diamond sintering process. These metal impurities expand to the extent that at about 750 degrees C they can break apart the diamond to diamond bond existing in the diamond mass. In the ballas cutter a process has been developed to leach out the majority of these impurities, thus giving the cutting element improved thermal stability up to 1200 degrees C. This allows the cutter to be furnaced into a matrix body bit in the same way a natural diamond bit is produced.
Basically the ballas bit offers a combination of a polycrystalline diamond cutter and a natural diamond bit design. The ballas bit is designed to have the cutting efficiency of a diamond compact but greater durability in harder more abrasive formations. The ballas material first became available in 1982 and early results were encouraging. Ballas core bits in the Midland, Texas area doubled the penetration rate of conventional diamond core bits, and were proving much more abrasion resistant than diamond compact bits.