It is well understood and documented that bottom-hole hydraulics plays an important role in conventional diamond bit performance. Recent industry experience has shown a similar importance with the new synthetic diamond compact bits. Certain types of polycrystalline diamond compact bits have cutters arranged in a reverse spiral mode. For optimum performance bit hydraulic design should be tailored to that reverse spiral mode. Theory reveals that a bit built with rectangular nozzles with a long axis in the tangential direction can give a fluid velocity envelope similar to the envelope of the reverse spiral. A rectangle can also reduce bit plugging by providing an increased perimeter in a non-concentric geometry. The introduction of rectangular nozzles into the bit renders the regular round nozzle pressure drop equation unsuitable for direct application. Extensive hydraulic tests were performed with different configurations of rectangular nozzles to establish more accurate pressure loss equations. The experiments included single nozzle studies and laboratory and field tests of bits. Field test results confirmed the reliability of the results obtained in the laboratory studies.

For the past few years the industry has been trying to extend the operating range of polycrystalline diamond compact (PDC) bits in terms of formations that can be drilled. Initially, the direction was uphole to the softer formations where the high standoff blade bits and large diameter PDC bits have proven economic. The next step is to extend downhole where the formations are harder and generate more heat. Typically, either tungsten carbide insert (TCI) bits or natural diamond bits are used in these harder formations because the heat degrades the polycrystalline diamond compact cutters. However, insert bits tend to be short-lived and require tripping more often than is satisfactory and diamond bits have penetration rates that are slower than desirable. Bits using PDC cutters with enhanced thermal stability and a radius of curvature on the diamond table have demonstrated a significant economic impact in many harder formations. The diamond wafer is shaped like a dome and set in the bit in place of the conventional, flat PDC cutters. The dome shape dissipates heat better, facilitates more efficient mechanical cuttings removal and drills with a constant rate of penetration throughout the life of the cutter due to the variable rake angle. In addition to extensive laboratory testing, a rigorous field-test program was undertaken to determine the significance of this new development. Field-test results are presented from Northern Europe and the lower Wilcox Frio sands and other formations of South Texas.

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