Diamond Technology

 

Polycrystalline Diamond Compact (PDC) Design Methodology Utilizing Strain Energy Capacity

by K.E. Bertagnolli and C.H. Cooley

Absract

Polycrystalline diamond compact (PDC) bits have gained wide commercial acceptance in oil and gas drilling due to their high rates of penetration, long life and mechanical simplicity. However, PDC bits have had limited success at drilling high compressive strength and abrasive rock formations. One of the limitations to hard rock drilling is the propensity of the cutters to fracture. This paper examines the use of strain energy capacity as an index of fracture resistance under dynamic and static loading. The cutter and the bit absorb energy during a down-hole impact event. This absorbed impact energy is converted to strain energy as the cutter deforms under the applied load. Cutters with higher strain energy capacity will be able to absorb more impact energy during drilling without exceeding the diamond tensile limit. PDC cutters with various diamond/carbide interface geometries and diamond thickness were modeled with finite element analysis (FEA). The FEA models included both residual stress loads and simulated impact loads. Strain energy capacity was calculated after adjusting the impact load to produce a critical tensile stress on the diamond surface. Laboratory drop-tower impact testing was done on each of the calculated designs. Designs were then ranked based on calculated strain energy capacity and experimental drop tower results. Agreement, though not perfect, is promising. These tools hold the potential to quickly screen cutter designs before undertaking the risk of testing a bit down-hole. They also allow the incorporation of residual and brazing stresses, which can be significant.

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