Paper: Bit Selection The Center Of A Good Drilling Program

As America's energy demands increase, major new responsibilities are placed on the industry to find new reserves. Hundreds of thousands of dollars, sometimes even millions, are spent on a single well. These high costs, coupled with limited equipment sources, make it critical that we use drilling equipment, especially rock bits, efficiently. Several years ago the cost and importance of the rotary rock bit were considered relatively insignificant to the overall cost of drilling an oil well. However, with the development of tungsten carbide inserts and sophisticated lubrication and bearing designs the rock bit has become expensive. Even though these bits can now drill through thousands of feet of rock, selection of each bit has become a very important factor in the cost of drilling operations. The proliferation of bearing designs and cutting structures since 1967 caused the International Association of Drilling Contractors (IADC) to adopt in 1973 a standard coding for rotary rock bits. This coding is summarized in Fig. 1. the new classification system was initiated to help eliminate some of the confusion among contractors and operating company personnel arising from different coding systems of the various manufacturers. The IADC selected a three-digit numerical system which classifies: 1. Cutting structure (milled tooth or insert) 2. Formation hardness, and 3. Design features. The first digit relates to the cutting structure of the bit. Series 1,2 and 3 in this position describe milled tooth bits for soft, medium, and hard formations, respectively. Series 5,6,7, and 8 describe insert bits for soft, medium, hard, and extremely hard formations respectively. The second digit is a formation hardness subclassification with numbers 1 through 4 designating formation hardness. The final digit, the bit feature classification, indicates mechanical or design features such as gauge inserts, sealed or frictiontype bearings. The IADC classification of l-l-4, for example, refers to a milled tooth bit (1) used to drill the softest formation (1) and having a standard mechanical feature of the sealed bearing (4). The IADC classification of 7-4-7 indicates an insert bit (7) designed to drill hard formation (4), and having friction bearing and gauge inserts (7).