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Designing the best drill bit programme for a well is also a specialised job. In exploration wells where the formations to be encountered and their thicknesses are liable to be uncertain a full selection of drill bits, which are highly engineered tools, must be available at the rig site to ensure the most effective drilling speeds through the entire section. These are usually on hire, to be returned after the well if unused. Bits excluding coreheads, plus associated specialised bit services if required, may comprise around 1% to 2% of the total well cost, depending on the type of well, its geology and its depth. There are three main types of drill bit. Tri-cone milled tooth bits have three rolling, self cleaning conical cutters; tri-cone insert (TCI) bits have diamond or tungsten carbide inserts for hard formations; and polycrystalline diamond compact (PDC) have no independent moving parts. Over the last decade materials improvements and better design with respect to hydraulics and cutting structure have resulted in longer lives and higher rates of penetration in both tri-cone and PDC bits. For tn-cone bits, improved wear resistance of seals, increased load capacity of bearings and better wear properties for diamond and tungsten carbide inserts are the main improvements. For PDC bits, better blade design and cutting structures and oriented cutting patterns for directional drilling have improved Raps. Improvements in POC bits, especially in materials, hard facing and new geometries, and their greater application have led to an overall increase in drilling penetration rates of around 30% over the last decade.
Selection of the correct bit optimises the rate of penetration and may save more than the bit cost in drilling time. Selection is based on the expected geology as well as the proven performance of different types of bit in nearby wells. Although a compromise choice is often required, in general, tri-cone milled tooth bits are selected in the softer upper part of a well, insert type tri-cone bits are used in harder formations and POC bits are used on long sections of soft uniform rock type (especially shales and chalk). At deeper levels when prolonged bit life is desired because of the long trip times POC bits are most often selected. Diamond bits are used rarely for very hard formations. Forces are generated at the bit/rock interface by the high-pressure flow of drilling fluid through the bit nozzles. These forces are given the term, bit hydraulics, and they control the efficient removal of cuttings as they are produced. Optimum bit hydraulics, related to the power expended at the bit, its impact force and the minimisation of nozzle blockage, will maximise the rate of penetration. The ultimate choice of bit hydraulic system depends on the rock type being drilled. In general new tri-cone bits are employed with mini extended nozzles, asymmetric nozzle configurations and diffusing centre-jets to create a fluid stream that gives a continuous sweeping action for hole cleaning and cuttings removal. POC bit design concentrates on optimum nozzle numbers, layouts and sizes to achieve the same objectives.
As a drill bit becomes dulled Rap reduces and eventually the bit needs to be changed. Not only does this increase the cost of drilling both in terms of slower penetration rates followed by the need for a trip but also badly worn bits will disguise formation changes that would be recognised from drilling breaks. Each bit type has different resistance to wear. Tri-cone milled tooth bits will gradually wear down but the bearings on the cones can rapidly deteriorate if the bearing seal is damaged. Insert bits also wear down but breaking teeth are a greater problem. PDC and diamond bits have long lives but will quickly dull as the last of their cutting surfaces wear away. Over the last decade there has been a shift in bit use with PDC bits replacing tri-cone bits for many applications in all depth ranges. PDC bits are essentially drag bits that work via a scraping action, as opposed to chipping and crushing. This long lasting scraping action and basic rugged nature is very suitable for extended reach wells with mud motors steering the bit, particularly in the softer formations found in the North Sea and Gulf of Mexico. They can convert more weight onto the rock face and the larger pipe sizes used enhance hole cleaning. Perhaps over 3,000m may be achieved in a single PDC bit run. The world record bit run stands at over twice this distance.
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