There are several factors that affect drill bit selection. Due to the high number of wells that have been drilled, using information from an adjacent well is most often used to make the appropriate selection.A major factor in drill bit selection is the type of formation that needs to be drilled. The effectiveness of a drill bit varies by formation type. There are three types of formations: soft, medium and hard. A soft formation includes unconsolidated sands, clays, soft limestones, red beds and shale. Medium formations include calcites, dolomites, limestones, and hard shale. Hard formations include hard shale, calcites, mudstones, cherty lime stones and hard and abrasive formations.Durability and rate of penetration are two key factors to clarify the bit quality.