Rock Bit Technology - The Effect On Drilling Costs

Abstract

Introduction The development of rock bits has been an evolutionary process. Modern-day rock bit technology had its beginning in 1909 with the introduction of the first rotary-cone rock bit. Since that time, rock bits have been continually improved. These improvements were highlighted by the introduction of the tricone bit in 1933, jet bits in 1948, tungsten carbide insert bits in 1951, sealed bearings in 1960, shaped and extended tooth-shape inserts in 1967, and journal bearings in 1970. The net effect of the rock bit evolution has been increased performance and reduced drilling costs. By ncorporating special lubricants in specially sealed cones, bearing life has been extended by maintaining a well-lubricated bearing and preventing the intrusion of mud and abrasives in the bearing. The development of tougher tungsten carbide materials has resulted in extended tooth-shape insert designs closely paralleling steel tooth bit configurations. This has permitted the use of longer-life insert bits in the faster drilling intervals. The long-life tungsten carbide cutting structure with sealed-lubricated bearing systems (roller and journal) has greatly extended bit life. Runs exceeding 100 hours and/or several thousand feet are not unusual. First Rotary Rock Bit On August 10, 1909, Howard R. Hughes patented the first rotary rock bit, which he called a "boring drill".(1) Prior to this time, the use of the rotary drilling process had been limited to soft formations because harder formations were difficult or impossible to drill using the scraping action of the drag bit. The initial rolling cutter bit resembled two pipe reamers set on journal bearings attached to a drill pipe sub (Fig. 1). This enabled the rotary drilling process to drill formations which were too hard to drill using the scraping action of the drag bit. The early designs utilized cones which did not mesh. Consequently, the bit "balled up" in soft shale rathereadily. This, along with the slower penetration rates, kept the bit from gaining wide acceptance during the early years and the drag bit remained as the more popular tool for drilling the soft formations encountered on the Texas Gulf Coast. Self-Cleaning Cones As the rotary process of drilling spread inland from Texas Gulf Coast, new formation combinations were encountered which created a serious problem. Inter-bedded layers of plastic shale and hard rock ledges often necessitated many round trips to alternate the drag bit with the rock bit. A bit was needed to drill both the hard and soft formations.(2) In 1925, this problem was solved with the introduction of a two-cone hit with intermeshing teeth (Fig. 2). As the teeth of one cone passed through the grooves on the opposite cone, accumulated formation was pushed from the grooves of the opposing cone – thus the name self-cleaning. The new cutting structure enabled the drilling industry, for the first time, to make hole through both soft and hard formations with a single bit, thus markedly reducing over-all time of each well. Anti-Friction (Roller) Bearings Until 1932, the basic bearing of rolling cutter hit remained the journal type.