By increasing penetration rates and bit life, especially in hard formation, the use of down-hole air hammers in the oil field has significantly reduceddrilling costs in the Northeast U.S. and West Texas (Ref. 1). Unfortunately, drilling by this percussion method has been limited mostly to straight holeapplications. This paper presents a new concept of a percussion drilling tool which performs both the function of a down-hole hammer as well as that of adown-hole motor. Such a drilling tool, being introduced here as Steerable Percussion Air Drilling System (SPADS), eliminates the necessity to rotate the drill string and, consequently, enables the use of downhole air hammers to drill directional wells.
In hard rock formations such as those found in the Appalachian mountains, the Permian basin, and the Rocky Mountain regions of the U.S. and Canada, the use of air or gas as the drilling medium is advantageous because of improved penetration rates. Conventional down-hole air hammer, on one hand, yields very high penetration rates. On the other hand, it depends on the rotation of the entire drill string to rotate the bit. The ability to control the direction of the well bore is therefore severely restricted. Directional drilling, undermost circumstances, requires the use of a PDM down-hole motor with a bent housing. Unfortunately, when employing gas or air as the drilling medium, PDMmotors, which were designed to operate with mud, suffer from poor stator life, due to high RPM and insufficient lubrication, low torque capacity, due to the compressibility of the drilling medium, and poor bit life, due to insufficient cooling of the seals and/or bearing while being subjected to high RPM.
