Field Results of Percussion Air Drilling
Authors: Bates R.E. 
Published: December 2007

Abstract
Pneumatic down-hole percussion tools have drilled many feet of hole under diverse conditions in varied formations. The analysis of the field records describes the tool's performance under general field operating conditions.

Introduction
Many attempts have been made to develop a down-hole percussion drilling tool-driven by the circulating fluid- which would work in conjunction with present rotary tools. It has proven easier to develop a working machine which is driven by a compressible fluid. These tools were used in mining before being applied to the oil field; however, air drilling in the petroleum industry has made a place for such a tool. Field testing of the percussion tool described here began in 1957. An interim period was then spent on design work and part modification developing the finished model for commercial application. While minor modifications are still bring made, the tool at the writing is the model which has performed all the drilling analyzed in this report. The purpose of this report is to describe the machine and conditions affecting its operation. An analysis has been made of the available field records which depicts the percussion tool performance under field operating conditions.

Air Percussion Drilling
Rotary air drilling is the common term describing the drilling technique wherein a compressible fluid (air, natural gas or inert gas) is used as the circulating medium for cleaning the hole of drilled cuttings, heaving formations, formation fluids and injected fluids or solids. Because air drilling does not specify a dry or wet hole, the terms "dusting" and "foam (mist) drilling" indicate dry and wet conditions, respectively. The term conventional air rotary is used to indicate a standard air drilling system, and the term air percussion rotary is used to indicate the addition of a percussion tool to the standard air drilling system.

Percussion Tool Operation
The percussion tool is a two-cycle air-engine (Fig. 1), with a single piston which is reciprocated by the net force of the air acting on alternate ends of the piston (hammer). At the bottom of the force-stroke the hammer delivers the percussion blow. Flow of air is controlled with fixed finger valves so that the piston is the only moving part. The tool has two positions which automatically control operation. In the open position the mandrel section of the anvil is extended and valve settings are changed so that the hammer will not operate. In the closed position the mandrel section of the anvil is enclosed within the tool, and valve settings are correct for proper reciprocation of the hammer. The bit connects directly to the percussion tool anvil, thereby becoming an integral part of the tool. Air in excess of tool requirements flows straight through the tool through the by-pass tube. Air flow is controlled by orifices. A safety float valve is installed in the upper end of the tool. Mud can be circulated through the tool when it is open.
Design Factors Affecting Percussive Energy
The equation for kinetic energy per hammer blow is:
Kinetic Energy, ft-lb = p x A x S
where p = pressure, psiA = piston area, sq in. S = length of stroke, ft.
The piston area and length of stroke are fixed on a given tool so the equation becomes:
Kinetic Energy, ft-lb = kp
where k = constantp = pressure, psi.
Thus, kinetic energy of the hammer blow varies directly with the pressure.
JPT
P. 257ˆ