Air Drilling Operations Improved by Percussion-Bit/Hammer-Tool Tandem
- Maxwell C. Whiteley | William P. England
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- October 1986
- Document Type
- Journal Paper
- 377 - 382
- 1986. Society of Petroleum Engineers
- 1.8 Formation Damage, 4.3.4 Scale, 4.2.3 Materials and Corrosion, 2.4.3 Sand/Solids Control, 1.6.1 Drilling Operation Management, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.10 Drilling Equipment, 1.6 Drilling Operations
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Summary. Contractors and operators air drill whenever possible to improve rate of penetration (ROP). This is done with pneumatic hammer tools (HT's) and various bit types used with standard rotary air rigs. The recent application of a "flat-bottomed" percussion bit (FPB) combed with a custom-designed HT originally developed for mining operations has significantly improved air drilling operations in the Arkoma basin. The improvements include a large increase in ROP, improved hole geometry, reduced drillstring stresses, and a substantial reduction in cost per foot. This paper provides (1) a discussion of the engineering design and operation of the FPB/HT tandem, (2) applications and limitations of the tools, (3) guidelines for optimization of performance, and (4) documentation of field performance on Arkoma basin wells to demonstrate the improvements in air drilling operations.
The Arkoma basin is notorious for hard drilling at the surface and through intermediate sections of the hole. These hard-rock conditions are ideal for air drilling. The major advantages of air drilling over conventional mud drilling are improved ROP, reduced water requirements, reduced mud costs, reduced reserve-pit capacity requirements, negligible formation damage, and immediate indication of zone productivity. Air drilling is limited only by the formation hardness, the available capacity and pressure rating of the air equipment, and the water influx from drilled formations. Soft formation cuttings are too large to lift with air at application depths and are not usually suitable for air drilling. Hard formation cuttings are virtually dust and can be lifted with an adequate volume of air. Water influx from drilled formations reduces the hole-cleaning efficiency of air, but the addition of soaps and other chemicals through mist injection into the drillstring allows air drilling to continue even with significant water influx. Air drilling without mist injection or water influx is called "dusting." Air drilling with mist injection and water influx is called "misting."
Most air drilling is done with three-cone carbide button bits and various types of pneumatic HT's. This method provides improved ROP over conventional mud drilling. However, the recent introduction of a carbide-insert FPB/HT has dramatically improved ROP over the threecone bits and HT's previously used.
A consistent, sizable reduction in cost per foot has been obtained with the FPB/HT tandem in the drilling of 12 1/4 -in. [311 -mm) -OD surface and intermediate holes in the Arkoma basin. The FPB/HT tandem (shown in Fig. 1) that was used to drill the 12 1/4-in. [31 1-mm] holes is a valveless 8 1/2-in. [216-mm] -bore hammer drill designed to operate efficiently on 125- to 200-psi [0.9- to 1.4-MPa) air pressure. The hammer drill assembly includes an integral water-check valve that prevents water from entering the tools and drillstring when the air supply is cut off. Because the HT is valveless, the piston functions as a sliding valve to control the operating air cycle (shown in Fig. 2).
Motion 1-Upstroke/Power. The drilling cycle starts when the supply air pressure opens the check valve and enters the drill. As the bit is fed to the rock, the bit moves upward until it seats on the chuck. This pushes the piston upward, uncovering the bottom air passages and allowing live air to enter Chamber A.
Motion 2-Upstroke/Momentum and Expansion. The air, acting on the surface of the piston flange, rapidly accelerates the piston upward. Continued piston travel cuts off the air supply to Chamber A, but air expansion and inertia cause the piston to continue movement to the top of the stroke.
Motion 3-Upstroke/Cushioning. The air passages supplying Chamber B are now opened, admitting live air above the piston. An increase in pressure will stop the upward piston travel and store energy for the power stroke. In Chamber A, the piston motion has uncovered the bit exhaust tube, and this air is exhausted out the face of the stroke.
Motion 4-Downstroke/Impact. Energy generated by the piston mass and expansion of compressed air is released, forcing the piston down to the striking force of the bit. When the top of the piston pulls away from the air distributor stem, the air in Chamber B is vented down the center of the piston out the face of the bit. The exhaust air blows cuttings away from the bit and up the hole annulus.
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