Modifications To and Experience With Air-Percussion Drilling
- C.A. Pratt (Shell Canada Ltd.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- December 1989
- Document Type
- Journal Paper
- 315 - 320
- 1989. Society of Petroleum Engineers
- 1.5.1 Bit Design, 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, 1.5 Drill Bits, 1.6 Drilling Operations
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Shell Canada Ltd. has air-percussion drilled 48 wells since Fall 1983. The industry hammer was used predominantly, but the older oilfield hammer was used occasionally. A total of 117 solid-head bits were run. Experiences with the industrial percussioammer indicated that tool modifications were required to reduce hole spiraling and the tendency of bits to go undergauge. High penetration rates (ROP's) with the new hammer required excessive blowing on connections, leading to the modification of a computer program for air-volume requirements. Because minor equipment failures were causing lost time, modifications were made to the jet system, sample catcher, and downhole floats.
There are two downhole air-percussion hammers. The older oil- field hammer, with specially reinforced oilfield three-cone bits, has been in use since the early 1960's. The newest oilfield air hammer uses a solid-head, relatively flat-bottomed bit with tungsten-carbide inserts. Manufacturers commonly call these industrial, or "mining, " hammers.
Fig. 1 shows the general makeup of an industrial hammer. In a study of mining and oilfield hammers, Finger l showed that industrial hammers could drill more than twice as fast as extremely high weight-on-bit (WOB) rotary air and that, in most cases, ROP with industrial hammers was three to six times as fast as with oil-field hammers. Fig. 2 shows a composite of the results of this study.
Oilfield and mining hammers are similar because air pressure causes a piston to move up and down. On the down stroke, the piston strikes the bit or anvil/bit that impacts on the formation. The mining hammer's kinetic energy and power are about six times that of the oilfield hammer.
Finger identified two potential problems with application of the industrial/mining hammer in the oilfield.
Sensitivity to WOB. Industrial/mining hammers are sensitive to WOB because a minimum weight is required to keep the tool closed, solid-head bits cannot withstand high WOB (tungsten-carbide buttons will fail in shear), and carbides wear quickly at high WOB.
Gauge Wear. Gauge wear is a potential problem because solid- head bits cannot be used to ream long sections and hole-size reduction, as practiced in the mining industry, may not be practical. Sheffield and Sitzman also mention a drawback of flat-bottomed solid-head bits: they have little gauge protection and are often pulled undergauge. Whitely and Englands indicate that gauge wear, not bit-face wear, dictates bit life in most cases. Their paper mentions manufacturers' efforts to improve gauge protection.
We had a service representative clean, adjust, and calibrate the automatic driller before air-hammer drilling. In the few cases where the automatic driller was incapable of maintaining constant WOB and constant pressure drops across the hammer, drillers successfully drilled manually. Weight-indicator sensitivity was of some concern because in a few cases WOB was zero while rapid ROP's were achieved. Close observation of air pressure on and off bottom gives a good indication of when the tool is completely closed. An air-pressure gauge is located in the driller's console to monitor pressure and WOB closely.
Gauge wear initially appeared to be a problem with standard industrial mining bits (Fig. 3a). To reduce the risk of bits going un dergauge, we worked with a manufacturer to redesign and build new bits for oilfield use. The resulting design was nothing more than an adaptation of the long gauge-protection section found on straight-hole diamond bits (Fig. 3b). This bit design eliminated the gauge wear problem without affecting ROP. Since Fall 1984, 95 bits with gauge protection have been successfully used on 38 wells. Gauge protection is also provided by a specially designed stabilized driver sub discussed later.
When a bit goes slightly undergauge, the driver sub causes noticeable torquing and hangs up sufficiently to reduce ROP. Also, mining bit tolerances and sizes allow the drilling of slightly oversized holes so that if gauge is lost, standard rotary or oilfield-hammer drilling can be continued.
In Fall 1983, a mining hammer was successfully tested in a Jumping Pound field well. Initial results were encouraging and led to an ongoing testing-and-development pregrain. To date, 117 percussion hammer bits have been used on 48 wells. Increased ROP's resulting from improvements in design led to reduced drilling time in several areas of Alberta, British Columbia, and the Northwest Territories (Fig. 4). Average time to total depth for recent air- and mud-drilled wells at Jumping Pound was 80 days (the best was 6 days) compared with 103 days for the record mud-drilled well.
Instantaneous ROP's (i.e., ROP calculated by use of time to drill a kelly down) with the mining hammer in 8%-in. [219-mm) hole exceeding 262 ft/hr [80 m/h] have occurred, and 187 ft/hr [57 mfh] is common. These instantaneous rates resulted in 1,443 ft/D [440 ni/d], while seven single-shot surveys were taken. At high instantaneous ROP, air volume based on calculations from Angel's work failed to clean the hole adequately.
At Jumping Pound, average ROP on wells drilled with the frilly modified industrial-hammer system is 560 ft/D [170 m/d] from surface casing to average mud-up depth of 7,546 ft [2300 m]. Mudup point is reached in an average of 12 days. From 1976 to 1980 (the last years that water/mud drilling was used extensively), average time to the same depth was 54 days (I 15 ft/D [35 m/d]). Rotaryair drilling from 1980 to 1983 averaged 312 ft/D [95 M/d] over the same interval. Fig. 5 shows increased ROP with air over mud and additional days saved during mining-hammer drilling at Jumping Pound. Figs. 6 through 8 show similar increases in ROP's in the Clearwater, Flathead Valley, and Arrowhead areas.
Mining Hammer Bottomhole Assembly/Tool Modifications
Most of our air drilling is undertaken in the Canadian Rockies or in the foothills belt along the eastern edge of the mountains (Fig. 4). Formation dips can exceed 40, and wells typically build angle. Most wells fail into the 10,500- to 16,400-ft [3200- to 5000-m] range and reach 15 to 25* inclination, with some wells building to 35. The main concern is how to minimize dogleg severity, particularly in the upper 5,000 to 6,600 ft [1500 to 2000 m]. Mechanisms that cause hole deviation during air drilling are not completely understood. In our air-drilling experience, 81/2-in. [216-niml holes build higher angle and 12 'A -in. [31 1 -mml holes generally maintain lower angles than they do in equivalent mud- drilled wells. Wilson and Cooper et al. found hole deviation to be more of a problem in air-drilled holes than in mud-drilled holes of the same size. Dogleg severities-and, if uncontrolled, hole angle-appear to be higher in air holes for three main reasons.
1. Overgauge hole resulting from erosion and instability of the hole prevents packed-hole-assembly techniques from working.
2. ROP and deviation are in some way tied together. Because the bit drills forward faster, it will also drill sideways faster.
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