Drilling rock

The flexibility of laser processing is well known. By tailoring the beam irradiance and the interaction time, a wide range of applications can be accommodated with the available beam power determining the speed of the process. Readers of ILS should be familiar with many of the more widely used processes. However, the use of high-power lasers for drilling natural gas wells may stretch your imagination a bit. How does a laser drill an 8-in. hole 5000 to 10,000 ft deep, that is, an aspect ratio >7500? Read on.

In 1997 the Gas Research Institute (currently, Gas Technology Institute or GTI, www.gastechnology.org) initiated a project to revolutionize the methods used to drill and complete natural gas wells.¹ It essentially examined the feasibility of using the Army's 1MW class Mid-Infrared Advanced Chemical Laser (MIRACL) and the Air Force 10kW Chemical Oxygen-Iodine Laser (COIL) to drill rock. Penetration rates obtained with MIRACL were approximately 450 ft/hr, that is ~100 times rotary drilling rates. The laser-drilled holes had walls of vitrified material.

To appreciate the potential of high-power lasers in drilling rocks, it is necessary to understand problems commonly encountered in rotary drilling. Boreholes must be lined to prevent collapse, influx of liquids or loss of drilling fluids. A laser-drilled hole with a vitrified wall would solve many of the problems that often occur before liners or casing can be implemented. Rotary drilling rates are subject to rotation rate, weight on the drill bit, horsepower, bit design and several other parameters. In addition, out-of-balance and off-axis problems occur. Laser drilling is expected to depend only on the power and irradiance available and is basically an "inertia-less tool." To extract the gas or liquids from a borehole, shape charges are often used to perforate the casing. Lasers may also supply a more precise technique for perforation.

In 2001 the Department of Energy provided funding to GTI to further the development of laser rock drilling in collaboration with Argonne National Laboratory (ANL) and the Colorado School of Mines. Additional funding was provided to ANL in 2002 to use its high-power laser processing facility to examine the effects of high-power CO₂ (6kW) and Nd:YAG (1.6kW pulsed) lasers on rock drilling.

To understand the fundamentals of how laser drilling can be applied to rocks and the optimization of the process, one has to take advantage of the mapping of beam irradiance and interaction times for different laser processes. Figure 1, an updated mapping based on Professor Steen's work, ² is based on metals and also applies to other materials with similar fluence requirements. The irradiance supplies the heat flux to the process. The product of the irradiance and the interaction time is the energy supplied per unit area or fluence to produce the heating, melting and/or vaporization. Short interaction times achieved with picosecond and femtosecond laser pulses result in insignificant melt zones as depicted on the upper left of the figure. As the interaction time increases, the melting regime grows. For drilling, high irradiance and short interaction times are required to minimize melt, whereas for transformation hardening (heat treating) the irradiance and interaction are limited to prevent melting. In general, the laser process deals with heating, melting or vaporizing the material while minimizing the heat affects on the base material such that undesirable affects are minimized.

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Limestone, sandstone and shale are the three major types of rocks encountered in gas well drilling. Limestone is essentially calcium carbonate and some magnesium oxide. Sandstone consists of quartz, feldspar and other minerals, while shale is similar with quartz, feldspar and clays. The porosity varies from 0.6 percent for limestone and 3 percent for shale to >20 percent for sandstone, and water is often present. Tests carried out on these rock types showed that the irradiance and interaction time have to be controlled to minimize melting for efficient rock removal. Limestone has to be drilled by conventional laser drilling with pulse beams and high-pressure gas assist requiring <10⁵ W cm^–2 for decomposition and melting. CW beams tend to heat up the limestone and cause melting. Unlike limestone, which is basically a solid material, sandstone and shale are made of quartz grains cemented by feldspar, clay and other minerals with some water in the pores. Quartz or silica tends to be more transparent to CO₂ and Nd:YAG radiation than the other minerals and water.

A relatively low irradiance pulse (10⁴ Wcm^–2) can heat up some of the rock constituents and water, causing expansion and vaporization, leading to fragmentation of the rock. A high-pressure gas assist can then remove the rock fragments. The beam can be translated to remove several mm per pulse over a larger area. Rock fragmentation is in the "heating regime" as applied to the quartz component of the rock even though some of the other constituents may be melted or vaporized. The low irradiance threshold allows the use of long focal lengths or collimated beams for deep penetration.

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Samples of rock drilled with 4kW of CO₂ beam power are shown in Figure 2. The limestone sample has a hole (~1.2 in) drilled to 4 in. deep by using a long focal length optic and high-pressure nitrogen assist. A CW beam (~1/2 in diameter) was applied while rotating the sample. The sandstone and shale samples were drilled (1 in diameter) to 2 in. deep by percussion drilling, also with gas assist but with pulses <1 sec duration. Drilling speeds exceeding 150 ft/hr were obtained for sandstone, which compares favorably with the rotary drilling speed of ~50ft/hr. The Nd:YAG laser had lower average power but was more effective in fragmenting the sandstone and shale samples because quartz particles have a substantially lower absorptivity for 1.06-µm than 10.6-µm radiation.

Tests at surface conditions with high-power CO₂, diode and Nd:YAG lasers demonstrate that lasers can drill rocks with relative ease. The challenge is to apply the technology in actual conditions and as the hole progresses where water is present and sludge is used as a fluid for rotary drilling. At depths of >5,000 ft, the pressure and the temperature may exceed 2000 psig and 100° C, respectively.

Preliminary data indicate that water and fluids that absorb and diffract the beam prevent effective drilling. A gas void may have to be created to allow effective processing. Hyperbaric conditions favor formation of intense plasma plumes that tend to block the beam but can be alleviated by using inert cover gas.6 Laser heads and beam delivery components have to withstand temperatures >100° C and operate in hyperbaric environments. Fiber can be used for flexible beam delivery for Nd:YAG and diode lasers but at a power loss of 50 percent/km length. Although there is still sufficient power for processing, the attenuation needs to be reduced to be more effective.

ANL in collaboration with GTI, Parker Geoscience Consulting and Colorado School of Mines has demonstrated and improved the efficiency of lasers for rock drilling for surface conditions. They are now working on the next phase for processing under more prototypical conditions to advance the development. Collaboration with industrial system suppliers to improve the operation range of beam delivery components and performance of silica fibers is key. If innovation prevails, hybrid technologies that possess the merits of conventional rotary drilling and laser processing may result.

Keng Leong is a frequent contributor to ILS. Zach Xu and Claude Reed, [email protected], are with Argonne's Laser Applications Laboratory.

Acknowledgement

This work is supported under FWP 49066 by the National Energy Technology Laboratory, Office of Fossil Energy, U.S. Department of Energy, under Contract No. W-31-109-ENG-38.

References

1. D.G. O'Brien, R.M. Graves and E.A. O'Brien, SPE 56625, Society of Petroleum Engineers Annual Technical Conference and Exhibition, Houston, Texas, 3–6 October, 1999.
2. W.M. Steen, Laser Materials Processing, Springer-Verlag, London, 1994.
3. R.M. Graves, A. Araya, B.C. Gahan and R.A. Parker, SPE 77627, SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 29 Sept.–2 Oct. 2002.
4. B.C. Gahan, R.A. Parker, S. Batarseh and H. Figueroa, SPE 71466 presented at Society of Petroleum Engineer Annual Technical Conference and Exhibition, New Orleans, Lousiana, 30 Sept.–3 Oct. 2001.
5. Z. Xu and C.B. Reed, 21st ICALEO, Scottsdale, AZ, October 14–17, 2002.
6. G.J. Shannon, W. McNaught, W.F. Deans and J. Watson, J. Laser Applications, 9, 129–136 (1997).