According to the Department of Energy, if drilling with lasers ultimately proves viable, it could be the most radical change in drilling technology in the last century. It was at the turn of the 20th century when rotary drilling supplanted cable tool drilling as the petroleum industry's standard method for reaching oil and gas formations. While major improvements have occurred since then, the basic mechanical drilling method has remained essentially the same.

Using lasers to bore a hole offers an entirely new approach. The novel drilling system would transfer light energy from lasers on the surface, down a borehole by a fiber optic bundle, to a series of lenses that would direct the laser light to the rock face.

Faster Drilling

Now, researchers believe that state-of-the-art lasers have the potential to penetrate rock at rates 10 to 100 times faster than conventional boring technologies provide - a huge benefit in reducing the high costs of operating a drill rig.

Today, a typical land-based oil or gas well costs around $400,000 to drill, while costs for an offshore well average nearly $4.5 million. But in some deeper or more difficult drilling terrains, costs can be much higher. Reducing the time a drill rig remains on site can significantly lower costs and make previously uneconomic gas or oil deposits commercially attractive.

Lasers could play a role as a vertical boring tool in gas well drilling. Courtesy of the Gas Technology Institute and Visual Designs Inc.

Past Work

During the two-year period, over 200 samples of various lithologies were exposed to a range of power levels, wavelengths, durations and beam diameters. Cores of sandstone, limestone, shale, granite, salt and concrete all were tested.

This earlier study showed that laser systems now can provide more than enough power to cut rock. Because the laser head does not contact the rock, there is no need to stop drilling to replace a mechanical bit. Those involved with the newer laser study commented on the earlier study in an October 2001 report entitled, "Laser Drilling: Determination of Energy Required to Remove Rock," which they presented at the 2001 Society of Petroleum Engineers Annual Technical Conference and Exhibition. Referring to the precursory study, this report says, "Fast penetration speeds were obtained as well as some fundamental changes in the properties of the samples. Also, the experiments indicated that at such high powers, there were deleterious secondary effects that increased as hole depth increased." The report goes on to say that included in these effects were "the melting and remelting of broken material, exsolving gas in the lased hole, and induced fractures, all of which reduced the energy transfer to the rock and therefore the rate of mass removal."

While the lure of laser drilling has been its speed, one major drawback has been the large amounts of energy experts assumed would be required. The 1997-99 Gas Research Institute study indicated, however, that conventional wisdom -much of it based on 20-year-old calculations - may have significantly overestimated the energy that would be required to break, melt or vaporize rock. Speaking of its predecessor, the recent report says, "The study showed clearly that current laser technology is more than sufficient to break, melt or vaporize any lithology that may be encountered in the subsufrace. It also showed that the energy required to accomplish these varies as much within litholgies as between them."

Three lasers were tested in the program:

• U.S. Army Mid-Infrared Advanced Chemical Laser (MIRACL) - MIRACL is the highest average power laser (megawatt class) in the United States. It was originally developed for shipboard defense and used extensively for testing StarWars concepts during the 1980s and 1990s at HELSTF. The existing MIRACL laser has propagated hole-burning power many miles through the atmosphere at flying tactical and strategic military targets, and has the demonstrated power levels needed to burn through solid materials such as soft rock minerals.

• U.S. Air Force Chemical Oxygen/ Iodine (COIL) laser - COIL, a high-powered laser invented by the U.S. Air Force in 1977 for air-to-air defense, appears to offer potential for natural gas drilling applications. COIL was originally developed as an airborne laser tactical weapon capable of tracking and destroying missiles. This same precision applied to drilling and completing gas wells at depths of more than 15,000 feet could eliminate problems with well control, side-tracks and directional drilling. One distinct advantage of a COIL is its potential for coupling into fiber optics.

• Laser Hardening Material Ex-perimental Laboratory CO₂ laser - In February, 1999, testing of the CO2 laser at Kirtland AFB indicated that, while effective, the CO2 laser is not as suited for rock cutting as the COIL laser tested previously.

More Research Needed

The Energy Department selected the current laser drilling project in a competition for new ideas to improve the way companies find and produce natural gas. The proposal was originally submitted by the Chicago-based Gas Research Institute, which subsequently combined with the Institute of Gas Technology, also of Chicago, to form the Gas Technology Institute.

Prior to its onset, several goals were set for the study. The Department of Energy Web site claimed that one of the primary objectives of the new study will be to obtain much more precise measurements of the energy requirements needed to transmit light from surface lasers down a borehole with enough power to bore through rocks as much as 20,000 feet or more below the surface.

Another aspect of the study will be to determine if sending the laser light in sharp pulses, rather than as a continuous stream, could further increase the rate of rock penetration. Pulsed lasers have been used for better performance in cutting steel, for example. It may be likely that the pulsing action will flex and break the physical bonds between the rock grains, boosting the cutting effectiveness.

A third aspect of the new project will be to determine if lasers can be used in the presence of drilling fluids. The technical challenge will be to determine whether too much laser energy is expended to vaporize and clear away the fluid where the drilling is occurring.

Later in the project, researchers could examine other ways to use lasers in oil and gas drilling. For example, after a well is drilled, perforations are created into the formation to start the flow of hydrocarbons. Part of the research effort will study ways lasers could be used to create these perforations.

Recent Findings

In the experiments, the group prepared sandstone, limestone and shale samples for interaction with a 1.6-kW pulsed Nd:YAG laser beam, and only shallow holes were created. The paper says these tests hoped to discover "how the beam's size, power, repetition rate, pulse width, exposure time and energy can affect the amount of energy transferred to the rock for purposes of spallation, melting and vaporization."

The experiments gave rise to the following major observations: "Absorption of radiant energy from the laser beam gives rise to the thermal energy transfer required for the destruction and removal of the rock matrix. Results from the tests indicate that each rock type has a set of optimal laser parameters to minimize specific energy values as observed in a set of linear track and spot tests.

"In addition, it was observed that the rates of heat diffusion in rocks easily and quickly are overrun by absorbed energy transfer rates from the laser beam to the rock. As absorbed energy outpaces heat diffusion by the rock matrix, local temperatures rise to the minerals' melting points and quickly increase specific energy values. The lowest specific energy values are obtained in the spalling zone just prior to the onset of melting."

Also noted was that the specific energy decreases as both the pulse repetition rate and pulse width increase, but of these two, pulse width is a bigger factor in reducing specific energy. Furthermore, out of limestone, sandstone and shale, the latter samples had the lowest specific energy values.

"The results of this investigation," the group reports, "provide a better understanding of the minimum energy requirements to break rock, and ultimately the determination of both the technical and economic feasibility of this revolutionary drilling application." However, the paper recommends further studies to better understand and hone this innovative process. n

For more information on drilling with lasers, visit the Department of Energy's fossil energy page at www.fe.doe.gov.

Sidebar: Why Lasers?

GRI's choice of laser technology was supported by two factors: the extensive amount of research performed by the U.S. military into high power lasers during the past two decades, and a window of opportunity provided by a 1994 Congressional action that mandated government-developed technology, even military technology, be available to U.S. industries for commercial use. At the same time, auxiliary technologies are reaching levels of development that encourage the prospect of economically feasible laser drilling (e.g., fiber optics and coiled tubing).

Links

• Department of Energy