Research team strikes it rich with a photon drill

20 Mar 2003

Oil is the lifeblood of the global economy, but drilling through thousands of feet of rock to reach it is a tough and expensive business. Michael Hatcher reports on a high-power laser project that could ultimately replace the traditional drill bit with a photon saw.

From Opto & Laser Europe April 2003

When the laser was invented in the 1960s, one of its many potential applications was thought to be drilling for oil. At the time, the idea was not followed up. Oil companies seemed unlikely to abandon their trusted rotary drills, which had served them so well for decades, for unproven laser technology.

A research group based at the Argonne National Laboratory in the US, however, is now producing results suggesting that the laser drill could work. The team has found that a 1.6 kW Nd:YAG source is capable of drilling through, cracking or melting any rock type it encounters.

For the oil industry, the benefits of laser drilling would be immense. Using conventional techniques, the annual cost of drilling onshore wells in the US alone has been estimated at around $15 bn. But according to Richard Parker, a member of the Argonne team, using laser drilling could reduce this cost dramatically, cutting drilling times by a factor of ten or more.

Layers of rock According to Parker, the drawbacks of current technology emerge when a well is being drilled through layers made of different types of rock. For example, shale - which accounts for about two-thirds of all the rock drilled by oil companies - requires a particular kind of drill bit. But if a seam of limestone is encountered while drilling through shale, the drill bit becomes useless. The drill has to be hauled back to the top of the well, fitted with a different bit designed for limestone cutting, and lowered back to the seam. When the limestone layer has been drilled through, the bit must be changed again. Parker thinks that with a laser drill, on the other hand, it should be possible simply to optimize the output parameters for each rock type as it is struck.

He and a team at the Gas Research Institute (GRI), Illinois, started looking seriously into laser drilling about 10 years ago. Calculations made in the 1960s had always suggested that a huge amount of energy would be needed to break through rocks with a laser. "The early work was based on the assumption that the laser would have to melt the rock to drill through it," Parker told Opto & Laser Europe. "This suggested that high energies were needed and that laser drilling would be very costly."

Parker and colleagues began testing this theory. They used three high-power military lasers operating at different wavelengths in the infrared to drill through the rock: the US Air Force's high-power carbon dioxide source; their 1.34 µm chemical oxygen-iodine laser (COIL - also known as the Airborne Laser); and the US Army's 3.4 µm mid-infrared advanced chemical laser (MIRACL), which is based on hydrogen fluoride.

The results showed that the "specific energy" - the energy required to remove 1 cm³ of material - for each rock type was surprisingly low. Less energy was needed to drill through the rock than had been predicted by the early calculations. The assumption that the rocks had to be melted had been wrong; thermal spallation, by which the heat from the laser and its shockwave cracks the rock into small splinters, was a far more efficient mechanism.

"We could cut any kind of rock we tried with the lasers - whether it was different granites, shales, sandstones or concrete," said Parker. As with the conventional drill, the team found that no two rocks reacted in the same way. For instance, the spallation mechanism tends not to work with limestone, which does need to be melted.

Parker was keen to follow up by investigating what effect using different wavelengths and pulse durations would have on the specific energy for each rock type. At around this time the GRI became the Gas Technology Institute, funding of the project passed to the US Department of Energy, and Parker moved to Argonne where he continued the project with an Electrox 1.6 kW Nd:YAG pulsed source.

Working with researchers at the Colorado School of Mines, Parker found that the specific energy of a rock decreases as the pulse repetition rate and pulse width increase. Changing the pulse width had a greater effect than changing the repetition rate. Parker and his colleagues also found that the most efficient drilling technique is to move the beam's focus between different points on the surface of the rock, which reduces the possibility of melting it.

"Drilling with a pulsed laser is more efficient than continuous-wave operation and increasing the repetition rate improves the speed of drilling - up to a point," explained Parker. "However, at very high repetition rates this decreases as the continuous-wave condition is approached."

Altering the wavelength appears to have a limited effect on efficiency. The team has now worked with sources ranging from a high-power diode laser's 800 nm emission to the 10.6 µm of a carbon dioxide source. The longer wavelength drills marginally better but, says Parker, the requirement for fibre-optic delivery currently favours sources working near to 1 µm.

The next step is to build a prototype instrument and perform some field tests. "I believe that we could assemble the tools to make a prototype with existing technology," said Parker. "The problem will be designing the laser head and finding a way to deliver the laser energy thousands of feet down, but these are engineering problems."

To make a standard 6-8 inch diameter well, Parker says that he would need seven or eight 4 kW Nd:YAG lasers to approach the acceptable cutting speed of 30 ft per hour in the hardest rocks.

The team is now busy calculating the optimal laser parameters required to cut through different rock types. In their latest work, they have found that the laser beam irradiance required for spallation of Berea grey sandstone is 920 W/cm², while for shale the figure is 784 W/cm².

Parker has set up a company, Subsurface Laser Applications (SLA), to help transfer intellectual property and identify areas for commercialization in the field of small-hole laser drilling for environmental applications. As president of SLA, Parker says that with $1 m funding he could have an instrument in the field within two years.

Promising applications Although complete laser-drilling of a well remains the long-term goal, one of the most promising applications in the short term is known as "well completion". This is the process by which the amount of oil gushing into the well is maximized. The current method of choice is somewhat low-tech: it involves setting off explosives at the foot of the well bore. Although this works, it can create problems when pieces of metal casing blow into the perforation. One of Parker's ideas is to use a laser to perforate the formations instead, and complete the well in a more targeted manner.

Recent advances in fibre lasers could help to make laser drilling a reality. IPG Photonics' high-power sources, for example, (see Opto & Laser Europe January p24) provide a more straightforward way to deliver the required optical power down the well. Parker says that he is keen to look into this technology.

The recent demonstration of a holey fibre that can efficiently deliver light from a carbon dioxide source also offers some promise. Researchers at the Massachusetts Institute of Technology, US, made hollow optical fibre that transmitted 10.6 µm laser light across tens of metres. Yoel Fink and colleagues measured a loss of 1 dB/m when they fired a 25 kW emission down the fibre. However, this figure would need to improve markedly to be of any use in deep drilling.