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Additional Laser Research Due

Last fall Brian Gahan and his co-authors reported on the new study involving lasers and drilling at the Society of Petroleum Engineers annual meeting.

This current study is using a six kilowatt carbon dioxide gas-type laser capable of both continuous wave and super-pulsed beams, and a 1.6 kilowatt neodymium yttrium aluminum garnet (Nd:YAG) solid-state laser capable of a wide range of pulse widths and repetition rates.

To date the researchers have studied the effects of the various Nd:YAG laser parameters on the specific energy for samples of shale, limestone and sandstone. Some of the major observations include:

• Measured specific energy increases very quickly with the beam exposure time, indicating the effects of energy consuming secondary processes.
• Shale samples recorded the lowest specific energy values compared to limestone and sandstone.
• As both pulse repetition rate and pulse width increase the specific energy decreases -- but pulse width is a more dominant mechanism for reducing the specific energy than the pulse repetition rate.
• Two rock removal zones, spallation and melting, were identified in the shale sample data, with the least required specific energy occurring at the point prior to melting.
• Each rock type has a set of optimal laser parameters to minimize specific energy as observed in the linear track tests.
• Rates of heat diffusion in rocks are easily and quickly 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.

Researchers believe additional work should be done to consider the amount of energy absorbed by secondary processes. Also identification of spallation/melting interfaces should be identified in sandstone and limestone samples to determine their respective minimum specific energy values. And a follow-up analysis should be conducted to determine the effects of the laser rock interaction on permeability.

Gahan said he hopes to get the petroleum industry involved in the research.

"Adding the expertise from oil companies to this work would be a huge benefit," he said. "I would love to see a consortium type project come together where we can develop prototypes in parallel with the scientific research to move the concept out of the laboratory and into the field."

-- KATHY SHIRLEY