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A laser drilling research team was formed from members of academia, industry and national laboratory to explore the feasibility of using modern high-powered lasers to drill and complete oil and gas wells. The one-year Phase 1 study discussed in this report had the goals of quantifying the amount of pulsed infrared laser energy needed to spall and melt rock of varying lithologies and to investigate the possibility of accomplishing the same task in water under atmospheric conditions. Previous work by some members of this team determined that continuous wave lasers of varying wavelengths have more than enough power to cut, melt and vaporize rock. Samples of sandstone, limestone, and shale were prepared for laser beam interaction with a 1.6 kW pulsed Nd:YAG laser beam to determine 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 the purposes of spallation, melting and vaporization. The purpose of the laser rock interaction experiment was to determine the threshold parameters required to remove a maximum rock volume from the samples while minimizing energy input. 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 (SE) 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 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 can rise to the melting points of the minerals and quickly increase observed SE values. The lowest SE values are obtained in the spalling zone just prior to the onset of mineral melt. The current study determined that using pulsed lasers could accomplish removing material from rock more efficiently than continuous wave lasers. The study also determined that reducing the effect of secondary energy absorbing mechanisms resulted in lower energy requirements in shale and, to some extent, in sandstones. These secondary mechanisms are defined as physical processes that divert beam energy from directly removing rock, and may include thermally-induced phase behavior changes of rock minerals (i.e., melting, vaporization, and dissociation) and fractures created by thermal expansion. Limestone is spalled by a different mechanism and does not seem to be as affected by secondary mechanisms. It was also shown that the efficiency of the cutting mechanism improved by saturating porous rock samples with water, and that a laser beam injected directly through a water layer at a sandstone sample was able to spall and melt the sample. |
