Laser Drilling: Determination of Energy Required to Remove Rock

Abstract

Abstract 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 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 rise to the mineral's melting points and quickly increase SE values. The lowest specific energy values are obtained in the spalling zone just prior to the onset of melting. Introduction The oil and gas industry introduced a radical change at the turn of the twentieth century, displacing cable tool drilling with rotary drilling. Since then, great strides have been made in refining the rotary technique, however no fundamental revolutionary changes have since been introduced. In 1997, Gas Technology Institute (GTI) initiated a two-year study exploring the feasibility of adapting high-powered military lasers for a revolutionary application in oil and gas exploration and production. The concept included the exposure of different rock types to the action of powerful lasers with the purpose of determining possible applications in drilling and perforating oil and gas wells. The application of laser technology to drilling has its detractors. The skepticism is based mostly on limited laboratory tests conducted and theories formulated more than 25 years ago. Since then, significant advances have been made in laser power generation, efficiencies and transmission capabilities. Results from GTI's initial investigation determined that these calculations significantly overestimated the energy required to spall, melt or vaporize rock. Initial laser drilling experiments used the U.S. Army's Mid-Infrared Advanced Chemical Laser (MIRACL) and the U.S. Air Force's Chemical Oxygen-Iodine Laser (COIL) laser systems. Both systems operated in the infrared optical region with power delivery capacities of 1 MW and 10 KW, respectively. Both of these lasers delivered only continuous wave (CW) beams, although the COIL is now capable of pulsed beam delivery. Cores of sandstone, limestone, shale granite, salt, and concrete were tested. Fast penetration speeds were obtained as well as some fundamental changes in the properties of the samples. For example, the porosity surrounding the lased hole in a Berea sandstone sample actually increased. Also, the experiments indicated that at such high powers, there were deleterious secondary effects that increased as hole depth increased. These effects included 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. The GTI study showed clearly that current laser technology is more than sufficient to break, melt or vaporize any lithology that may be encountered in the subsurface. It also showed that the energy required to accomplish these varies as much within lithologies as between them. However, no quantitative results as to minimum power required or determination of factors that control power requirements were obtained. It became clear from these experiments, for example, that there is a need to control the amount of material melted during the laser exposure, as well as to determine quantitatively the minimum laser power needed to drill rocks for oil and gas applications.