Application of advanced high power laser technology to oil and gas well drilling has been attracting significant research interests recently among research institutes, petroleum industries, and universities. Potential laser or laser-aided oil and gas well drilling has many advantages over the conventional rotary drilling, such as high penetration rate, reduction or elimination of tripping, casing, and bit costs, and enhanced well control, perforating and side-tracking capabilities. The energy required to remove a unit volume of rock, namely the specific energy (SE), is a critical rock property data that can be used to determine both the technical and economic feasibility of laser oil and gas well drilling. When a high power laser beam is applied on a rock, it can remove the rock by thermal spallation, melting, or vaporization depending on the applied laser energy and the way the energy is applied. The most efficient rock removal mechanism would be the one that requires the minimum energy to remove a unit volume of rock. Samples of sandstone, shale, and limestone were prepared for laser beam interaction with a 1.6 kW pulsed Nd:yttrium–aluminum–garnet laser beam to determine how the beam 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 optimal 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. 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. Tests also clearly identified the spallation and melting zones for shale samples while changing the laser power. The lowest SE values are obtained in the spalling zone just prior to the onset of mineral melt. The laser thermally spalled and saw mechanically cut rocks show similarity of surface microstructure. The study also found that increasing beam repetition rate within the same material removal mechanism would increase the material removal rate, which is believed due to an increase of maximum temperature, thermal cycling frequency, and intensity of laser-driven shock wave within the 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 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.

Specific energy is often used as a measure of a drilling methods performance. Specific energy as defined in this paper is the amount of energy required to remove a given volume of rock (energy/volume). This is a normalization method used to compare the efficiency of one rock destruction technique to another. This paper compares specific energy between drilling with high power lasers and other drilling techniques such as traditional rotary and other novel methods such as water jets. Specific energy calculations made from laboratory measurements taken using four high power lasers, are compared to specific energy calculations reported in the literature. Although many other rock types were lased in this research, all comparisons are made on Berea sandstone, as it is the most commonly tested rock. A discussion of how the sample type, shape of test sample, and experimental conditions effect the calculations and results is included. It was found that there could be, and probably is, confusion caused by the way that specific energy is defined and calculated. The definition of specific energy, as used in this paper, is often compared to specific kerfing energy that is defined as power per kerf depth multiplied by the speed the cutting mechanism is moving across a rock surface. It was also found, by reviewing published work, that there have been many comparisons made in the literature that did not take into consideration such things as rock type and shape. Experimental conditions such as the atmosphere where the "cutting" tool and the rock interact have not been accounted for when many of the previous comparisons were made. Sample size in relationship to power density has also not been accounted for. Introduction In 1997, a research project funded by the Gas Research Institute (now Gas Technology Institute) revitalized the interest in revolutionizing well drilling using lasers1,2,3. This research demonstrated the feasibility of laser/rock destruction using three U.S. military lasers; MIRCAL, COIL, and CO2 and two Russian lasers; CO and CO2. Tests were conducted on 11 different rock types under varying conditions of sample size and shape, saturation, stress, purge gas, lasing time, pulsing, etc. Details of the study are explained in the above references. The second phase of the research used what was learned in the first phase and focused on three rock types; Berea sandstones, limestone, and shale, two saturations; air and water and one laser; the Nd:YAG which is located at the Argonne National Laboratory in Argonne, IL. The project was funded by the U.S. Department of Energy (DOE), GTI, PDVSA, and Halliburton. Preliminary work was also done using a Direct Diode Laser located at Native American Technologies in Golden, CO. During the many technical presentations made over the last five years, many questions regarding topics such as economics, efficiency, beam deliverability, environmental issues, and comparison to other novel drilling methods, as well as traditional rotary methods, arose. These questions have been or are currently being addressed in on-going projects. This paper gives some answers regarding the comparisons to other methods using specific energy as the benchmark. The confusion and varying ways to calculate specific energy are demonstrated and "true" comparisons are made.

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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractConventional wellbore perforation techniques are designed to establish flow from the hydrocarbon-bearing reservoir through the cemented casing into the wellbore. The explosive force of shaped charges is focused and intensified into a smalldiameter jet that penetrates the casing and cement into the reservoir rock. This process reduces reservoir rock porosity and permeability as metal and carbon debris are forced into the perforation tunnel, while very fine grain particles plug or reduce the pore throat size. As a result, it is necessary to perform time-consuming and costly post-perforation operations to minimize flow restrictions into the wellbore. Developing alternative perforation methods that reduce or eliminate formation damage could significantly boost production rates, cumulative production and overall economic returns.The research team led by Gas Technology Institute (GTI) has demonstrated, through the application of high-energy lasers to rock samples, that damage to permeability and porosity of the adjacent zones cannot only be reduced, but that near-hole permeability in a reservoir sandstone can be increased up to 171%. By applying this technique downhole, perforations and other directionally controlled completion and stimulation methods could be employed without damaging the reservoir. This paper presents the experimental methods and results of exposing sandstone, limestone, and shale rock samples to high-power laser beams. A grid pattern was applied to the rock samples from which acoustic velocity and permeability measurements in and near the perforated tunnel where taken. In addition, rock mineralogy and rock properties were analyzed before and after the test.