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.
The mechanism and efficiency of ablating concrete surfaces with a pulsed Nd:YAG laser were studied. Ablation efficiency and material removal rates were determined as functions of irradiance and pulse overlap. The ablation mechanism was dominated by fragmentation and disaggregation of the concrete. The ablation efficiency was insensitive to peak laser irradiance over a range from 0.2 to 4.4 MW/cm2. Excessive pulse overlap (>60%) caused a significant decrease in ablation efficiency by inducing melting. In concrete samples, the cement phase of the material responds in various ways to the laser energy, including disaggregation, melting, and vaporization, but the aggregate portion (sand and rock) mostly fragments. The ablation effluent therefore consists of both micron-size aerosol particles and chunks of fragmented aggregate material.
Hydraulic fracturing is considered necessary for economically efficient gas production in low natural permeability shale resources. Flow control devices such as setting balls or plugs are used for sleeve actuation or stimulation diversion during fracturing. After fracturing, these in-flowpath devices must be disposed of by methods such as drilling out or flowing back to open the flowpath for production. Traditional low-strength ball or plug materials are prone to shape changes. Severe deformations present flowback issues and potentially require costly intervention. Additionally, it is believed that smaller balls—particularly from the well's toe section—do not always flow back to surface, leading to potential restrictions in the tubing. Operations value the concept of fully degradable ball material. Traditional degradable material, though, lacks the high material strength necessary for high-pressure fracturing and has unreliable degradation rates. This paper will present a newly-developed nanostructured material technology called controlled electrolytic metallics (CEM), which makes high-strength and lightweight metallic composites possible. This new class of composites is completely corrodible in typical downhole environments at a predictable and controllable rate. CEM balls have passed multiple impact tests at speeds exceeding 100 mph. In 3% potassium chloride (KCl) at 200°F, these balls can completely corrode away in-situ in days. Material composition and associated processing can be changed to increase material strength by several times and the corrosion rate by several hundred times. CEM balls performed as designed in Bakken shale field testing. Material can be designed to rapidly corrode in 5 to 15% hydrogen chloride (HCl), giving a foolproof technology with operational flexibility. The fundamentals of innovative material design and processes, along with lab test and field application data of this material will be presented. CEM-based completion tools eliminate drilling out, guarantee an open flow path for each fractured zone, and enhance well productivity. This truly interventionless technology is also being researched for high-pressure, high-temperature (HP/HT) applications.
Laser ablation was investigated as a means of removing radioactive contaminants from the surface and near-surface regions of concrete from nuclear facilities. We present the results of ablation tests on cement and concrete samples using a pulsed Nd:YAG laser with fiber optic beam delivery. The laser–surface interaction was studied on model systems consisting of type I Portland cement with varying amounts of either fine silica or sand in an effort to understand the effect of substrate composition on ablation rates and mechanisms. The neat cement matrix melts and vaporizes when little or no sand or aggregate is present, and energy dispersive x-ray spectroscopy showed that some chemical segregation occurs in the effluent of ablated cement. The presence of sand and aggregate particles causes the material to fracture and disaggregate on ablation, with particles on the millimeter size scale leaving the surface.
V-Cr-Ti alloys are among the leading candidate materials for the frost wall and other structural materials applications in fusion power reactors because of several important advantages including inherently low irradiation-induced activity, good mechanical properties, good compatibility with lithium, high thermal conductivity and good resistance to irradiation-induced swelling and damage [1]. However, weldability of these alloys in general must be demonstrated, and laser welding, specifically, must be developed. Laser welding is considered to be an attractive process for construction of a reactor due to its high penetrating power and potential flexibility. This paper reports on a systematic study which was conducted to examine the use of a pulsed Nd:YAG laser to weld sheet materials of V-Cr-Ti alloys and to characterize the microstructural and mechanical properties of the resulting joints. Deep penetration and defect-free welds were achieved under an optimal combination of laser parameters including focal length of lens, pulse energy, pulse repetition rate, beam travel speed, and shielding gas arrangement. The key for defect-free welds was found to be the stabilization of the keyhole and providing an escape path for the gas trapped in the weld. An innovative method was developed to obtain deep penetration and oxygen contamination free welds. Oxygen and nitrogen uptake were reduced to levels only a few ppm higher than the base metal by design and development of an environmental control box. The effort directed at developing an acceptable postwelding heat treatment showed that five passes of a diffuse laser beam over the welded region softened the weld material, especially in the root region of the weld.
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