Approximately 40 different reservoir and surface rock samples were lased using high power COIL (1.315 micrometer), CO2 (wavelength = 10.6 micrometer) and Nd:YAG (wavelength = 1.06 micrometer) lasers. Spectrum of the samples (sandstones, shales, and limestones) in the wavelength region from 0.35 to 15.387 wavelength were obtained. Spectral signatures and optic coefficients of the reservoir and surface sandstones were discussed by the authors in a previous paper (SPIE 5273-97). In this contribution, a detailed study of the spectral properties and optic signatures of shale and limestone samples is presented. The optic coefficients (extinction/reflection (E), scattering (S), absorption (K) and emission (F)) of these rocks are mathematically and statistically calculated and are critically investigated against rock chemistry, grain size, porosity, cementing matrix and rock textures, and total organic content.
Our investigations show that: 1) Porosity and grain size are the only rock properties that exhibited a strong statistical relation with the absorption and reflection coefficients. 2) Rocks with high porosity have greater reflection coefficients (at the COIL and Nd:YAG wavelengths) compared to those having lower porosity. 3) The reflectance at the CO2 laser wavelength (10.6 wavelength) is not a function of porosity or grain size. 4) Surface and reservoir shales have almost the same spectral features and hence similar optic coefficients. This indicates that mode of occurrence does not influence the spectral signatures of rocks. 5) Spectrum of limestones is dominated by the four (v1 v2, v3, and v4) fundamental stretching carbonate absorption bands.
With interest growing in the earth engineering industries for practical laser applications, research in using lasers for rock destruction progresses every year. This research yields tremendous amounts of data that must be properly documented and analyzed. However, with nearly 20 laser parameters and rock properties to consider, the analysis requires powerful analytical tools as well as proper selection of influential parameters. In an effort to understand laser-rock interaction, this paper presents analysis of several years of data and yields new understanding of the results. Attempts are made to reduce the number of variables used in experiments by performing data analysis with commercial statistical software tools. By using these tools, trends and correlations of variables identified the significant variables that control rock removal by laser. The significant variables that govern the success of rock removal are: the time the rock is exposed to the laser energy and the laser power settings. Furture experiments will be tailored to optimize these variables. Likewise, the variables such as bulk density, which introduce little or no error in resulting calculations, can be eliminated from consideration and can save valuable testing time. From the analyses performed we found that rock density can be ignored in mineralogically simple rock, although further tests must be done on complex rocks in order substantiate its omission in such cases. The observations presented in the paper should aid in developing future rock destruction experiments.
Approximately 40 different reservoir and surface rock samples were lased using high power COIL (λ = 1.315 μm), CO2 (λ = 10.6 μm) and Nd:YAG (λ = 1.06 μm) lasers. Spectrum of the samples in the wavelength region from 0.35 to 15.387 μm was obtained. The objectives of this research are to make a detailed study of the spectral properties and optic signatures of rock samples, including reservoir rocks collected from a depth of more that 8,000 ft, in order to predict the energy absorbed when a laser hits a rock. The optical coefficients [extinction/reflection (E), scattering (S), absorption (K) and emission (F)] of these rocks are critically investigated against rock chemistry, grain size, mode of occurrence, porosity, cementing matrix and rock textures, and total organic content. This research, initiated for the petroleum industry, develops a relationship between reflectance and rock properties that are commonly known and used as correlation parameters for other reservoir characterization uses. Our results show that: (1) More than 25% of the COIL and Nd:YAG laser energy is reflected and/or scattered by rocks with more than 85% SiO2 content. (2) Surface and reservoir sandstones have almost the same spectral features and hence similar optic coefficients. (3) Rocks with high porosity have greater reflection coefficients (at the COIL and Nd:YAG wavelengths) compared to those having lower porosity. (4) The reflectance at the CO2 laser wavelength (10.6 μm) is not a function of porosity or grain size.
KEYWORDS: Computed tomography, Chemical analysis, Laser drilling, Analytical research, Thermal analysis, High power lasers, Scanning electron microscopy, Signal attenuation, Laser energy, Minerals
The focus of this paper will be on the analytical techniques used to study laser/rock destruction. High-power COIL, CO2, and Nd:YAG lasers were used to drill holes, melt, crack, and vaporize rock samples. Rock types used in the research include: surface sandstones, reservoir sandstones, surface shale, reservoir shale, surface limestone and dolomite, and granite. Physical and chemical properties of the unlased and lased rocks are analyzed. X-Ray Fluorescence (XRF) is used to determine rock chemistry while carbon content is determined by a coulometer. Permeability is determined by the Pressure-Decay Profile Permeameter (PDPK), and Computerized Tomography (CT) imaging is used to calculate the volume of material removed by laser and pixel porosity and density around the lased holes. Thermal properties, such as endothermic and exothermic reactions, clay disassociation, and melting temperatures, are determined using Simultaneous Thermal Analysis (STA). Scanning Electron Microscope with Energy Dispersive Spectrometer (SEM-EDS) is used to map laser-induced fractures and mineralogical transformations. Mineral assemblages, rock texture, and average porosity are determined using petrographic thin sections.
This paper describes the experimental results of selective rock removal using different types of high power lasers. US military owned continuous wave laser systems such as MIRACL and COIL with maximum powers of 1.2 MW and 10 kW and wavelengths of 3.8 and 1.3 mm respectively, were first used on a series of rock types to demonstrate their capabilities as a drilling tool for petroleum exploitation purposes. It was found that the power deposited by such lasers was enough to drill at speeds much faster than conventional drilling. In order to sample the response of the rocks to the laser action at shorter wavelengths, another set of rock samples was exposed to the interaction of the more commercially available high power pulsed Nd:YAG laser. To isolate the effects of the laser discharge properties on the rock removal efficiency, a versatile 1.6 kW Nd:YAG laser capable of providing pulses between 0.1 millisec and 10 millisec in width, with a maximum peak power of 32 kW and a variable repetition rate between 25 and 800 pulses/sec was chosen. With this choice of parameters, rock vaporization and melting were emphasized while at the same time minimizing the effects of plasma shielding. Measurements were performed on samples of sandstone, shale, and limestone. It was found that each rock type requires a specific set of laser parameters to minimize the average laser energy required to remove a unit volume of rock. It was also found that the melted material is significantly reduced in water saturated rocks while the drilling speed is still kept higher than conventional drilling.
Experiments on laser-rock-fluid interaction have been carried out by using pulsed CO and CO2 lasers which irradiated rocks typical for oil field: sandstone, limestone, shale and granite. Energy fluence and laser intensity on rock surface were up to 1.0 kJ/cm2 and 107W/cm2, respectively. The dependencies of specific energy consumption (i.e. energy per volume needed for rock excavation) on energy fluence, the number of pulses, saturated fluid, rock material and irradiation conditions have been obtained for various rock samples. The dependencies of momentum transferred to the rock on energy fluence for dry rocks and rocks with surface saturated by water or mineral oil have been measured. High-speed photography procedure has been used for analyzing laser plasma plume formation on a rock surface. Infrared spectra of reflectivity and absorption of rocks before and after irradiation have been measured.
Experiments on laser-rock-fluid interactions have been carried out by using pulsed CO and CO2 lasers which irradiated rocks typical for oil field: sandstone, limestone, shale and granite. Energy fluence and laser intensity on rock surface were up to 1.0 kJ/cm2 and 107 W/cm2, respectively. The dependencies of specific energy consumption (i.e. energy per volume needed for rock excavation) on energy fluence, the number of pulses, saturated fluid, rock material and irradiation conditions have been obtained for various rock samples. The dependencies of momentum transferred to the rock on energy fluence for dry rocks and rocks with surface saturated by water or mineral oil have been measured. High-speed photography procedure has been used for analyzing laser plasma plume formation on a rock surface. Infrared spectra of reflectivity and absorption of rocks before and after irradiation have been measured.
KEYWORDS: Laser drilling, Free electron lasers, High power lasers, Laser applications, Laser energy, Chemical oxygen iodine lasers, Laser safety, Chemical lasers, Continuous wave operation, Water
The results of the first year of a Gas Research Institute funded research program to study laser-rock-fluid interaction will be presented. The overall purpose of this research is to determine the feasibility, costs, benefits, and the environmental impact of using laser technology to drill and complete oil and gas wells. When drilling and completing petroleum wells, many rock types (sandstone, limestone, dolomite, granite, shale, salt, concrete) and fluids (fresh water, salt water, oil, hydrocarbon gas, drilling fluids) must be penetrated by the laser. The Free-Electron Laser (FEL) technology is attractive because of the ability to tune the laser to different wavelengths. Laser energy absorbed by rocks is related to the wavelength of the laser source. The mechanisms of rock destruction (spalling, melting and vaporization) are therefore a function of the wavelength. The ability to transmit laser energy over long distances (up to 5000 m or 15,000 ft) is also a function of wavelength. Results of tests conducted at the U.S. Air Force and the U.S. Army's high power laser facilities are presented. The challenges ahead to advance a fundamental change in the methods currently used to drill and complete petroleum wells are discussed.
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