Analysis of Thermally Altered Rock Properties Using High-Power Laser Technology (2005)

Abstract Lasers have potential for rock destruction applications in mining, petroleum, tunneling, and trenching. The high temperatures induced using high power lasers cause an increase in porosity and permeability, while reducing rock strength making it easier to break-up rocks. These results are part of a more extensive research project studying the application of laser technology in the oil and gas industry. Pre- and post-lasing analysis included mineralogy determination, clay characterization and original fractures, as well as porosity, permeability, elastic moduli and strength. The characteristics of unlased rocks were compared to the characteristics of lased rocks in order to determine the effect the high temperature caused by lasers has on altering the rocks. Over 300 core samples where exposed to laser energy in laboratory settings. Rocks types lased include Berea sandstone, shaley sandstone, limestone, concrete, granite, and salt. Permeability and porosity of the reservoir rocks ranged from 0.02 to 6000 md and 2% to 23%, respectively. It was found that high temperature caused by lasers enhance porosity and permeability because high temperature vaporizes or breaks cementation leading to more pore space. On the other hand, the strength of the rock was reduced as a function of temperature. The changes are related to the thermal properties of the rocks. For example, sandstones that have a comparatively high thermal conductivity have a wide range of temperature distribution and therefore higher permeability distribution. Introduction This paper discusses the effect of high-power lasers (greater than 1 kW power) on porosity, permeability and rock strength. This study is part of more extensive research into applying laser technology to drill and complete oil and gas wells. Basic research of high power laser-rock-fluid interaction determined certain lasers have the required power, portability, reliability, durability, safety and minimal environmental impact for economically drilling and completing oil and natural gas wells.1 Sandstone, limestone and shale were lased varying the laser parameters. In order to determine the effect of high-power lasers on rock porosity, permeability, and strength, analysis was conducted on the rock core samples before lasing as well as after lasing. When possible, more than one analytical method was used to measure the desired rock property. Laser/rock interactions are highly complicated and numerous variables control the rate and path of the reaction and reaction by-products. Laser parameters that control the laser/rock interaction are: laser type; laser power; and lasing mode. 2 However, the physical and chemical properties of therocks also play a major role in determining the nature of the laser/rock interaction. In petroleum engineering applications, certain rock properties are critical for determining flow rate, storage capacity, and overall effectiveness of enhanced oil recovery methods. This paper will focus on the rock property alteration. The magnitude of change can be related to the laser parameters, but relative change is always the same. Porosity Measurement Methods The porosity of the samples was obtained using three different methods: petrographic thin section and applying fractal fragmentation theory, compressional and shear wave velocities using acoustic profiles, and CT scanning technology.