Experimental study of thermal fracturing of Hot Dry Rock irradiated by moving laser beam: Temperature, efficiency and porosity

Abstract

A new laser irradiation fracturing method is employed to crack the Hot Dry Rock (HDR) and variations of rock temperature, specific energy (SE) and modified specific energy (MSE), thermal damages and open porosity of granite samples caused by moving laser beams with various irradiating conditions including laser power, diameter and moving speed of laser beam were investigated. Results indicate that rock temperature and the corresponding temperature gradients near the laser beam spots are strongly dependent on the laser power, beam diameter and irradiation time. The high temperature generated by the laser irradiation melts and cracks the HDR samples. The removed mass, cracked mass and size of grooving kerf induced by laser irradiation are also related to various irradiation conditions. SE and MSE are found nonlinearly reduced with the increased laser power density. Laser irradiation has a greater enhancement to thermal fracturing of granite than it does to thermal drilling. The open porosity (OP) of irradiated HDR samples increases with increasing laser power, decreasing diameter and moving speed of laser beam. The results can provide some guidance to those seeking a new economical and reasonable fracturing method for the HDR geothermal exploitation.

Experimental Investigation on Compressive Strength, Ultrasonic Characteristic and Cracks Distribution of Granite Rock Irradiated by a Moving Laser Beam

“…The uniaxial compressive strength of granite ranged between 120 and 140 MPa, with the main minerals of quartz and albite. The detailed technical data were presented in our previous paper [33].…”

“…In addition, hydraulic fracturing also has some deficiencies, including high-initiation pressure, pore plugging in the water-sensitive stratum and groundwatercontamination risk [18]. In order to avoid these problems caused by hydraulic fracturing, a series of waterless fracturing measures were developed by researchers to crack the hard rock, such as gas fracturing [19,20], liquid-nitrogen fracturing [21][22][23], thermal spallation fracturing [24,25], microwave fracturing [26][27][28][29] and laser fracturing [30][31][32][33][34].…”

“…They found that the ultimate tensile stress achieved by laser irradiation ranged from 481 to 536 MPa, which was much greater than that of the granite specimen. Wang et al [33,34] experimentally investigated the temperature change in and mechanical properties of rock specimens generated by laser irradiation. They found that the temperature gradient of irradiated material can reach 5000 • C/mm, which led to a significant reduction in the compressive strength.…”

The situation analysis of hot dry rock geothermal energy development in China-based on structural equation modeling

“…The roughness of rock will affect the stability of rock. Some scholars [ 25 ] Hot and dry rocks may occupy a large amount of land and even destroy forests in the process of development. In addition, groundwater is polluted during heat exchange between water and rocks [ 26 ].…”

The fragmentation mechanism of granite in electrical breakdown process of plasma channel drilling

“…While increasing the density of crown cutter can effectively improve the drilling efficiency of PDC bit in HDR (Zhu and Dan 2019). Furthermore, gas drilling (Zhukov et al 2018), laser drilling (Wang et al 2020), thermal-mechanical combined drilling (Rossi et al 2020) and other novel technologies that aim at HDR formations are constantly appearing. However, although these novel technologies have made some breakthroughs, most of them still regard the hard interaction between the bit and rock as the main and basic driving force for fragmentation, and others are used as supplements.…”