Abstract
Liquid nitrogen (LN2), which can greatly improve the efficiency of hot dry rock (HDR) mining, is commonly used as a cooling material in the enhanced geothermal system (EGS). Physical property, triaxial compression, and permeability tests were undertaken on treated granite samples, for a better scientific understanding of the effect of the LN2 cooling method on the mechanical and permeability properties of the rocks after heat treatment. The experimental results indicated that the physical properties of the treated granite change significantly, such as the density and wave velocity are substantially reduced. Meanwhile, with the increase of treatment temperature, the macroscopic cracks on its surface are gradually generated and the volume is expanded clearly. In addition, the surface wettability of granite gradually increases with increasing temperature. Compared with the air/water cooling methods, under LN2 cooling condition, the mechanical properties decrease markedly. When the temperature exceeds 600°C, the granite strength decreases significantly to only 56.16% of the reference value. The deformation properties also change significantly, with a final strain of about 3% at failure for a sample at 800°C, showing an obvious ductile deformation characteristic. Further, an appreciable correlation also exists between the initial permeability of granite and temperature. Once the temperature exceeds 200°C, the increase in temperature contributes to the increase in initial permeability. In addition to the effect of temperature, the increase in load also leads to a change in the permeability coefficient. When the temperature reaches 600°C, the permeability of granite first decreases and then increases with the increases in axial stress. The results of this paper are valuable in understanding the effect of thermal shock by LN2 on the fracturing efficiency and permeability characteristics of dry hot rocks.
Highlights
Development of hot dry rock (HDR) resources plays an important role in meeting the demand for energy [1,2,3,4,5]
Rapid thermal shock can greatly improve the efficiency of HDR fracturing, especially with the use of LN2
The influence of different thermal shocks on the physical, mechanical, and permeability properties of granite samples was systematically studied
Summary
Development of hot dry rock (HDR) resources plays an important role in meeting the demand for energy [1,2,3,4,5]. The geothermal energy is mainly stored in a hot granite reservoir, which is distributed at the depth of 2–6 km with a temperature above 150°C [6]. Liquid nitrogen (LN2) cryogenic fracturing has been proven to be an efficient stimulation method, which has a great potential in establishing pathways with high permeability [9, 10]. During the process of cryogenic fracturing, the use of LN2 can produce a high thermal gradient and a rapid temperature change in the rock inner and outer body, leading to a greater thermal stress and more thermal cracks in HDRs compared with traditional water/air cooling [11]. Water was often used as a coolant in previous works, but water has the disadvantage that can even cause important reductions
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