Thermomechanical Fracture Evolution in Granite Subjected to Thermal Shock from Liquid Nitrogen after High-Temperature Heating

Abstract The application of liquid nitrogen ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="upper L upper N 2" display="inline" overflow="scroll"> <mml:msub> <mml:mi>LN</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:math> ) fracturing offers a promising thermofluid approach for improving the permeability characteristics of hot dry rock (HDR) reservoirs during geothermal resource development. Unlike conventional hydraulic fluids, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="upper L upper N 2" display="inline" overflow="scroll"> <mml:msub> <mml:mi>LN</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:math> rapidly vaporizes at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="negative 196 degree normal upper C" display="inline" overflow="scroll"> <mml:mo form="prefix">−</mml:mo> <mml:mn>196</mml:mn> <mml:mo>°</mml:mo> <mml:mi mathvariant="normal">C</mml:mi> </mml:math> , inducing steep thermal gradients, intense thermal stresses, and phase transition effects that strongly interact with the rock matrix. Uniaxial compression coupled with Brazilian splitting characterization was executed within this research, utilizing granite specimens following sequential exposure to elevated thermal conditions and subsequent <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="upper L upper N 2 hyphen i n d u c e d" display="inline" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>LN</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mtext>-</mml:mtext> <mml:mi>induced</mml:mi> </mml:math> cryogenic treatment. Granite specimens were subjected to heating at varying temperature levels, followed by exposure to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="upper L upper N 2" display="inline" overflow="scroll"> <mml:msub> <mml:mi>LN</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:math> , with their responses evaluated using uniaxial compression and Brazilian splitting tests, ultrasonic velocity measurements, acoustic emission (AE) monitoring, and surface profilometry. Notably, under the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="400 degree normal upper C plus upper L upper N 2" display="inline" overflow="scroll"> <mml:mn>400</mml:mn> <mml:mo>°</mml:mo> <mml:mi mathvariant="normal">C</mml:mi> <mml:mo>+</mml:mo> <mml:msub> <mml:mrow> <mml:mi>LN</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:math> condition, the reductions in mass and density ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="normal upper Delta upper M" display="inline" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mi>M</mml:mi> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="normal upper Delta rho" display="inline" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mi>ρ</mml:mi> </mml:math> ) increased by 21.5 and 17 times, respectively, compared to the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="25 degree normal upper C plus upper L upper N 2" display="inline" overflow="scroll"> <mml:mn>25</mml:mn> <mml:mo>°</mml:mo> <mml:mi mathvariant="normal">C</mml:mi> <mml:mo>+</mml:mo> <mml:msub> <mml:mrow> <mml:mi>LN</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:math> condition. Similarly, the reduction in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="upper P hyphen w a v e" display="inline" overflow="scroll"> <mml:mi>P</mml:mi> <mml:mtext>-</mml:mtext> <mml:mi>wave</mml:mi> </mml:math> velocity ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="normal upper Delta upper V Subscript p" display="inline" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:msub> <mml:mi>V</mml:mi> <mml:mi>p</mml:mi> </mml:msub> </mml:math> ) was amplified by a factor of 10.5. Maximum declines in uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS) reached 30.58% and 50.3%, respectively. The observed stress thresholds indicated that elevated temperature gradients induced substantial thermal stress, resulting in initial microstructural damage. The combination of higher thermal stress and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="upper L upper N 2" display="inline" overflow="scroll"> <mml:msub> <mml:mi>LN</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:math> cooling accelerated microcrack development, as evidenced by distinct acoustic emission patterns. Under the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="400 degree normal upper C plus upper L upper N 2" display="inline" overflow="scroll"> <mml:mn>400</mml:mn> <mml:mo>°</mml:mo> <mml:mi mathvariant="normal">C</mml:mi> <mml:mo>+</mml:mo> <mml:msub> <mml:mrow> <mml:mi>LN</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:math> condition, the quiet phase of acoustic emission ringing counts was shortened by 54.3%, and the onset of the rapid decline phase of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="r hyphen value" display="inline" overflow="scroll"> <mml:mi>r</mml:mi> <mml:mtext>-</mml:mtext> <mml:mtext>value</mml:mtext> </mml:math> occurred 13 seconds earlier than under the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="25 degree normal upper C plus upper L upper N 2" display="inline" overflow="scroll"> <mml:mn>25</mml:mn> <mml:mo>°</mml:mo> <mml:mi mathvariant="normal">C</mml:mi> <mml:mo>+</mml:mo> <mml:msub> <mml:mrow> <mml:m