Experimental study on evaluation of porosity, thermal conductivity, UCS, and elastic modulus of granite after thermal and chemical treatments by using P-wave velocity

Abstract

Hot dry rocks (HDRs) are subjected to thermal and chemical stimulation caused by water cooling and chemical actions during reservoir reconstruction. The damage caused by stimulation will change the physical and mechanical characteristics of HDRs, which brings potential risks to the operating life and safety of artificial geothermal systems. The material properties of granite specimens treated by heating and long-term static chemical immersion are studied by ultrasonic tests (UT), nuclear magnetic resonance (NMR), thermal conductivity tests, and uniaxial compression tests. Five temperature levels (from 25 to 600 °C) and four chemical immersion conditions (heat treatment only, distilled water immersion, acid stimulation, and alkali stimulation) are considered. The findings demonstrate that porosity rises with the heat treatment temperature, whereas P-wave velocity, thermal conductivity, uniaxial compressive strength (UCS), and elastic modulus fall. Acid stimulation can improve the porosity of heat-treated granite while decreasing P-wave velocity and thermal conductivity. Alkali stimulation easily forms precipitates or colloidal materials to fill the microcracks and micropores inside the heat-treated granite, resulting in reduced porosity and increased P-wave velocity and thermal conductivity. The mechanical properties of the granite that has undergone heat treatment will be further weakened by acid or alkali stimulation. Regression analysis shows an excellent nonlinear logarithmic relation between P-wave velocity and other measured properties. In addition, the damage characteristics and micro-mechanism of the treated granite are discussed by mineral composition analysis and scanning electron microscope (SEM) observation.