Thermal effect on the geo-engineering characteristics of a rock salt

by Nazlı Tunar Özcan

Rock salt caverns are considered one of the best hosts to store oil, natural gas, radioactive and toxic wastes due to their low permeability, self-healing characteristics and wide distribution on the Earth. Stored nuclear waste in rock salts will radiate for many years. Therefore, the thermal energy and also temperature in the host environment will increase depending on time. In this study, P-wave velocity (V_p), Brazilian tensile strength (σ_t), uniaxial compression strength (σ_c) of Çankırı rock salt were investigated under different temperatures ranging from 20°C to 250°C since the temperature is a factor that causes changes in some physical and geo-mechanical properties of rocks. The acoustic emission technique was utilized during uniaxial compression strength tests, to monitor the crack accumulation. Additionally, X-ray micro-computed tomography technique was employed to observe the microstructure and determine the porosity of rock salt samples depending on the temperature. The V_p and the σ_t of Çankırı rock salt decrease with increasing temperatures of samples whereas the σ_c increases. The ductility of rock salt tends to increase with augmented temperature and the axial strain at the ultimate stress level is 2.96% at 20°C whereas it reaches up to 6.29% at 250°C. The AE activity of rock salt generates at the early stages of loading and AE count prominently increases with the increasing temperature of samples. Therefore, the stress levels of crack initiation (σ_i) and crack damage (σ_cd) thresholds were reached earlier than the previous one with each temperature increment. According to X-ray micro-CT images of rock salts, the number of cracks increased markedly in thermally treated rock salt samples and therewith the porosity increases from 1.12% to 2.73% with an increase in temperature from 50°C to 250°C.