Experimental and simulation study of cryogenic stimulation of granites from Akmola region in Kazakhstan

The purpose of this research is to investigate the effects of elevated heating and cooling with liquid nitrogen (LN₂) on the mechanical properties of two different granites from the Akmola region in Kazakhstan. While trends regarding behavior of treated samples are similar, the differences observed between the two compositions of granite are asserted to be due to the difference in quartz versus feldspar ratios and the contrast in coefficients of thermal expansion at heterogeneous grain contacts. The results showed that high initial temperature along with LN₂ cooling increased the deformation of specimens leading to smaller peak stress values under load in both granite specimens. For granite 1, stress values decreased from a baseline value of 158.13 MPa to 123.24 MPa after heating to 400 °C and immersion in LN₂; while for granite 2, the stress values decreased from 181.42 MPa to 118.33 MPa under similar conditions. There was a positive correlation observed between Young's modulus and the degree of thermal shock, whereas a negative correlation was noted for Poisson's ratio. The failure mode classification in Brazilian tests progressed with increasing initial sample initial temperature from Type I to Type III, indicating increasing complexity in rock integrity with larger thermal shock. Adding to the study's novelty, numerical simulations were performed to investigate the distribution of stress in granite samples that were cryogenically treated utilizing von Mises stress analysis to monitor changes over time. The close match between the simulation results and the experimental data confirmed the model's reliability. These findings offer strong evidence in favor of the feasibility of increasing granite permeability by LN₂-based treatments, which may improve the effectiveness and security of geothermal extraction techniques.