Investigation of Nanomechanical Properties of Heat-Treated Material Using Atomic Force Microscopy (AFM)

ABSTRACT: Enhanced geothermal systems (EGS) technology holds considerable promise for significantly increasing the recoverable geothermal energy capacity in the United States. However, developing geothermal resources within subsurface fault zones, characterized by thin layers of gouge material, introduces complex geomechanical interactions that can markedly increase the risk of induced seismicity. We employ novel atomic force microscopy (AFM) techniques to investigate the morphological and structural changes of coal and granite samples heat treated at 50, 75, and 100 °C. A probe is used to raster scan the surfaces of gouge samples while monitoring the interactions between the probe and the sample. Our results reveal a decline in coal's modulus with increasing temperature, from 5.2 GPa to 3.3 GPa, potentially due to microcracking and softening of its matrix. In contrast, granite exhibits an increase in modulus, from approximately 10 to 65 GPa, which may be attributed to the closure of micro-voids resulting from thermal expansion under increasing temperature conditions. These findings demonstrate that while differential expansion of mineral phases might have led to microcracking and softening in coal, granite may have experienced enhanced grain-to-grain contact. In addition, this study demonstrates the applicability of AFM as a tool for examining the nanomechanical properties of geomaterials, offering invaluable insights into the mechanisms of induced seismicity and helping to refine fault friction and sliding models for improved risk mitigation.