An experimental investigation of hydraulic fracturing mechanisms in menderes metamorphic rocks: Prospects for enhanced geothermal systems

Hydraulic fracturing is a critical technique for stimulating low-permeability geothermal reservoirs, such as Enhanced Geothermal Systems (EGS) and Hot Dry Rock (HDR). This study investigates the hydraulic fracturing mechanisms in metamorphic rocks such as marble and schist commonly found in western Turkiye, a region with a gross geothermal power capacity exceeding 1,700 MW. For the first time, fracturing experiments using pressurized nitrogen gas and uniaxial compression tests were conducted on both reservoir outcrop and core plug samples of metamorphic reservoir rocks from this region. Vertical and horizontal fractures formed at varying breakout pressures, with the fracture surfaces exhibiting irregular morphologies and apertures facilitating fluid flow. Uniaxial compression tests determined the mechanical properties, including Young's modulus and Poisson's ratio. Vertical crack propagation was observed in outcrop samples, while diagonal fractures appeared in core samples. Mineralogical analysis through thin-section, Scanning Electron Microscopy - Energy Dispersive Spectroscopy (SEM-EDS), and X-Ray-Diffraction (XRD) revealed that the core sample was grayish-green mica schist with muscovite, quartz, garnet, and staurolite, while the outcrop sample was white marble composed primarily of calcite. Results indicate that marble and schist exhibit lower breakdown pressures compared to granite, making them promising candidates for EGS development. This study represents a novel contribution to understanding the interplay between rock type, fracture mechanisms, and experimental findings in Turkiye's metamorphic geothermal systems.