Surface Mechanical Properties of Acid-Etched Siliceous Limestone Fractures Under Deep, High-Temperature Conditions

Acid etching alters both the geometry and the near-surface mechanical properties of carbonate fracture surfaces, yet surface morphology alone may not reliably indicate mechanical weakening. This study investigated siliceous limestone through rock-slab acid-etching tests at 140 °C, three-dimensional laser scanning, and nanoindentation within calcite grains, comparing gelled acid, a premixed self-generating-acid/gelled-acid system, and integrated variable-viscosity (IVV) acid under different injection conditions. Surface roughness, fractal dimension, and directional tortuosity were evaluated alongside elastic modulus, hardness, and indentation-derived fracture toughness, and a comprehensive damage index was introduced to couple etching depth with mechanical-property loss. The results show that low macroscopic roughness can coexist with substantial local mechanical weakening: the comparatively smooth surface obtained with gelled acid at 20 mL/min still exhibited a reduction of nearly 90% in calcite-grain hardness. Among the morphological parameters examined, fractal dimension showed the strongest association with the measured mechanical parameters, most notably with hardness. These findings support joint morphological and micromechanical characterization of acid-etched fracture surfaces and caution against assessing acid-etching quality from surface roughness alone.