An Experimental Study on the Quantitative Mesoscopic Roughness Characteristics of Rock Using the Size Reduction Method

The development of emerging clean energy technologies, including CO2 geological sequestration, enhanced geothermal systems (EGS), and underground hydrogen storage, faces a critical scientific and engineering challenge: the accurate characterization of complex fracture morphology in subsurface rock formations. Accurate quantification of fracture morphology is a fundamental prerequisite for engineering design and numerical simulation. However, current methodologies for fracture characterization often fail to effectively distinguish between detailed features such as fracture orientation and surface roughness, leading to imprecise characterization results that directly impact the quality of engineering implementation. This study introduces an innovative theoretical framework─the size reduction method─which isolates detailed roughness from surface tortuosity. Additionally, a novel detailed roughness index, Ra-r, is proposed to complement this method. Utilizing this advanced approach, the research conducts a comprehensive comparison of surface roughness across various fracture types: different regions within a single fracture, natural versus hydraulic fractures within the same specimen, fractures of identical lithology, and fractures of differing lithologies. The findings reveal several critical insights: First, within a single fracture, the roughness at turning positions is significantly lower than that in positions with a unidirectional trend, approximately 31.9% of the latter. Second, hydraulic fractures in carbonatite specimens exhibit higher roughness compared to natural fractures. Third, the presence of quartz mineral fillings reduces the roughness of carbonatite hydraulic fractures to 69.4%. Lastly, a comparison between carbonatite and shale fractures indicates that the roughness of shale hydraulic fractures is about 50.6% of that of carbonatite, while the roughness of natural fractures in shale is approximately 51.4% of that of carbonatite. This research offers significant contributions to the theoretical understanding and practical applications of fracture flow dynamics, providing a robust foundation for future studies and industrial implementations in the field of resource extraction.