Advancements in CT Three-Dimensional Reconstruction of Rock Samples

ABSTRACT: Understanding the internal discontinuities, joints, and fissures within rock samples is crucial across multiple engineering fields, including geotechnical engineering and energy exploration. Computed tomography (CT) technology, with its non-destructive and high-resolution capabilities, has become indispensable for studying rock internal structures, overcoming the limitations of traditional methods such as mercury intrusion porosimetry, scanning electron microscopy, and nuclear magnetic resonance. This paper reviews the recent progress in the application of CT for three-dimensional (3D) reconstruction in rock mechanics. The CT technology's operational principles, encompassing X-ray penetration, signal conversion by detectors, and computer-aided reconstruction, are outlined. The discussion focuses on critical steps like image preprocessing, threshold segmentation, and 3D reconstruction, underscoring the selection of appropriate segmentation thresholds for precision. The integration of CT with laboratory investigations of rock permeability and numerical simulations is also explored. Despite advancements, challenges persist in aligning sample size with scanning resolution, threshold determination, and the development of 3D reconstruction algorithms. Future research should consider applying CT in geological CO2 utilization and storage and merging CT with other analytical techniques to broaden its scope and enhance geological research methods. 1. INTRODUCTION The mechanical properties, damages and seepage characteristics of rocks are critical research areas in fields such as oil and gas extraction, geology, geotechnical engineering and coal mining. Currently, most studies on these characteristics are based on basic assumptions that the spatial distribution of rock structures and physical-mechanical properties is homogeneous, layer-wise homogeneous or artificially given as statistically non-homogeneous distributions(Yue, 2022; Wang et al.,2021). However, rocks are composed of different mineral particles bonded or embedded together, and the prolonged sedimentation and geological processes lead to numerous discontinuous and irregular joints, fractures, and sedimentary interfaces within the rocks. Research into the heterogeneous and discontinuous characteristics of rock mechanics must first obtain the heterogeneous and discontinuous structure inside the rocks. With the advancement and development of testing equipment, as well as the requirements for the precision and imaging methods of pore and fracture observations, techniques such as mercury intrusion porosimetry (MIP) (Turturro et al.,2022), scanning electron microscopy (SEM) (Ma et al.,2022), nuclear magnetic resonance (NMR) (Cai et al.,2018), and X-ray diffraction (XRD) (Zhang et al.,2023) have gradually been applied to pore characterization studies, as detailed in Table 1. MIP is a destructive experiment and tends to underestimate the content of large pores due to its limitation on the maximum pore radius it can measure (Tang et al.,2021; Chu et al.,2022; Zhu et al.,2016). The pressure measured reflects the diameter of the pore entrance rather than the pore itself. SEM can observe the mineral occurrence form, crystal form, surface morphology, and composition, but it requires pre-treatments like gold coating and sectioning, and the observation range is very small, limiting the structural analysis to a small sample area, making it difficult to provide detailed information on the porosity and connectivity of large mineral bodies (Dehestani et al.,2020;Zhang et al.,2014;Du et al.2019). NMR, as a non-destructive testing method, is widely used to characterize pore structures with high measurement accuracy, but it is challenging to reconstruct the 3D pore structure and to obtain further information on other minerals in coal-rock bodies (Luo et al.,2022;Zhao et al.,2019).