ABSTRACT:
Despite extensive research on permeability characterization of rocks, the micro-mechanisms controlling the permeability evolution during triaxial loading is poorly understood. This is mainly related to inability to characterize the processes that change the porosity and therefore flow paths. This particularly pertains to permeability evolution due to formation of compaction bands. The formation of compaction bands and its associated permeability loss has been reported in both laboratory experiments and field observations. However, the micro-scale mechanisms deriving the macro-scale permanent deformation are not yet fully understood nor systematically investigated. As a result, the permeability loss is often correlated to the macro-scale observations without micro-scale. In this study, triaxial compression experiments were conducted on high porosity (up to 50%) limestone samples in ductile region. The permeability of the sample was continually measured throughout the experiment and the sample was scanned at different displacement stages using a micro-CT scanner to map the internal 3D structure of the sample. The Digital Image Correlation (DIC) technique was performed on filtered, transformed, and segmented 3D images to investigate the strain field and identify the evolution of compaction bands with stresses. The pore connectivity and the change in permeability due to overall compaction were also simulated using pore network modelling (PNM) and the micro-macro scale physical links were established.
1. INTRODUCTION
Compaction bands are localized planar deformation features that developed orthogonal to the maximum principal stress (Mollema and Antonellini 1996, Issen and Rudnicki 2000, 2001). These geological features were first reported in detail in Jurassic Aztec Sandstone of south-eastern Nevada by Hill (1993) and later documented by others (Sternlof et al. 2005, Eichhubl et al. 2010). The previous studies suggest that the compaction bands may be a common feature in high porosity materials. These localized features have been verified by both laboratory and field investigations. Compaction bands observed in laboratory are typically generated in high porosity rocks using triaxial compression test. Their micro-mechanism is attributed to local grain crushing and pore collapse causing permeability reduction. The laboratory observation showed that the compaction bands are formed in ductile region and may grow by thickening or extension of multiple compaction bands. In addition, the compaction bands were seen in nature (Holcomb et al. 2007, Rustichelli et al. 2012).
