Abstract
Standard techniques for computed tomography imaging are not directly applicable to a carbonate rock because of the geometric complexity of its pore space. In this study, we first characterized the pore structure in Majella limestone with 30 per cent porosity. Microtomography data acquired on this rock was partitioned into three distinct domains: macropores, solid grains, and an intermediate domain made up of voxels of solid embedded with micropores below the resolution. A morphological analysis of the microtomography images shows that in Majella limestone both the solid and intermediate domains are interconnected in a manner similar to that reported previously in a less porous limestone. We however show that the macroporosity in Majella limestone is fundamentally different, in that it has a percolative backbone which may contribute significantly to its permeability. We then applied for the first time 3-D-volumetric digital image correlation (DIC) to characterize the mode of mechanical failure in this limestone. Samples were triaxially deformed over a wide range of confining pressures. Tomography imaging was performed on these samples before and after deformation. Inelastic compaction was observed at all tested pressures associated with both brittle and ductile behaviors. Our DIC analysis reveals the structure of compacting shear bands in Majella limestone deformed in the transitional regime. It also indicates an increase of geometric complexity with increasing confinement-from a planar shear band, to a curvilinear band, and ultimately to a diffuse multiplicity of bands, before shear localization is inhibited as the failure mode completes the transition to delocalized cataclastic flow. (Less)
Highlights
Recent advances in rock mechanics research in this area was reviewed by Wong & Baud (2012), who emphasized the importance of integrating data on mechanical deformation and failure mode with systematic microstructural observations of both the pre-existing pore structure and damage development to gain insights into the operative mechanisms
Our study shows that macroporosity in Majella limestone has a percolative backbone (Fig. 6b) that may contribute to significant enhancement of its permeability
As for mechanical deformation, recent studies concluded that macropores and micropores have fundamentally different influences on the elastic properties (Baechle et al 2008; Knackstedt et al 2009) and compaction (Ji et al 2012; Vajdova et al 2012) of porous limestone
Summary
The transition of failure mode in porous rock from brittle faulting to distributed cataclastic flow is a topic of importance in many geological applications, including the mechanics of faulting and deformation band formation (Aydin & Johnson 1978; Shipton & Cowie 2001; Aydin et al 2006), tectonic evolution and fluid flow in sedimentary formations (Antonellini et al 1994; Casey & Butler 2004; Eichhubl et al 2004, 2010; Sheldon et al 2006), as well as reservoir compaction and subsidence (Bouteca et al 1996; Fisher et al 1999). Recent advances in rock mechanics research in this area was reviewed by Wong & Baud (2012), who emphasized the importance of integrating data on mechanical deformation and failure mode with systematic microstructural observations of both the pre-existing pore structure and damage development to gain insights into the operative mechanisms. Pore space in rock is characterized using optical and scanning electron microscopes (SEM). Advances in 3-D imaging techniques such as X-ray computed tomography (CT) and laser scanning confocal microscopy (Fredrich et al 1995) have furnished enhanced perspective on pore geometry complexity. X-ray CT has been used widely for characterizing porous clastic rocks such as sandstone, whose void space is dominated by relatively equant pores connected by throats that are sufficiently large for direct imaging
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