Mechanical behavior, failure mode, and transport properties in a porous carbonate

Abstract

We performed a systematic investigation of mechanical compaction, strain localization, and permeability in Leitha limestone. This carbonate from the area of Vienna (Austria) occurs with a broad range of grain sizes and porosity, due to changes in depositional regime and degree of cementation. Our new mechanical data revealed a simple relation between porosity and mechanical strength in both the brittle and ductile regimes. Increasing cementation and decreasing porosity led to a significant increase of the rock strength in both regimes. Micromechanical modeling showed that the dominant micromechanisms of inelastic deformation in Leitha limestone are pore‐emanated microcracking in the brittle regime, and grain crushing and cataclastic pore collapse in the ductile regime. Microstructural analysis and X‐ray computed tomography revealed the development of compaction bands in some of the less cemented samples, while more cemented end‐members failed by cataclastic flow in the compactant regime. In contrast to mechanical strength, permeability of Leitha limestone was not significantly impacted by increasing cementation and decreasing porosity. Our microstructural and tomography data showed that this was essentially due to the existence of a backbone of connected large macropores in all our samples, which also explained the relatively high permeability (in the range of 2–5 darcies) of Leitha limestone in comparison to other carbonates with significant proportion of micropores.

Inelastic Compaction in High‐Porosity Limestone Monitored Using Acoustic Emissions

“…Taken together, the microstructural analysis of Baud et al () and our new microstructural observations (Figures ) show that grain crushing was the main micromechanism of inelastic compaction at the yield point in SML. A recent study made comparable observations in Leitha limestone of porosity between 21 and 31% and also identified grain crushing as the dominant micromechanism of inelastic compaction in the less cemented and macroporous samples (Baud et al, ). Their results were consistent with the Hertzian fracture model of Zhang et al ().…”

“…Our microstructural observations (Figure ) suggest that the diffuse bands could be made of several compaction band segments. SML is a weak, poorly cemented limestone and in this sense similar to the Leitha limestone in which a recent study observed compaction bands (Baud et al, ). Studying samples with different cement content, they observed that increasing cementation inhibited the development of compaction localization.…”

“… The mechanical strength of a quartz‐rich limestone such as SML was not influenced by the quartz content. As in other limestones studied by previous authors (Baud et al, ; Baud, Wong, & Zhu, ; Zhu et al, ), microstructural attributes such as initial porosity, grain size, and pore size exert a first‐order control on strength. We monitored the development of stress‐induced damage and strain localization using AE in SML when previous studies suggested that it was not possible in a porous limestone. Since SML stands out as an exception in this aspect, and because our quantitative microstructural analysis did not reveal extensive microcraking in quartz grains in this rock, we speculate that the recorded AE activity is likely to be due to microcraking at the interface of the quartz grains.…”

“…Carbonate rocks are widely recognized to have microstructure, and in particular pore structure, that is significantly more complex than siliciclastic rocks (Choquette & Pray, ; Folk, ; Lucia, ). Hence, pilot laboratory studies mostly selected relatively pure carbonates (i.e., close to 100 wt % calcite) and focused on somehow simpler end‐members such as micritic (Nicolas et al, ; Vajdova et al, ), allochemical (Dautriat et al, ; Vajdova et al, ), microporous (Regnet et al, ; Baud et al, ), or macroporous limestones (Baud et al, ). The question of the impact of the composition and in particular the presence of secondary minerals (such as quartz or dolomite) on the strength and rheology of high‐porosity limestone has so far remained understudied, although high‐pressure, high‐temperature torsion experiments on low‐porosity synthetic limestones have shown that the addition of dolomite (Delle Piane, Burlini, & Kunze, ; Kushnir et al, ) or quartz (Rybacki et al, ) serves to increase the strength of limestone.…”

Microstructural Control of Physical Properties During Deformation of Porous Limestone

“…In oolithic limestones, Regnet et al (2015) demonstrated that the distribution of microporosity Journal of Geophysical Research: Solid Earth 10.1029/2018JB015636 exerts a strong influence on the macroscale mechanical behavior. Recently, Baud, Exner, et al (2017) showed the degree of cementation could also be at the origin of large variability in mechanical strength and could promote or inhibit different localized failure modes during compaction of carbonates.…”