Thierry Reuschlé

Mechanical behaviour and failure mode of bentheim sandstone under triaxial compression

Abstract Hydrostatic and triaxial compression tests have been conducted on nominally dry samples of Bentheim sandstone, a homogeneous quartz-rich sandstone with porosity of about 23%. A broad range of confining pressures were used to observe the transition from the brittle faulting to cataclastic flow regime. Mechanical data for the brittle strength and compactive yield stress can be fitted with empirical envelopes that have been shown to be applicable to other porous sandstones. However, the Bentheim sandstone is somewhat unusual in that quasiductile failure (characterized by an overall hardening trend punctuated by episodic strain softening and compaction band formation) was observed over a wide range of confining pressures from 120 MPa to 300 MPa. Since this failure mode is similar to observations in honeycombed cellular solids, it is speculated that the prevalence of quasiductile failure in the Bentheim sandstone arises from its relatively homogeneous mineralogy and grain size. Compaction band formation may be inhibited in other sandstones with higher fractions of feldspar and clay, as well as more disperse grain sizes.

Microstructural controls on the physical and mechanical properties of edifice-forming andesites at Volcán de Colima, Mexico

The reliable assessment of volcanic unrest must rest on an understanding of the rocks that form the edifice. It is their microstructure that dictates their physical properties and mechanical behavior and thus the response of the edifice to stress perturbations during unrest. We evaluate the interplay between microstructure and rock properties for a suite of edifice-forming rocks from Volcan de Colima (Mexico). Microstructural analyses expose (1) a pervasive, isotropic microcrack network, (2) a high, subspherical vesicle density, and (3) a wide vesicle size distribution. This complex microstructure severely impacts their physical and mechanical properties. In detail, porosities are high and range from 8 to 29%. As a consequence, elastic wave velocities, Youngs moduli, and uniaxial compressive strengths are low, and permeabilities are high. All of the rock properties demonstrate a wide range. For example, strength decreases by a factor of 8 and permeability increases by 4 orders of magnitude over the porosity range. Below a porosity of 11–14%, the permeability-porosity trend follows a power law with a much higher exponent. Microstructurally, this represents a critical vesicle content that efficiently connects the microcrack population and permits a much more direct path through the sample, rather than restricting flow to long and tortuous microcracks. Values of tortuosity inferred from the Kozeny-Carman permeability model support this hypothesis. However, we find that the complex microstructure precludes a complete description of their mechanical behavior through micromechanical modeling. We urge that the findings of this study be considered in volcanic hazard assessments at andesitic stratovolcanoes.

Stylolites in limestones: Barriers to fluid flow?

Stylolites—products of intergranular pressure-solution—are laterally extensive, planar features. They are a common strain localization feature in sedimentary rocks. Their potential impact on regional fluid flow has interested geoscientists for almost a century. Prevalent views are that they act as permeability barriers, although laboratory studies are extremely rare. Here we report on a systematic laboratory study of the influence of stylolites on permeability in limestone. Our data demonstrate that, contrary to conventional wisdom, the studied stylolites do not act as barriers to fluid flow. In detail, when a stylolite occurs perpendicular to the direction of flow, the permeability simply follows the same power law permeability-porosity trend as the stylolite-free material. We show, using a combination of high-resolution (4 µm) X-ray computed tomography, optical microscopy, and chemical analyses, that the stylolites of this study are not only perforated layers constructed from numerous discontinuous pressure solution seams, but comprise minerals of similar or lower density to the host rock. The stylolites are not continuous high-density layers. Our data affirm that stylolites may not impact regional fluid flow as much as previously anticipated.

Interpretation of porosity and LWD resistivity from the Nankai accretionary wedge in light of clay physicochemical properties: Evidence for erosion and local overpressuring

In this study, we used porosity to assess the compaction state of the Nankai accretionary wedge sediments and any implications for stress and pore pressure. However, hydrous minerals affect porosity measurements, and accounting for them is essential toward defining the interstitial porosity truly representative of the compaction state. The water content of sediments was measured in core samples and estimated from logging data using a resistivity model for shale. We used the cation exchange capacity to correct the porosity data for the amount of water bound to clay minerals and to correct the porosity estimates for the surface conductivity of hydrous minerals. The results indicate that several apparent porosity anomalies are significantly reduced by this correction, implying that they are in part artifacts from hydrous minerals. The correction also improves the fit of porosity estimated from logging-while-drilling (LWD) resistivity data to porosity measured on cores. Low overall porosities at the toe of the accretionary wedge and in the splay fault area are best explained by erosion, and we estimated the quantity of sediments eroded within the splay fault area by comparing porosity-effective stress relationships of the sediments to a reference curve. Additionally, a comparison of LWD data with core data (resistivity and P wave velocity) obtained at Site C0001 landward of the mega-splay fault area, suggested a contribution from the fracture porosity to in situ properties on the formation.

Physical property relationships of the Rotokawa Andesite, a significant geothermal reservoir rock in the Taupo Volcanic Zone, New Zealand

BackgroundGeothermal systems are commonly hosted in highly altered and fractured rock. As a result, the relationships between physical properties such as strength and permeability can be complex. Understanding such properties can assist in the optimal utilization of geothermal reservoirs. To resolve this issue, detailed laboratory studies on core samples from active geothermal reservoirs are required. This study details the results of the physical property investigations on Rotokawa Andesite which hosts a significant geothermal reservoir.MethodsWe have characterized the microstructure (microfracture density), porosity, density, permeability, elastic wave velocities, and strength of core from the high-enthalpy Rotokawa Andesite geothermal reservoir under controlled laboratory conditions. We have built empirical relationships from our observations and also used a classical micromechanical model for brittle failure. Further, we compare our results to a Kozeny-Carman permeability model to better constrain the fluid flow behavior of the rocks.ResultsWe show that the strength, porosity, elastic moduli, and permeability are greatly influenced by pre-existing fracture occurrence within the andesite. Increasing porosity (or microfracture density) correlates well to a decreasing uniaxial compressive strength, increasing permeability, and a decreasing compressional wave velocity.ConclusionsOur results indicate that properties readily measurable by borehole geophysical logging (such as porosity and acoustic velocities) can be used to constrain more complex and pertinent properties such as strength and permeability. The relationships that we have provided can then be applied to further understand processes in the Rotokawa reservoir and other reservoirs worldwide.

Dynamic fracturing by successive coseismic loadings leads to pulverization in active fault zones

Previous studies show that pulverized rocks observed along large faults can be created by single high-strain rate loadings in the laboratory, provided that the strain rate is higher than a certain pulverization threshold. Such loadings are analogous to large seismic events. In reality, pulverized rocks have been subject to numerous seismic events rather than one single event. Therefore, the effect of successive “milder” high-strain rate loadings on the pulverization threshold is investigated by applying loading conditions below the initial pulverization threshold. Single and successive loading experiments were performed on quartz-monzonite using a Split Hopkinson Pressure Bar apparatus. Damage-dependent petrophysical properties and elastic moduli were monitored by applying incremental strains. Furthermore, it is shown that the pulverization threshold can be reduced by successive “milder” dynamic loadings from strain rates of ~180 s−1 to ~90 s−1. To do so, it is imperative that the rock experiences dynamic fracturing during the successive loadings prior to pulverization. Combined with loading conditions during an earthquake rupture event, the following generalized fault damage zone structure perpendicular to the fault will develop: furthest from the fault plane, there is a stationary outer boundary that bounds a zone of dynamically fractured rocks. Closer to the fault, a pulverization boundary delimits a band of pulverized rock. Consecutive seismic events will cause progressive broadening of the band of pulverized rocks, eventually creating a wider damage zone observed in mature faults.

The mechanical behaviour of synthetic, poorly consolidated granular rock under uniaxial compression

Abstract In order to isolate the effect of grain size and cementation on the mechanical behaviour of poorly consolidated granular rock, we prepared synthetic rock samples in which these two parameters were varied independently. Various proportions of sand, Portland cement and water were mixed and cast in a mold. The mixture was left pressure-free during curing, thus ensuring that the final material was poorly consolidated. We used two natural well-sorted sands with grain sizes of 0.22 and 0.8 mm. The samples were mechanically tested in a uniaxial press. Static Youngs modulus was measured during the tests by performing small stress excursions at discrete intervals along the stress–strain curves. All the samples exhibited nonlinear elasticity, i.e., Youngs modulus increased with stress. As expected, we found that the uniaxial compressive strength increased with increasing cement content. Furthermore, we observed a transition from grain size sensitivity of strength at cement content less than 20–30% to grain size independence above this value. The measured values of Youngs modulus are well explained by models based on rigid inclusions embedded in a soft matrix, at high cement content, and on cemented grain-to-grain contacts, at low cement content. Both models predict grain size independence in well-sorted cemented sands. The observed grain size sensitivity at low cement content is probably due to microstructural differences between fine- and coarse-grained materials caused by small differences in grain sorting quality.

Mechanical compaction and strain localization in Bleurswiller sandstone

We performed a systematic investigation of mechanical compaction and strain localization in Bleurswiller sandstone. Our data show that the effective pressure principle can be applied in both the brittle faulting and cataclastic flow regimes, with an effective pressure coefficient close to but somewhat less than 1. Under relatively high confinement, the samples typically fail by development of compaction bands. X-ray computed tomography (CT) was used to resolve preexisting porosity clusters, as well as the initiation and propagation of the compaction bands in deformed samples. Synthesis of the CT and microstructural data indicates that there is no casual relation between collapse of the porosity clusters in Bleurswiller sandstone and nucleation of the compaction bands. Instead, the collapsed porosity clusters may represent barriers for the propagation of compaction localization, rendering the compaction bands to propagate along relatively tortuous paths so as to avoid the porosity clusters. The diffuse and tortuous geometry of compaction bands results in permeability reduction that is significantly lower than that associated with compaction band formation in other porous sandstones. Our new data confirm that Bleurswiller sandstone stands out as the only porous sandstone associated with a compactive cap that is linear, and our CT and microstructural observation show that it is intimately related to collapse of the porosity clusters. We demonstrate that the anomalous linear caps and their slopes are in agreement with a micromechanical model based on the collapse of a spherical pore embedded in an elastic-plastic matrix that obeys the Coulomb failure criterion.

Bedding-parallel stylolites in shallow-water limestone successions of the Apulian Carbonate Platform (central-southern Italy)

Bedding-parallel stylolites typically represent the product of chemical compaction (overburden weight-induced pressure solution) experienced by carbonate successions during their burial history, when bedding is still horizontal. Due to their common occurrence in carbonate rocks, with lateral extents that can exceed 1 km, bedding-parallel stylolites are of special interest for the hydrocarbon industry because they may affect the regional fluid flow in the subsurface. Aimed at assessing the development and distribution of bedding-parallel stylolites in shallow-water, platform limestone successions, field and laboratory studies were carried out on Cretaceous limestones originally pertaining to the Apulian Carbonate Platform realm and now exposed in three distinct Italian locations: Maiella Mountain, Gargano Promontory and Murge Plateau. Results point to a prominent role played by the geological characteristics of limestones on development and localization of bedding-parallel stylolites within shallow-water, platform limestone successions. In particular, bedding-parallel lamination and fine rock grain size, co-occurring in stromatolitic limestones, determined there laterally more extensive and closely spaced stylolites than in the associated calcilutites and calcarenites. Large fenestral pores, which are ubiquitous in stromatolitic limestones, represent rock heterogeneities able to influence the roughness of individual stylolites. Laboratory measurements revealed that the permeability of the studied Cretaceous limestones is very low (<10 μD). Pilot tests suggest that bedding-parallel stylolites in stromatolitic layers are not barrier to fluid flow but may represent pathways through low-permeability, platform limestone successions in the subsurface.

Effect of pore and confining pressures on VP in thermally pre‐cracked granites

The acoustic P-waves velocity VP is measured in a water saturated fine-grained granite as a function of pore pressure for a series of confining pressures ranging from 10 to 75 MPa. Prior to VP measurements, different crack contents were generated in the granite specimens by means of controlled thermal pre-treatments at 450, 510, and 555°C. VP variations are interpreted as resulting from crack behaviour evolution in response to imposed pressures. An effective pressure law with an effective pressure coefficient close to 0.9 is derived for VP from a delimited pressure domain. This behaviour is interpreted in terms of crack deformation under pressure.