Patrick Baud

Failure mode and weakening effect of water on sandstone

Previous studies have shown that brittle strength of a rock is generally reduced in the presence of water. However, for siliciclastic rocks, there is a paucity of data on the water-weakening behavior in the cataclastic flow regime. To compare the weakening effect of water in the brittle faulting and cataclastic flow regime, triaxial compression experiments were conducted on the Berea, Boise, Darley Dale, and Gosford sandstones (with nominal porosities ranging from 11% to 35%) under nominally dry and saturated conditions at room temperature. Inelastic behavior and failure mode of the nominal dry samples were qualitatively similar to those of water-saturated samples. At elevated pressures, shear localization was inhibited, and all the samples failed by strain hardening. The compactive yield strengths (associated with the onset of shear-enhanced compaction) in the saturated samples were lower than those in the dry samples deformed under comparable pressure conditions by 20% to 70%. The reductions of brittle strength in the presence of water ranged from 5% to 17%. The water-weakening effects were most and least significant in the Gosford and Berea sandstones, respectively. The relation between water weakening and failure mode is consistently explained by micromechanical models formulated on the basis that the specific surface energy in the presence of water is lowered than that in vacuum by the ratio λ. In accordance with the Hertzian fracture model the initial yield stress in the compactive cataclastic flow regime scales with the grain-crushing pressure, which is proportional to λ 3/2 In the brittle faulting regime, damage mechanics models predict that the uniaxial compressive strength scales with λ 1/2 . In the presence of water the confined brittle strength is lower due to reductions of both the specific surface energy and friction coefficient.

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.

Localized failure modes in a compactant porous rock

Since dilatancy is generally observed as a precursor to brittle faulting and the development of shear localization, attention has focused on how localized failure develops in a dilatant rock. However, recent geologic observations and reassessment of bifurcation theory have indicated that strain localization may be pervasive in a compactant porous rock. The localized bands can be in shear or in compaction, and oriented at relatively high angles (up to 90°) to the maximum compression direction. Here we report microstructural characterization of the spatial distribution of damage in failed samples which confirms that compaction bands and high-angle conjugate shears can develop in sandstones with porosities ranging from 13% to 28%. These failure modes are generally associated with stress states in the transitional regime from brittle faulting to cataclastic ductile flow. The laboratory results suggest that these complex localized features can be pervasive in sandstone formations, not just limited to aeolian sandstone in which they were first documented. They may significantly impact the stress field, strain partitioning and fluid transport in sedimentary formations and accretionary prisms. While bifuraction theory provides an useful framework for analyzing the inception of localization, our data rule out a constitutive model that does not account for the activation of multiple damage mechanisms in the transitional regime.

Dilatancy, compaction, and failure mode in Solnhofen limestone

Failure mode is intimately related to porosity change, and whether deformation occurs in conjunction with dilatation or compaction has important implications on fluid transport processes. Laboratory studies on the inelastic and failure behavior of carbonate rocks have focused on the very porous and compact end-members. In this study, experiments were conducted on the Solnhofen limestone of intermediate porosity to investigate the interplay of dilatancy and shear compaction in controlling the brittle-ductile transition. Hydrostatic and triaxial compression experiments were conducted on nominally dry samples at confining pressures up to 435 MPa. Two conclusions can be drawn from our new data. First, shear-enhanced compaction can be appreciable in a relatively compact rock. The compactive yield behavior of Solnhofen limestone samples (with initial porosities as low as 3%) is phenomenologically similar to that of carbonate rocks, sandstone, and granular materials with porosities up to 40%. Second, compactive cataclastic flow is commonly observed to be a transient phenomenon, in that the failure mode evolves with increasing strain to dilatant cataclastic flow and ultimately shear localization. It is therefore inappropriate to view stress-induced compaction and dilatancy as mutually exclusive processes, especially when large strains are involved as in many geological settings. Several theoretical models were employed to interpret the micromechanics of the brittle-ductile transition. The laboratory data on the onset of shear-enhanced compaction are in reasonable agreement with Curran and Carrolls [1979] plastic pore collapse model. In the transitional regime, the Stroh [1957] model for microcrack nucleation due to dislocation pileup can be used to analyze the transition from shear-enhanced compaction to dilatant cataclastic flow. In the brittle faulting regime the wing crack model provides a consistent description of the effect of grain size on the onset of dilatancy and brittle faulting.

Micromechanics of compressive failure and spatial evolution of anisotropic damage in Darley Dale sandstone

The micromechanics of compressive failure in Darley Dale sandstone (with initial porosity of 13%) was investigated by characterizing quantitatively the spatial evolution of anisotropic damage under the optical and scanning electron microscopes. Two series of triaxial compression experiments were conducted at the fixed pore pressure of 10 MPa and confining pressures of 20 and 210 MPa, respectively. For each series, three samples deformed to different stages were studied. Failure in the first series was by brittle faulting. In contrast, failure in the second series was ductile, involving shear-enhanced compaction and distributed cataclastic flow. In the ductile series, crack density and acoustic emission activity both increased with the development of strain hardening. The stress-induced cracking was relatively isotropic. In the brittle series, crack density increased with the progressive development of dilatancy, with spatial distributions indicative of clustering of damage at the peak stress and shear localization in the strain softening stage. Dilatancy was associated with significant anisotropy in stress-induced cracking, that was primarily due to intragranular and intergranular cracking with a preferred orientation parallel to the maximum principal stress. Compared with published data for Westerly granite and San Marcos gabbro (with porosities of the order of 1%) and for Berea sandstone (with porosity of 21%), there is an overall trend for the stress-induced anisotropy (in a sample deformed to near the peak stress) to decrease with increasing porosity. The sliding wing crack model was adopted to analyze the evolution of anisotropic damage, using a friction coefficient and fracture toughness inferred from stress states at the onset of dilatancy. Significant discrepancy exists between the model prediction and microstructural data on stress-induced anisotropy, which is possibly due to limitations intrinsic to the microscopy technique as well as the sliding wing crack model.

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.

Damage accumulation during triaxial creep of darley dale sandstone from pore volumometry and acoustic emission

Abstract We performed triaxial creep tests on water-saturated samples of Darley Dale sandstone to investigate the effect of pressure on the process of time-dependent brittle deformation under all-round compression. Axial strain, acoustic emission (AE) output and pore volume change were monitored continuously during each test. We observed the three classical creep regimes (primary, steady-state and tertiary). The level of applied differential stress has a crucial effect on the creep rate and on the time-to-failure; from 30 minutes at 90% of the short-term strength to almost one day at 80%. For the experiments performed at the lower levels of stress, the duration of the primary creep phase increases, while the acoustic emission level during the steady-state phase decreases dramatically. The variations of axial strain and differential pore-fluid volume are more regular when the tests are conducted at stresses closer to the strength of the material. AE measurements suggest that the final stage of the deformation occurs for similar levels of cumulative events and cumulative AE energy, regardless of stress level. The same comment can be made for the pore-fluid volumometry results. This suggests that the final stage that leads to failure occurs for almost the same level of damage in all samples.

Mechanical Compaction of Porous Sandstone

In many reservoir engineering and tectonic problems, the ability to predict both the occurrence and extent of inelastic deformation and failure hinges upon a fundamental understanding of the phenomenology and micromechanics of compaction in reservoir rock. This paper reviews recent research advances on mechanical compaction of porous sandstone, with focus on the synthesis of laboratory data, quantitative microstructural characterization of damage, and theoretical models based on elastic contact and fracture mechanics. The mechanical attributes of compaction in nominally dry and saturated samples have been studied under hydrostatic and nonhydrostatic loadings over a broad range of pressure conditions. Specific topics reviewed herein include: comparison of mechanical and acoustic emission data with continuum plasticity theory; microstructural control of onset and development of compaction; strain hardening and spatial evolution of damage during compaction; and the weakening effect of water on compactive yield and porosity change.

Influence of temperature on brittle creep in sandstones

The characterization of time-dependent brittle creep, promoted by chemically active pore fluids, is fundamental to our understanding of the long-term evolution and dynamics of the Earths crust. Here we report results from a study of the influence of temperature on both short-term strength and time-dependent brittle creep in three sandstones under triaxial stress conditions. We show that an increase in temperature from 20 degrees to 75 degrees C significantly enhances stress corrosion cracking in all three sandstones, leading to (1) a systematic reduction in strength during constant strain rate experiments and (2) an increase by several orders of magnitude in brittle creep strain rates during stress-stepping creep experiments. We also show that a conventional creep experiment performed at 75 degrees C exhibits a qualitatively similar three-stage brittle creep curve as that observed at ambient temperature. Extrapolation of our results suggests that temperature is likely to be the dominant influence on the evolution of creep strain rate with depth in the shallow crust. Citation: Heap, M. J., P. Baud, and P. G. Meredith (2009), Influence of temperature on brittle creep in sandstones, Geophys. Res. Lett., 36, L19305, doi: 10.1029/2009GL039373.

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.