Mean Deviatoric Stress Research Articles

Differences in the plastic and brittle properties of Danian and Maastrichtian reservoir chalk from the North Sea

Wellbore collapse, seafloor subsidence, and seismic events are some of the main technical and societal issues related to the deformation of producing chalk reservoirs. The triaxial tests represent valuable input data for geomechanical and rock physics models that document the properties and mechanical response of the rock under in situ conditions. Most of the experimental studies are related to chalk from the Upper Maastrichtian Tor Fm. that is the main oil-bearing unit in the North Sea. The overlying Danian Ekofisk Fm. that also contains a non-negligible part of the hydrocarbon reserves has however received little attention. The aim of this study is to assess the plastic and shear behaviour of oil-saturated, Danian chalk containing different amount of quartz by using an extensive experimental dataset. The hydrostatic yield stresses and the Mohr-Coulomb criteria are quantified across a 30–45% porosity range for specimens with a low (<3%) and high amount of quartz (>4%), referred to as clean and impure chalk. Porosity-dependent functions are proposed to estimate the geomechanical properties and the yield surface is reconstructed in a plot of mean-deviatoric stress versus porosity. These results are compared with previous studies on Maastrichtian chalk. The plastic and shear behaviour of Danian and Maastrichtian chalk appears similar for porous specimens and differs as porosity declines. The differences are of significance changing the geomechanical properties by a factor of 20%–60%. The mineralogical and diagenetic parameters that can explain these observations are discussed. The outcomes can be integrated in constitutive models to improve the prediction on the contribution of the Ekofisk Fm. to the total strain occurring in depleted reservoirs from the North Sea. This study is also of interest for other sectors of industry such as CO 2 and energy storage in chalk that involve economical and societal risks associated with rock deformation. • Relationships between porosity and mechanical properties proposed for Danian rock. • The yield surfaces of Danian and Maastrichtian chalk reconstructed and compared. • Quartz and calcite cementation impact the mechanical contrasts between chalk types. • Constitutive models applied to Maastrichtian chalk is not valid for Danian chalk. • The outcomes assist engineers in developing simulators designed for the Ekofisk Fm.

Relating microstructure and particle-level stress in colloidal crystals under increased confinement.

The mechanical properties of crystalline materials can be substantially modified under confinement. Such modified macroscopic properties are usually governed by the altered microstructures and internal stress fields. Here, we use a parallel plate geometry to apply a quasi-static squeeze flow crushing a colloidal polycrystal while simultaneously imaging it with confocal microscopy. The confocal images are used to quantify the local structure order and, in conjunction with Stress Assessment from Local Structural Anisotropy (SALSA), determine the stress at the single-particle scale. We find that during compression, the crystalline regions break into small domains with different geometric packing. These domains are characterized by a pressure and deviatoric stress that are highly localized with correlation lengths that are half those found in bulk. Furthermore, the mean deviatoric stress almost doubles, suggesting a higher brittleness in the highly-confined samples.

Strength of a matrix with elliptic criterion reinforced by rigid inclusions with imperfect interfaces

Abstract Elliptic effective strength criteria in the mean-deviatoric stress plane are encountered in porous media for a granular material made of rigid grains with cohesive frictional interfaces or a material with pores in a Drucker–Prager matrix. The macroscopic strength criterion of a heterogeneous material comprising a matrix with elliptic strength criterion reinforced by rigid inclusions with perfect or imperfect interfaces is studied. Considered imperfect interfaces follow either a Tresca or a Mohr–Coulomb strength criterion. Derived macroscopic criteria are shown to be a combination of a larger ellipse, which corresponds to the criterion for perfectly bounded interfaces, conditionally truncated by a smaller ellipse resulting from the activation of interfacial mechanisms. The activation of the interfacial mechanisms depends on the matrix and interfaces strength properties, inclusions concentration, as well as the macroscopic strain triaxiality ratio.

Multiaxial fatigue life estimation based on combined deviatoric strain amplitudes

This contribution presents a model for multiaxial fatigue life estimation where a combination of deviatoric strain amplitudes defines the fatigue parameter. It also takes into account the influences of both hydrostatic stresses and mean deviatoric stresses. Assessment considers 211 proportional and non-proportional strain-controlled programs reported in the literature, including a number of cases with mean strains/stresses, for three steels and two aluminum alloys. The resulting life estimates correlates well with the experiments, falling in most cases within a factor of two bandwidth.

Bounds on the viscosity coefficient of continental lithosphere from removal of mantle lithosphere beneath the Altiplano and Eastern Cordillera

The rapid rise of the central Andean plateau between ∼10 and 6.8 Ma implies that mantle lithosphere, including eclogitized lower crust, was removed from beneath the region in that time interval; we infer from that removal that the average viscosity coefficient of mantle lithosphere was quite low when removal occurred. Using scaling laws for the growth of perturbations to the thickness of a dense layer over an inviscid substratum (Rayleigh‐Taylor instability), we place bounds on the average viscosity coefficient for central Andean lithosphere. When compared with laboratory measurements of flow laws for olivine and eclogite, the allowed range of viscosity coefficients yields bounds on the temperature of ∼500–800°C at the Moho beneath this region and suggests that mean stresses across mantle lithosphere during continental deformation are less than ∼50 MPa. This range of temperature is comparable with, if a slightly lower than, we might expect for lithosphere approximately doubled in thickness and not yet equilibrated with the doubled crustal radioactivity. The mean deviatoric stress is comparable to that associated with stresses that drive plates and hence shows that lithospheric material is not too strong to prevent removal of its mantle part.

The effect of ambient pressure and impactor geometry on low speed penetration of unconsolidated materials

The dynamic behaviour of projectiles impacting planetary surfaces can be measured to derive mechanical properties of the target material. Several such dynamic penetrometers will be used on Mars, the Moon, Titan, and a comet nucleus. However, solid bodies in the Solar System exhibit a wide range of surface atmospheric and gravitational conditions and previous workers have shown that changes in ambient pressure or gravity can significantly alter penetration dynamics. This presents a challenge for the terrestrial calibration of penetrometers. We have applied a penetration model to low speed impacts in air and vacuum, with the aim of quantifying any differences in a target's measured properties. A 1.05 kg instrumented penetrometer was dropped onto two cohesionless granular materials at speeds of around 2.7 m s −1. The apparatus was located in a vacuum chamber, allowing tests to be made at low pressures. An initial upper limit for the mean deviatoric stress (a measure of material strength) was found in each case by dividing the gravitational potential energy lost (during the penetrometer's fall and penetration) by the volume penetrated. This value can be reduced using the projectile's recorded deceleration and a penetration model that includes friction and dynamic resistance. Good fits between the recorded and modelled deceleration were obtained for a range of values of dynamic drag coefficient and coefficient of friction. Initial comparison of the air and vacuum drops performed so far suggests behaviour consistent with that described by previous workers, namely that pore pressure aids penetration in loose materials but inhibits penetration in heavily compacted materials, and that these effects are larger for smaller grain sizes.

Measuring physical properties at the surface of a comet nucleus

The European Space Agency’s cornerstone mission Rosetta is due for launch in January 2003. It will perform a rendezvous with comet 46P/Wirtanen beyond 3 AU and, following an initial mapping phase, deploy a lander to a selected site on the nucleus surface. The Rosetta Lander will provide unprecedented access to cometary material. Some of the most uncertain characteristics of the nucleus material are physical properties such as its density, the structure of the surface layers and its mechanical strength. MUPUS (Multi-Purpose Sensors for Surface and Sub-Surface Science) is one of the experiment packages selected for the Lander payload which will address certain physical properties and their evolution with time. This thesis focuses on the in situ measurement of the density of the surface layers by a radiation densitometer incorporated into the MUPUS thermal probe, and on the penetrometry measurements to be performed by an accelerometer mounted in the Lander’s anchoring harpoon. A concept for incorporation of a gamma ray attenuation densitometer into the thermal probe is presented and explored. A 137Cs radioisotope source will be mounted near the tip of the probe and semiconductor radiation detectors situated at the top of the probe will monitor the transmitted count rate during probe insertion, as the intervening material attenuates the radiation. Preliminary experiments to evaluate cadmium telluride (CdTe) detectors for this purpose are presented, as well as results from a specially-developed Monte Carlo computer code designed to model the absorption and scattering of photons in bulk material. Also presented is a control algorithm to dynamically re-budget the integration time and depth resolution of the instrument as it is inserted by the hammering mechanism. This is required due to: a) the wide range of possible densities the instrument may encounter, b) the variation vs. depth of required integration time, and c) the limited time in which the measurement must be performed. For lower than nominal densities, integration time may be wasted when it could be used to improve the accuracy and depth resolution. For higher densities the integration time at particular depths may not be sufficient to obtain acceptable accuracy; in this case some depth resolution could be sacrificed to improve the accuracy. The proposed algorithm uses the density measured at each point to update the time budget and depth resolution for the remaining stages of penetration. Although the use of the gamma ray backscatter type of densitometer was eventually rejected in favour of the aforementioned attenuation technique, investigation of the backscatter technique resulted in an extension to the Single Scattering Model– an analytic approximation of its operation. This extended model adds to our understanding of these devices' response to spatial inhomogeneity. Calculations show that anchoring of the Lander is necessary to avoid possible ejection from the nucleus by gas drag in the case of a landing in an active area. The use of the Lander’s anchoring harpoon to perform penetrometry measurements is reported, including the results of preliminary experiments and techniques for analysing the accelerometry data. It is shown that layers with distinctly different strengths may be identified, and that the mean deviatoric stress– a strength parameter– may be constrained to within a factor of about 2.2. This would be a significant improvement on current estimates, which vary by several orders of magnitude. Together with other investigations on the Rosetta mission the densitometry and penetrometry measurements will serve to constrain models of the physical state and evolution of the cometary material found at the landing site. In particular both instruments are sensitive to near-surface layering, which may be expected from theoretical models of cometary activity.

Tectonomagnetism and tectonoelectricity

The last four years has seen a considerable waning of efforts in the U.S. to use the inherent stress sensitivity of the magnetization of rocks in tectonically active regions as a tool for understanding and, perhaps predicting, earthquake activity and volcanic eruptions. The primary reasons for this decreased interest derive from accumulating evidence that the mean deviatoric stress release during earthquakes (and the ambient shear stress near active faults) is lower than than had been previously assumed and, secondly, the observational limits of magnetometers separated by about 10 km, after application of a variety of noise reduction techniques, appear to be no better than about 0.1 nT. Lower estimates of the average change in deviatoric stress associated with earthquake rupture correspondingly reduce the fields calculated from tectonomagnetic models. At the same time, some unexpected and exciting results have been obtained which indicate many aspects of magnetic field changes near active faults and volcanoes are still not completely understood. The more important of these results are: 1) the observation of correlated magnetic, strain, uplift, and gravity changes during the latter stages of a remarkable deformation episode near the San Andreas fault in southern California, 2) indications of anomalous change in magnetic fields along the San Andreas fault to the southeast of Palm Springs, 3) an unambiguous observation of a volcanomagnetic effect at Mt. St. Helens prior to an extrusive eruption, and 4) the first observations of coseismic seismomagnetic effects on two independent magnetometers during the July 8, 1986, ML 5.9 North Palm Springs earthquake. A resurgence of interest has occurred during the quadrennium in a related area concerning electrical phenomena and electromagnetic effects related to earthquakes and volcanoes. Mechanisms have been proposed to explain the occurrence of earthquakes lights and the possibility of electromagnetic emission prior to the 1960 Chilean earthquake has been proposed. Self potential anomalies have been observed before and during several eruptions on Kilauea volcano in Hawaii.

Influence of cyclic load input on the mechanical properties of a sensitive soil from Orleans, Ontario

The importance of disturbance and remoulding to the alteration of mechanical properties of sensitive soils has been well documented in the geotechnical literature both in terms of laboratory and field behaviours. Man-made transient dynamic input such as dynamite blasting, heavy vehicles, and train movement have been suspected of being capable of causing a reduction in the in situ strength parameters of sensitive clays. A laboratory test program was undertaken to determine whether dynamic loading at peak stress levels below normal failure strength caused similar changes in the mechanical properties, and specifically to quantify the phenomena.In order to simulate highly overconsolidated conditions most of the tests were carried out under conditions of no confining pressure, although supplemental data were obtained from consolidated undrained tests. Some of the variables examined in this program were confining pressure, mean deviatoric stress, cyclic deviatoric stress, cyclic strain, number of applications, frequency, and reference strength. In order to compare the effect of dynamic input with the long-term creep phenomena, a simultaneous constant load program was undertaken.In general terms, the study indicates that under the prestated laboratory test conditions no major reduction in peak strength was found under dynamic loading, and that failure would occur at comparative stress levels under dead-load conditions, but required a greater time. In addition, examination of the sample after failure revealed that any remoulding of the sample appeared to be restricted to the area adjacent to the shear zone.