Temperature induced pore fluid pressurization in geomaterials

The theoretical basis of the thermal response of the fluid-saturated porous materials in undrained condition is presented. It has been demonstrated that the thermal pressurization phenomenon is controlled by the discrepancy between the thermal expansion of the pore fluid and of the solid phase, the stress-dependency of the compressibility and the non-elastic volume changes of the porous material. For evaluation of the undrained thermo-poro-elastic properties of saturated porous materials in conventional triaxial cells, it is important to take into account the effect of the dead volume of the drainage system. A simple correction method is presented to correct the measured pore pressure change and also the measured volumetric strain during an undrained heating test. It is shown that the porosity of the tested material, its drained compressibility and the ratio of the volume of the drainage system to the one of the tested sample, are the key parameters which influence the most the error induced on the measurements by the drainage system. An example of the experimental evaluation of undrained thermoelastic parameters is presented for an undrained heating test performed on a fluid-saturated granular rock

Experimental artefacts in undrained triaxial testing

For evaluation of the undrained thermo-poro-elastic properties of saturated porous materials in conventional triaxial cells, it is important to take into account the effect of the dead volume of the drainage system. The compressibility and the thermal expansion of the drainage system along with the dead volume of the fluid filling this system, influence the measured pore pressure and volumetric strain during undrained thermal or mechanical loading in a triaxial cell. A correction method is presented in this paper to correct these effects during an undrained isotropic compression test or an undrained heating test. A parametric study has demonstrated that the porosity and the drained compressibility of the tested material and the ratio of the vol-ume of the drainage system to the one of the tested sample are the key parameters which influence the most the error induced on the measurements by the drainage system

Thermo-Poro-Mechanical Properties of Clayey Gouge and Application to Rapid Fault Shearing

In this paper, the mechanism of fault pressurization in rapid slip events is analyzed on the basis of a complete characterization of the thermo-poro-mechanical behavior of a clayey gouge extracted at 760m depth in Aigion fault in the active seismic zone of the Gulf of Corinth, Greece. It is shown that the thermally collapsible character of this clayey gouge can be responsible for a dramatic reduction of effective stress and a full fluidization of the material. The thickness of the 'ultra localized' zone of highly strained material is a key parameter that controls the competing phenomena of pore pressure increase leading to fluidization of the fault gouge and temperature increase leading to pore fluid vaporization

Coupled thermo-hydro-mechanical processes in fault zones during rapid slip

International audienceThe physical processes which occur during an earthquake exhibit several coupled phenomena as large variations of stress, pore pressure and temperature take place in the slip zone. Thermo-poro-mechanical couplings due to shear heating can be associated to phase transition such as vaporization of the pore fluid, melting of fault gouge and to chemical effects such as dehydration of minerals or decarbonation of calcite. Different competing effects may influence dynamic slip and affect the weakening of the shear stress. In this paper, we show how thermal pressurization of the pore fluid and thermal decomposition of minerals induced by shear heating limit the co-seismic temperature rise which may explain the lack of pronounced heat outflow, and the lack of shallow frictional melting, along major tectonic faults

Bifurcation theory and localization phenomena

International audienceThe principles of strain localization analysis as applied to geomaterials are presented. Emphasis is given to the effects of fluid and temperature in the occurrence and development of instabilities. RÉSUMÉ. Dans cet article, on présente les principes de l'analyse de localisation des déformations dans les géomatériaux. En particulier, on met l'accent sur les effets du fluide et de la température sur l'apparition et le développement des instabilités

The effect of undrained heating on a fluid-saturated hardened cement paste

The effect of undrained heating on volume change and induced pore pressure increase is an important point to properly understand the behaviour and evaluate the integrity of an oil well cement sheath submitted to rapid temperature changes. This thermal pressurization of the pore fluid is due to the discrepancy between the thermal expansion coefficients of the pore fluid and of the solid matrix. The equations governing the undrained thermo-hydro-mechanical response of a porous material are presented and the effect of undrained heating is studied experimentally for a saturated hardened cement paste. The measured value of the thermal pressurization coefficient is equal to 0.6MPa/°C. The drained and undrained thermal expansion coefficients of the hardened cement paste are also measured in the heating tests. The anomalous thermal behaviour of cement pore fluid is back analysed from the results of the undrained heating test.Comment: Cement and Concrete Research (2008) In pres

Existence of a threshold for brittle grains crushing strength: two-versus three- parameter Weibull distribution fitting

International audienceGrain crushing plays an important role in the mechanical behavior of granular media, in chemo-hydro-thermo-mechanical couplings, in instabilities related to strain localization such as shear bands and compaction bands, in geophysical and geotechnical processes, in reservoir and petroleum engineering and in many other domains. The strength of brittle particles seems to be quite well described by a two-parameter Weibull distribution. Nevertheless, such a distribution predicts that failure is possible under any level of applied stress. On the contrary a three-parameter Weibull distribution contains a stress threshold under which grain failure is unlikely. Based on existing experiments on crushing of individual grains from various geomaterials and surrogate materials, and on new experiments performed on rock sugar particles, the present paper explores and compares the applicability of a two-versus a three parameter Weibull distribution. It is shown that in most of the cases the three-parameter Weibull distribution better describes the experimental results

Chemically induced compaction bands: Triggering conditions and band thickness

International audienceDuring compaction band formation various mechanisms can be involved at different scales. Mechanical and chemical degradation of the solid skeleton and grain damage are important factors that may trigger instabilities in the form of compaction bands. Here we explore the conditions of compaction band formation in quartz- and carbonate-based geomaterials by considering the effect of chemical dissolution and grain breakage. As the stresses/deformations evolve, the grains of the material break leading to an increase of their specific surface. Consequently, their dissolution is accelerated and chemical softening is triggered. By accounting for (a) the mass diffusion of the system, (b) a macroscopic failure criterion with dissolution softening and (c) the reaction kinetics at the micro level, a model is proposed and the conditions for compaction instabilities are investigated. Distinguishing the micro-scale (grain level) from the macro-level (Representative Elementary Volume) and considering the heterogeneous microstructure of the REV it is possible to discuss the thickness and periodicity of compaction bands. Two case studies are investigated. The first one concerns a sandstone rock reservoir which is water flooded and the second one a carbonate rock in which CO2 is injected for storage. It is shown that compaction band instabilities are possible in both cases

THERMAL PRESSURIZATION AND ANOMALOUS THERMAL EXPANSION OF THE PORE FLUID OF A HARDENED CEMENT PASTE

International audienceTemperature increase in a fluid-saturated porous material in undrained condition leads to pore pressure increase. This phenomenon of thermal pressurization is studied experimentally for a saturated hardened cement paste. The measured value of the thermal pressurization coefficient is found equal to 0.6MPa/°C. The experimental observation that this coefficient does not change with temperature between 20°C and 55°C is attributed to the anomalous thermal behaviour of cement paste pore fluid. It is shown that the thermal expansion of the cement paste pore fluid is higher than the one of pure bulk water and is much less sensitive to temperature changes. This anomalous thermal behaviour is due to the confinement of the pore fluid in the very small pores of the microstructure of the cement paste, and also to the presence of dissolved ions in the pore fluid

Shear-banding in drained and undrained triaxial tests on a saturated sandstone ; porosity and permeability evolution.

International audienceDetailed analysis of shear band formation and shear band microstructure formed in drained and undrained triaxial tests on Fontainebleau sandstone is presented. It is shown that under globally undrained conditions, local fluid exchanges inside the sample occur at shear banding resulting into an heterogeneous damage pattern along the shear band. At high confinement, pore pressure generation inside the band leads locally to fluidisation of the crushed material which results into the formation of connected channels in the heart of the band. Image processing analysis is used for evaluation of porosity inside the shear band and estimation of the permeability is performed using the Walsh and Brace [26] model. It is shown that, porosity increase as observed in the band at low confining pressure and porosity decrease as observed at high confining pressure are both accompanied by a reduction of permeability inside the shear band due to the increase of tortuosity and specific surface. However, this permeability reduction is much more important at high confining pressure and can reach values three orders of magnitude smaller than the permeability of the intact material