Valentina Favero

1D Compression Behaviour of Opalinus Clay

One of the main concerns related to tunnel excavations, drilling operations and wellbore stability in shales is the generation of excess pore water pressure due to changes in mechanical stress; therefore the consolidation of shales is a fundamental process that must be considered. This paper presents a comprehensive methodology for analysing the compression and consolidation behaviour of shales. An apparatus to perform high-pressure oedometric tests is presented and an analytical method is introduced to analyse the shale consolidation behaviour, which allows information to be gathered on the coefficient of consolidation, stiffness, poroelastic properties, and permeability of the tested material as a function of the applied stress conditions. Results obtained on Opalinus Clay shale using the developed methodology are presented and discussed.

An Insight on the Thermo-mechanical Behaviour of a Shale

The most recent developments in the field of geo-energy (CO2 sequestration, enhanced geothermal system technologies and nuclear waste disposal) have highlighted a need to examine the effects of temperature on the mechanical behaviour of shales. This work investigates the thermo-mechanical behaviour of Opalinus Clay in relation to different stress conditions and yield stress ratio (YSR) values and evaluates the impact of temperature on the hydro-mechanical properties of this material. To this end, a focused experimental campaign consisting in high-temperature/high-pressure oedometric tests has been carried out. The results show that the thermo-mechanical behaviour of Opalinus Clay is heavily affected by the YSR. The study of the thermal cyclic behaviour shows that expansive irreversible strains can occur upon first heating at high YSR and that a quasi-reversible behaviour follows during subsequent thermal cycles. A decrease in the yield threshold is observed when compression is applied at high temperature with respect to the yield threshold found at low temperature. Compressibility and swelling indexes are not significantly influenced by thermal changes, as well as the oedometric modulus and the secondary compression coefficient, whereas consolidation processes are found to occur faster at high temperature.

Hydro-mechanical Behaviour of Reconstituted and Intact Shales

The geo-energy sector is nowadays bringing ahead advanced technologies such as shales gas extraction, CO2 sequestration and nuclear waste geological storage, where the exploitation of shale formations is considered. Due to the great depth involved in the mentioned applications and to the difficulties in retrieving intact samples, reconstituted shale specimens are often adopted for hydro-mechanical testing. Reconstituted and intact shales may substantially differ in their hydro-mechanical behaviour due to the particular structure of the natural material; such peculiar structure is the result of diagenesis and burial history. This paper presents and experimental campaign aimed at (i) characterizing the role of diagenesis for Opalinus Clay shale from the northern region of Switzerland and (ii) understanding how representative the behaviour of reconstituted material is respect to the one of the natural shale. The investigation program comprises a series of low and high-pressure oedometric tests. The results of the tests on the reconstituted material are compared to those on the intact one and the major aspects related to the effect of structure on the geomechanical behaviour have been highlighted. Particular attention is given to the compressibility, swelling response, permeability and secondary consolidation of the material at the reconstituted and intact states.

On the Fluid Retention Properties of Shales

The development of engineering activities involving shales such as the extraction of shale gas and shale oil, the nuclear waste geological storage and the sequestration of CO2, has led to an increasing interest toward the geomechanical behaviour of this geomaterial. In the context of such engineering applications, a deep understanding of the hydro-mechanical behaviour of shales is of primary significance. The water retention mechanisms play a major role in either fluid trapping due to the capillary forces present in low permeability formations or in the resaturation of shale formations after desiccation. The paper presents the experimental methodologies that have been developed by the authors for the analysis of the retention behaviour of shales. Selected test results are presented for two shales from Switzerland.

On the Water Retention Behaviour of Shales

The involvement of shales in engineering fields such as the extraction of shale gas or the nuclear waste geological disposal is mainly driven by the low permeability and the high retention properties of these geomaterials. The high capillary forces developed in the matrix of the shales allow the fluid trapping with the consequence that the material remains saturated until significant values of suction. However, in the context of several engineering applications, the shale formations are exposed to relative humidity values which might induce changes in the degree of saturation. As a consequence, the investigation of the air entry value and of the retention behaviour of the material is of primary significance.The following paper presents an investigation on the water retention properties of shales. The experimental technique developed for such analysis is described in detail and selected results are presented. The testing methodology is based on the control of the water content and on the subsequent measurement of the total suction at equilibrium.The volumetric behaviour of the material is monitored in order to compute the retention curve in terms of degree of saturation. The results allow to identify a relationship between the air entry values and the porosity for Swiss shales.

An Insight into the Fluid Retention Capabilities of Shales

The hydro-mechanical behaviour of shales is becoming one of the most important issues in modern geomechanics, largely driven by petroleum industries (i.e., the extraction of shale gas), the sequestration of CO2 and the nuclear waste geological storage. In any such application, a deep understanding of the hydro-mechanical behaviour of the involved materials is of primary significance. The water retention mechanisms play a major role in either fluid trapping due to the capillary forces present in low permeability formations or in the resaturation of shale formations after desaturation. The paper presents different experimental methodologies that have been developed by the authors for the analysis of the retention behaviour of shales. They involve the direct control of the shale water content and the subsequent measurement of the suction at equilibrium by a psychrometer. A fluid displacement technique with a non-polar liquid is then used to assess the volume changes and compute the degree of saturation. Selected test results are presented for two shales from the northern region of Switzerland.