Frédéric Skoczylas

Laboratory Investigation on Physical and Mechanical Properties of Granite After Heating and Water-Cooling Treatment

High-temperature treatment may cause changes in physical and mechanical properties of rocks. Temperature changing rate (heating, cooling and both of them) plays an important role in those changes. Thermal conductivity tests, ultrasonic pulse velocity tests, gas permeability tests and triaxial compression tests are performed on granite samples after a heating and rapid cooling treatment in order to characterize the changes in physical and mechanical properties. Seven levels of temperature (from 25 to 900xa0°C) are used. It is found that the physical and mechanical properties of granite are significantly deteriorated by the thermal treatment. The porosity shows a significant increase from 1.19% at the initial state to 6.13% for samples heated to 900xa0°C. The increase in porosity is mainly due to three factors: (1) a large number of microcracks caused by the rapid cooling rate; (2) the mineral transformation of granite through high-temperature heating and water-cooling process; (3) the rapid cooling process causes the mineral particles to weaken. As the temperature of treatment increases, the thermal conductivity and P-wave velocity decrease while the gas permeability increases. Below 200xa0°C, the elastic modulus and cohesion increase with temperature increasing. Between 200 and 500xa0°C, the elastic modulus and cohesion have no obvious change with temperature. Beyond 500xa0°C, as the temperature increases, the elastic modulus and cohesion obviously decrease and the decreasing rate becomes slower with the increase in confining pressure. Poisson’s ratio and internal frictional coefficient have no obvious change as the temperature increases. Moreover, there is a transition from a brittle to ductile behavior when the temperature becomes high. At 900xa0°C, the granite shows an obvious elastic–plastic behavior.

About the interest of using gas to evaluate permeability of damaged granite

Granite rock is foreseen for the underground storage of radioactive waste in China. The water and gas permeabilities of granite from Sichuan province have been tested in two different laboratories in China and in France. It has been found that water permeability is not a convenient tool to evaluate mechanical damage since gas permeability is considerably more useful to evaluate micro-structural changes of such a material submitted to different levels of confining pressure. The variation of the ratio (K(Pc)/Ko) with the confining pressure was found to be similar for all the samples whatever their initial state was, either intact, micro-cracked or macro-cracked.

Effects of gas pressure on failure and deviatoric stress on permeability of reservoir rocks: initial studies on a Vosges sandstone

The geomechanical behaviour of a specific type of sandstone is investigated using tri-axial compression tests, a hydrostatic compression test to measure Biot’s coefficient, tri-axial tests with pressurised gas, and permeability tests under deviatoric stress. The tri-axial compression tests reveal a nonlinear stress–strain relationship at low confining pressures. Biot’s coefficient initially decreases with increasing confining pressure, and then gradually increases if the confining pressure is increased beyond a certain level. At constant confining pressure and deviatoric stress, the gas pressure loading path has a significant influence on the mechanical failure of the sandstone sample. If the gas pressure is increased by comparatively small increments, the critical pressure the sample can withstand is found to be close to the theoretical value, whereas if the gas pressure increments are comparatively large, the sample is found to fail at lower values of gas pressure. The injection and release of gas also accelerate the sample’s failure. At high confining pressures, there is a significant decrease in permeability with increasing deviatoric stress. However, at low confining pressures, the decrease in permeability is not as significant as that observed under high confining pressures. The reloading–unloading of deviatoric stress does not have a predictable effect on permeability.

Poromechanical Properties of a Sandstone Under Different Stress States

Poromechanical properties of a sandstone from an underground gas storage site are investigated with the use of a neutral gas to control the pore pressure. The poroelastic theory of anisotropic medium is used to evaluate elastic properties and coupling coefficients under different stress states. In-situ CT observations have been made under uniaxial compression test and they are used to underline some effects due to the occurrence of damage and microcraks. Under hydrostatic loading, pores and micro-cracks are gradually compressed. The initial state of the sandstone is “slightly” transversely isotropic but in a first approach the Biot’s tensor reduces to a scalar. This coefficient decreases with the increase in confining pressure that can be attributed to the closure of micro-cracks. The effect of damage and cracking is investigated with two series of conventional triaxial tests conducted at different confining pressures, either with an increase in pore pressure (series 1) or with a decrease in pore pressure (series 2), to evaluate the coupling coefficients. The results, derived from these two test series, are consistent as regards the effects of cracking on the material behavior. There is an obvious damage due to the triaxial loading which induces a parallel decrease in the ratio E3/(1xa0−xa0ν3) and in the modulus H3. Axial compaction results in the continual increase in the H1 modulus observed for every sample whatever the measurement technique was.

About adsorption effects on the poroelastic properties and gas permeability of COx claystone

Abstract Argillaceous rocks have long been selected as privileged host rock candidates for the nuclear waste disposal. For the sake of evaluating the sealing capacity of these natural barriers, one urgent issue is to study the poromechanical properties of them in the long-term gas migration process. As the key parameter of poroelasticity, Biot’s tensor of argillaceous rocks should be firstly studied. In this paper, a series of poromechanical tests was firstly carried out to measure Biot’s tensor components. ‘Non-logical’ values that Biot’s tensor components were larger than unity (1.0) were obtained in the tests. It was then suspected that the process of injecting pore pressure is not a purely poromechanical problem. In order to deeper study about these values, complementary poromechanical tests were handed on two samples with helium and argon or helium and carbon dioxide (CO2), respectively, to confirm the adsorption-swelling phenomenon. The results showed that Biot’s tensor components are obviously affected by the gases used, which is owing to their different adsorption capacities. To evaluate these swelling affect consequences, a series of permeability and triaxial compression tests was performed with different type of gases. The results obtained reveal that gas adsorption-swelling deeply effect the transport and (poro)mechanical properties of argillite.