Poroelasticity of the Callovo–Oxfordian Claystone

Abstract

International audienceThis work is devoted to an experimental investigation of the poroelastic behavior of the Callovo–Oxfordian claystone, a potential host rock for the deep underground repository of high-level radioactive waste in France. Drained, undrained, pore pressure loading and unjacketed tests were carried out in a specially designed isotropic compression cell to determine the poroelastic parameters of fully saturated specimens. Great care was devoted to the saturation procedure, and small loading rates were used to ensure full drainage conditions in drained and pore pressure tests (0.5 kPa/min) and in the unjacketed test (2 kPa/min). High-precision strain measurements were performed by ensuring direct contact between the LVDT stems and the specimen. An analysis in the framework of transverse isotropic poroelasticity provided the Biot effective stress coefficients b1 (perpendicular to bedding) between 0.85 and 0.87 and b2 (parallel to bedding) between 0.90 and 0.98 under different stress conditions (pore pressure 4 MPa, total isotropic stresses of 14 and 12 MPa, respectively). A set of equivalent isotropic poroelastic parameters was also determined and a very good compatibility between the results of different tests was found, giving confidence in the parameters determined. The unjacketed test provided a directly reliable measurement of the unjacketed modulus (Ks = 21.7 GPa) that was afterward confirmed by an indirect evaluation that showed the non-dependency of Ks with respect to the stress level. These parameters were obtained for specimens cored and trimmed in the laboratory. A parametric study was then conducted so as to provide an estimation of the parameters in situ, i.e., not submitted to the damage supported by laboratory specimens. A minimal value b = 0.77 seems to be a reasonable lower bound for the equivalent isotropic Biot parameter

Active porosity in swelling shales: insight from the Callovo-Oxfordian claystone

“…The experimental device used for the investigation of the poroelastic properties of the COx claystone, already described in details in Belmokhtar et al (2016), is presented in Figure 2. It consists in a thermal isotropic compression cell connected to two pressure-volume controllers (PVC, GDS Brand) imposing pressures up to 60 MPa.…”

“…The drained bulk compressibility and the unjacketed compressibility of the sample were determined by Belmokhtar et al (2016) and were equal to c d = 0.335 GPa -1 and c s = 0.046 GPa -1 , respectively. The water compressibility at 25°C is c w = 0.447 GPa -1 (Spang, 2002).…”

“…In a purpose of simplicity and to help in numerical calculations that are most often conducted within an isotropic equivalent framework (Gens et al 2007), equivalent isotropic values have also been proposed. It was shown in Belmokhtar et al (2016) that the equivalent isotropic value of the Biot parameter b of the COx claystone was larger than 0.77.…”

“…This work is based on the data of two recent works carried out in the same research group about, on the one hand, the status of water in swelling claystones or shales (Menaceur et al, 2016) and, on the other hand, the poroelastic behaviour of the Callovo-Oxfordian (COx) claystone (Belmokhtar et al 2016). The COx claystone is considered in France as a possible host rock for the long-term disposal of high activity exothermic radioactive wastes at great depth.…”

“…Following previous works that proposed for the COx claystone values of the Biot b parameter along a rather large range (between 0.4 and 0.9, Vincké et al 1998, Escoffier 2002, Bemer et al 2004, Escoffier et al 2005, Homand et al 2004), a better knowledge of the poroelastic properties of the COx claystone has recently been gained by Belmokhtar et al (2016), through a detailed determination of the various poroelastic parameters within a transverse isotropic framework (Biot parameters b 1 and b 2 , Biot modulus H, compression modulus of the solid phase K s ). Recent contributions on this topic also include Noiret et al (2017) on the Opalinus clay.…”