A coupled THC model of the FEBEX in situ test with bentonite swelling and chemical and thermal osmosis

Abstract

The performance assessment of a geological repository for radioactive waste requires quantifying the geochemical evolution of the bentonite engineered barrier. This barrier will be exposed to coupled thermal (T), hydrodynamic (H), mechanical (M) and chemical (C) processes. This paper presents a coupled THC model of the FEBEX (Full-scale Engineered Barrier EXperiment) in situ test which accounts for bentonite swelling and chemical and thermal osmosis. Model results attest the relevance of thermal osmosis and bentonite swelling for the geochemical evolution of the bentonite barrier while chemical osmosis is found to be almost irrelevant. The model has been tested with data collected after the dismantling of heater 1 of the in situ test. The model reproduces reasonably well the measured temperature, relative humidity, water content and inferred geochemical data. However, it fails to mimic the solute concentrations at the heater-bentonite and bentonite-granite interfaces because the model does not account for the volume change of bentonite, the CO 2 (g) degassing and the transport of vapor from the bentonite into the granite. The inferred HCO 3 -and pH data cannot be explained solely by solute Page 2 of 54 transport, calcite dissolution and protonation/deprotonation by surface complexation, suggesting that such data may be affected also by other reactions.

Coupled Thermo-Hydro-Mechanical-Chemical Modeling of Water Leak-Off Process during Hydraulic Fracturing in Shale Gas Reservoirs

“…We assume the hydraulic fractures are evenly spaced. The inputs of the reservoir, fracture, fluid and heat properties [35,36,[55][56][57][58] are given in Table 1.…”

“…Besides the salinity difference between the fracturing-fluids and shale formation brine, a temperature gradient across the shale-fluid interface exists during the hydraulic fracturing. Therefore, thermal osmosis occurs, causing the preferred transport of water molecules from the low-temperature side to the high-temperature side until the heat content reaches an equilibrium on both sides of the shale membrane [33][34][35].…”

Generic Argillite/Shale Disposal Reference Case

“…The exponent n ranges from 3 to 4.5 (ENRESA 2000), but most models (Zheng et al 2011a;Sánchez et al 2012) have used 3. The relative permeability curve for MX-80 bentonite is the same as in equation (1), with n ranging from 2 to 4 (Hökmark 2004).…”

“…ENRESA (2000) lists a quite large range of  and P 0, but in some modeling works (Zheng et al 2011a;Sánchez et al 2012),  is 0.18 and P 0 is 2×10 7 Pa; Villar et al (2008) used the same value of  but a slightly different value for P 0 (2.8×10 7 Pa). The parameters  and P 0 for MX-80 bentonite are from Hökmark (2004).…”

“…Generic Argillite/Shale Disposal Reference Case August 2014 13 , which is about 56% in terms of volumetric water content (Zheng et al 2011a). However, as mentioned in Bradbury and Baeyens (2003a), obtaining the pore-water chemistry of compacted bentonite is not a straightforward task, and therefore there is considerable uncertainty is associated with the concentrations of ions in the pore-water.…”