Modelling rock-water buffering

The task for the DECOVALEX research teams was to predict the THM effects in the buffer material inside the test pit and in the surrounding rock, both during excavation of the test pit and the heater testing. The test case was divided into three main tasks Tasks 2A, 2B. and 2C. Task 2A was to predict the HM effects in the rock cau,sed by the excavation of the test pit. Geometrical, mechanical and hydraulic rock properties, as well as hydraulic conditions before excavation, were given to the research teams, and they were asked to predict water inflow distribution in the test pit. Task 2B was a model calibration of rock and fracture properties and the hydromechanical boundary conditions, based on actual measured results predicted in Task 2A. Task 2C was to predict the THM effects in the rock and buffer during the heating experiment. The rock model was presumed to have properties based on the calibration in Task 2B. with the calibrated permeability distribution in the near-field rock. At... 

In this chapter we construct geochemical models to consider how the availability of oxygen and the buffering of host rocks affect the pH and composition of acid drainage. We then look at processes that can attenuate the dissolved metal content of drainage waters. 

If the standard states for the solid phases are the pure phases at T and P, their activities are both one, and the equilibrium constant equals the activity of water, i.e., the activity of water is a constant (fixed) as long as (pure) gypsum and anhydrite coexist. Water introduced along cracks in this rock will tend to change the activity of water in the system, but no change will in fact take place (at least in the model) until all the anhydrite is converted to gypsum, at which point the system is no longer buffered and the water activity can assume a new value fixed by the introduced water. 

Processes being studied in the modeling of the Kamaishi Test included groundwater and heat flow in the rock matrix, fractures, buffer, and their interfaces under varying unsaturated conditions. Before emplacement of buffer and heater, the inflow of water into the test pit was affected, not only by the presence of fractures, but also by the unsaturated condition of the rock near the test pit. Strong variation in the areal distribution of inflow was observed on the walls of the test pit. After the heater and bentonite were emplaced, diffusion of water into the bentonite from the rock occurred simultaneously with drying of the bentonite near the... 

A buffer of compacted bentonite is planned to be used to prevent the movement of groundwater and the consequential escape of material from a geological repository for spent nuclear fuel. Fluid flow, phase changes, mechanical behaviour of the buffer, rock, and the waste canisters, and the heat produced by the waste constitute a coupled thermohydromechanical system. The aim of the study is to derive a general thermodynamically consistent THM model for an arbitrary mixture. The general theory is applied to the thermohydraulic modelling of a mixture of compacted bentonite, liquid water, vapour, and air. 

The numerical model ROCMAS was applied to predict coupled THM processes in a bentonite barrier at the FEBEX in situ test. The results indicate that numerical modeling can provide highly reliable predictions for temperature distribution, and reasonably reliable predictions for moisture flow and stress in a bentonite barrier. Moreover, field observations and modeling shows that resaturation of the buffer was controlled by the properties of the bentonite barrier whereas the permeability of the rock was sufficiently high to act as an unrestriced water source. Therefore, the wetting of the bentonite took place uniformly from the rock and was not impacted by the permeability difference between the Lamprophyres dykes and surrounding rock. 

Figure 5b shows that the time to full resaturation of the buffer varies between 3 to 10 years. The different resaturation times among these models are in line with simulation results obtained for the well-defined axisymmetric problem (Chijimatsu et al., 2003) and for the case of a homogenous rock (Millard et al., 2003). This indicates that the variation of the resaturation time among these models are caused by slightly different input hydraulic properties of the buffer, such as relative permeability and water retention curves. 

Initially the first phase of the experiment was simulated so that the seepage into the rock prior to emplacement of the buffer material could be analysed. The simulation was carried out for a period of three years to represent the time when the borehole was open before buffer emplacement. At the end of this phase the model estimated that steady state pore water pressure profiles had been achieved within the rock. Inflow from the rock into the empty borehole was then calculated. 

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