Problems and research performed
BMT2 was a near-field model of fractured rock of a size 0.75 m in horizontal length and 0.5 m in vertical height, containing four perpendicular fractures, as smooth parallel fractures, cutting the model into nine blocks (Fig.3a). An initial temperature t° = 15° C and initial horizontal and vertical stress components = - 4 MPa were assumed, with the factures having a constant aperture of 300 micrometers. The aim of BMT2 was to investigate the coupled behavior of fractures and intact rock matrix in fully coupled THM processes, including optional consideration for forced heat convection along the fractures. Five research teams participated in BMT2 (see Table 2), using different numerical computer codes and numerical methods: MOTIF (FEM by ARCL), UDEC (DEM by CNWRA and INERIS), ROCMAS (FEM by LBL) and ADINA-T/JRTEMP (FEM by VTT). Figure 3b and 3c show two examples of the simulated results of displacements and water head at monitoring locations.
Main achievements and outstanding issues
Besides the achieved aim of comparing performance and relative validation of computer codes and models for such a near-field problem with explicit representation of fractures, the other main scientific/technical findings include the significant differences caused by the thermal convection by water flow in the fractures, importance of thermal expansion inside the host rock far from repository or near ground surface, and the dominating fracture closure under confined mechanical conditions during heating and thermal expansion processes. The main outstanding issues included: importance of the mechanical behavior generated by discrete and continuum models that require further study on the fundamental aspects of the numerical modeling using different material characterizations of fractured rocks, and not achieving a realistic model set-up with too small a power input and over-constrained boundary conditions—leading to a less realistic mechanical behavior of the model.