Influence of cyclic thermal processes on gas migration in saturated GMZ01 bentonite

To ensure the performance efficiency and long-term operational safety of the engineering barrier in deep geological repositories, it is of great importance to investigate gas migration behavior in the water-saturated bentonite subjected to decay heat generated by the nuclear waste. In this work, experimental investigations were conducted to find out influences of multi-step thermal cycles on variations of the effective gas permeability, gas breakthrough pressure and microstructure of the bentonite specimens. A temperature range of 20-60 degrees C was selected and each temperature step was kept constant for at least 3-4 days to allow the gas flow to be equilibrated. Results show that effective gas permeability increases with heating, while decreases with cooling. The effective gas permeability decreases from 7.9 x 10(-23) to 2.01 x 10(-23) m(2) for the specimen tested under a constant injection pressure of 1.52 MPa after experienced one to three thermal cycles. Meanwhile, the gas breakthrough pressure for GMZ bentonite after experiencing three thermal cycles was 2.43 MPa. Microstructural observations by the MIP tests and paraffin-coated tests indicate that the heating-cooling cycles induce decreases of void space, resulting from the shrinkage of the bentonite matrix. Decrease of macro-pores and a little increase of micro-pores induced by the thermal cycles applied could make it difficult for gas to break through the bentonite matrix. The interfacial effect is the dominant gas breakthrough mechanism for specimens experienced thermal cycles.