Analyzing structural changes induced by gas migration in heterogeneous pellet/powder bentonite mixtures through X-ray computed micro-tomography

Abstract

Understanding the mechanisms of gas transport and the resulting preferential pathways formation through bentonite-based barriers is essential for their performance evaluation. In this experimental study, gas migration within a heterogenous mixture of MX80 bentonite pellets and powder with a ratio of 80/20 in dry mass was investigated. A novel X-ray transparent constant volume cell has been developed to assess the effect of gas pressure, material heterogeneities, and water vapor gas saturation on breakthrough pressure and gas pathways. The new cell allows to perform high-resolution X-ray computed micro-tomography (X-ray μCT) scans to track microstructural changes during different phases of saturation and gas injection. Experimental results showed that the gas breakthrough occurred when the pressure was raised to 3 MPa. This is slightly higher than the expected swelling pressure (2.9 MPa) of the bentonite sample. Each gas injection was followed by a long resaturation phase restoring material homogeneity at μCT resolution scale (16 μm). However, the elapsed time needed for gas to breakthrough at 3 MPa diminished at each subsequent injection test. X-ray μCT results also revealed the opening of the specimen/cell wall interface during gas passage. This opening expanded as the injection pressure increased. The gas flow along the interface was associated with the development of dilatant pathways inside the sample, although they did not reach the outlet surface. It was observed that the water vapor gas saturation had no effect on the breakthrough pressure. These findings enhance the understanding of the complex mechanisms underlying microstructural evolution and gas pathway development within the highly heterogeneous mixture. The experimental outcomes highlight the effectiveness of X-ray μCT to improve quality protocols for engineering design and safety assessments of engineered barriers.

Keywords

• Bentonite pellet-powder mixture
• Gas migration
• Breakthrough pressure
• Gas pathways
• X-ray computed tomography
• Heterogenous bentonite mixture