Effect of gas pressure on the sealing efficiency of compacted bentonite-sand plugs

This research relates to the assessment of the sealing ability of bentonite/sand plugs when swollen in presence of both water and gas pressures, in the context of deep underground radioactive waste storage. Compacted bentonite/sand plugs are placed inside a constant volume cell, and subjected to swelling in presence of both water and gas: swelling kinetics and effective swelling pressure Pswell are identified. Secondly, the gas breakthrough (GB) characteristics of swollen plugs are assessed to determine their ability for gas migration, which has to be minimal for sealing radioactive waste repositories. We show that gas pressure Pg does not affect significantly Pswell until a threshold Pg>2MPa. When swelling occurs inside a tube with a smooth (turned) inner surface, continuous GB occurs when Pg is equivalent to the effective Pswell (obtained without gas pressure, at 7.32MPa±0.11). When the plug swells inside a grooved tube, continuous GB does not occur up to Pg≥10.5MPa: smooth interfaces are a preferential gas migration pathway rather than grooved interfaces, and rather than water-saturated bentonite-sand plugs. With smooth tubes, in presence of Pg≥2MPa, although Pswell is not affected, gas passes through the sample at significantly lower values than Pswell, due to partial sample saturation. It is concluded that GB pressure is a more accurate indicator of partial sample saturation than swelling pressure Pswell alone.