Technical Report NTB 93-35

Site characterization activities at Nagra's potential low- and intermediate-level waste repository at Oberbauenstock have indicated the presence of an anomalous pressure zone within the potential host rock (Valanginian Marl). Two conceptual models have been proposed to relate the observed pressure anomaly to the presence of natural gas within the marl. Model A assumes that the underpressured conditions have originated due to the transient discharge of gas from a relatively localized gas "bubble" into the adjacent Seelisberg tunnel. Model B assumes that the observed low pressures are caused by regional (quasi) steady-state upward gas migration from some deeper source.

This report presents a set of quantitative analyses, using analytical and numerical models, to evaluate the two conceptual models. Material balance calculations with simple analytical equations suggest that gas depletion from a localized region into the tunnel can result in pressure profiles similar to those observed during testing. Detailed site-scale modelling with the multiphase Simulator TOUGH, using a 2-D vertical cross section of the Oberbauenstock region, also suggests that the scenario presented in Conceptual Model A is more likely than the Model B scenario. Thus, both local- and regional-scale modelling studies indicate that the underpressuring is most likely the effect of gas discharges into the Seelisberg tunnel via some high-conductive flow paths. The presence of gas alone does not appear to be a sufficient explanation for the observed anomalous pressure conditions.