Summary Report of FORGE WP4.1.2: verification of critical stress theory applied to repository concepts

This report outlines the major conclusions from an experimental study of 48 separate experiments with the primary aim to verify critical stress theory. Two main types of experiment were conducted: 1). Loading-unloading tests, where fracture flow was monitored at constant injection pressure as normal load was increased in steps to a given level and then reduced back to the starting stress state; 2). Gas breakthrough experiments, where gas injection pressure was increased in a pressure ramp at constant vertical load. These were conducted with and without active shear. It was found that critical stress theory is valid in predicting the preferential flow of gas in relation to the orientation of the fracture plane with respect to the maximum horizontal stress direction. However, loading unloading experiments showed that understanding the stress history of the rocks is of paramount importance and a mere knowledge of the current stress state is insufficient in accurately predicting the nature of fluid flow.