Effect of fault activation on the hydraulic connectivity of faults in mudstone

Abstract

An injection test and repeated packer tests were conducted for a fault in siliceous mudstone in order to activate the fault and investigate changes in hydraulic connectivity of the fault before and after fault activation. The injection test successfully induced a significant dilational shear failure within the fault. Pressure changes measured by the repeated packer tests were analyzed before and after the failure, where log-log plots of the pressure derivatives changed after the failure from an upward trend indicating a limited extent of fluid flow paths to a horizontal trend suggesting well-connected fluid flow paths. After the borehole had been open for six weeks, the pressure derivatives were restored to an upward trend. This reversible change in pressure derivatives means that the hydraulic connectivity of the fault increased temporarily during and just after the injection test, but fault activation did not irreversibly affect the initially low hydraulic connectivity of the fault. This transition in the hydraulic connectivity of the fault is also consistent with the variation in fluid pressure monitored at a neighboring observation hole. We propose that analyzing the pressure derivatives obtained by repeated packer tests before and after an injection test in a single borehole is effective for assessing the sensitivity of the hydraulic disconnectivity of faults to fault activation, which is key information for risk assessment of radioactive waste disposal.