Estimation of the heterogeneity of fracture permeability by simultaneous modeling of multiple air-injection tests in partially saturated fractured tuff

Abstract

Air-injection tests were used to investigate the flow characteristics of the fractured volcanic tuffs at Yucca Mountain, Nevada, the potential site for a high-level nuclear waste repository. Because the tuff matrix pores are saturated over 90% with water and the matrix permeability is on the order of microdarcies, the air component of flow is mainly in the fractures. Air-injection tests can therefore help to determine the flow characteristics and heterogeneity structure of the densely fractured welded tuff. The tests were carried out in the Exploratory Studies Facility, an 8 km long underground tunnel at the Yucca Mountain site, in twelve 40 m long boreholes, forming three clusters within a cubic rock volume of approximately 40 meters on each edge. Each borehole in the test block was packed off (or isolated) into four sections (or zones) by inflatable packers. The in situ field tests consisted of constant-rate air injection into one of the isolated borehole zones while the pressure response was monitored in all the isolated zones. The pressure data showed an almost universal response in all monitored zones to injection into any borehole -zone, indicating that the fractures are well connected for airflow. Air-injection tests were performed in succession for all isolated zones. A simultaneous inversion was performed for the pressure response of all the monitoring zones for all the injection tests in the test block. TOUGH2, a 3D numerical code for multiphase, multicomponent transport, was used for this purpose. Spatially variable fracture permeability was used as an adjustable parameter to fit the simulated pressure responses to those measured, assuming fixed fracture porosity. For most of the pneumatic experiments, the calculated pressure changes matched the data well, and the estimated permeability ranged over four orders of magnitude, from 10 -15 m 2 to 10 -11 m 2 .