Changes in brine and gas permeability of rock salt as a result of nearby excavation (mine-by) have been measured from the underground workings of the WIPP facility. Prior to the mine-by, the formation responds as a porous medium with a very low brine permeability, a significant pore (brine) pressure and no measurable gas permeability. The mine-by excavation creates a dilated, partially saturated zone in the immediate vicinity of the excavation with an increased permeability to brine and a measurable permeability to gas. The changes in hydrologie properties are discussed in the context of pore structure changes.
A Disturbed Rock Zone (DRZ) develops around the excavations of the Waste Isolation Pilot Plant (WIPP), a US Department of Energy research and development facility in bedded salt (halite) near Carlsbad, New Mexico. The DRZ has been defined as the zone of rock in which mechanical and hydrologie properties have changed in response to excavation (Borns and Stormont, 1989). The presence of a DRZ has numerous implications for the performance of the WIPP. The DRZ is relatively permeable compared to the undisturbed formation, and must be considered in seal systems designed to help isolate waste. The increased porosity of the DRZ may also serve as a sink within which fluids (brine or gas) accumulate. Most research has focused on the properties and response of the rock mass outside the DRZ. Current mechanical and hydrologie models for rock salt do not account for the observed behavior in the DRZ. An in situ experiment was conducted between 1988 and 1990 which monitored the hydrologie response of a halite layer to nearby excavation and provided a hydrologie measure of the DRZ. Sometime later, gas and brine injection tests were conducted in the boreholes. The emphasis of measurements and analyses was to quantify the changes in gas and brine permeability as a result of excavation. The data also provide qualitative information regarding changes in dilation and saturation in response to excavation. These results suggest a more fundamental definition of the DRZ in terms of pore structure changes.
The twelve small-diameter "monitoring" boreholes were drilled vertically down from the floor of the Ll room in Communication the experimental portion of the WIPP facility. These 4.8-cm diameter bore- holes were drilled to a depth of 8 m with air as the drilling fluid. A test interval was created in the bottom of each borehole by placing an inflatable rubber packer nominally 65 cm from the bottom of the borehole. A schematic diagram of the monitoring boreholes is given in Figure 1. In order to minimize the volume of test interval, a 4.3-cm diameter steel rod was placed near the bottom of the bore- hole. The packers have a tubing feed- through to allow access to the test interval for fluid injection or withdrawal. The test interval pressure is measured by means of a strain-gaged diaphragm pressure transducer. A nearby data acquisition shed houses the excitation, signal conditioning and data recording instrumentation.