The development of nuclear power requires safe disposal of large quantities of nuclear waste. A committee of the National Academy of Sciences-National Research Council proposed storing the waste in natural salt formations. However, oil production activities and geologic exploration have left boreholes in the salt beds.
A report by Battelle Northwest Laboratories (Schneider and Platt 1974) suggested that. in order to protect waste repositories in salt it would be necessary to restore the formations surrounding the salt to essentially their original state. They proposed the use of compacted natural earthen materials to achieve such a high quality plug. Compacting borehole plugs using natural materials has been evaluated (Martin 1975; Olsen and Martin 1976). Since the essential function of a compacted plug is to keep water out of the salt, the permeability of a compacted plug is its most important property. The referenced studies indicated that adequate plugs (permeability, k ≤.1 μD) could be compacted in metal molds using the parent shale with high compactive efforts or montmorillonite with low compactive effort. However, when borehole plugging was modelled by compacting a plug into a shale specimen using high effort impact compaction, flow of water in a permeability test of. the composite section was concentrated in the interface zone between the plug. and the surrounding shale. This zone consists of both the outer portion of the plug and the inner portion of shale at the hole wall. The interface flow suggested a difference in lateral confining stress for Plugs compacted in metal molds compared to plugs compacted into the shale formation. The lower confining stress may result from damage to the rock formation during plugging or the lower modulus of the shale. Any cracking or crack propagation during plugging would also allow flow to take place in the shale portion of the interface zone.
This paper presents the results of a program to determine the relationship between changes in lateral stress during plugging and the ability of a plug-formation system to restrict the flow of water into the salt. A complete treatment of this subject is presented elsewhere (Olsen 1978).
Model tests of coring and plugging were run in a lateral stress cell - shown schematically in Figure 1. This cell was constructed to accommodate a 3.75-inch diameter by 2-inch high cylindrical sample which could be loaded by the top piston'. It was also possible to remove the central portion of the base and top piston for coring, plugging and extrusion while the remainder of the sample remained loaded. A sample borehole l.l-inches in diameter could thus be constructed and plugged. Both ends of the cell have porous stones and fittings to apply water pressures for backpressuring and/or permeability testing.
A 0.030-inch stainless steel membrane transferred the lateral pressure on the sample to a fluid in an annular chamber connected to a 1000 psi pressure transducer. Vertical loads were transmitted through and measured by a 10,000 pound capacity load cell.
