Possibility of triggering earthquakes by injection of radioactive wastes in shale at Oak Ridge National Laboratory, Tenn

Conditions for producing earthquakes by fluid injection are (1) underlying rock stressed to the verge of fracturing by tectonic stresses and/or on the brink of sliding on pre-existing fault planes, (2) the presence of potentially active faults, and (3) an increase of pore pressure in the rock caused by fluid injection or seepage from reservoir(s). The mechanism of disposal of radioactive wastes in shale by grout injection and hydraulic fracturing used at ORNL is different from injection of fluid. The injected grout is in its liquid phase only during the injection period and hopefully confined in the hydraulically induced bedding-plane fractures. The grout is then allowed to solidify under pressure after termination of the injection, and, after solidification, it becomes an integral part of the shale. Significant characteristics of the disposal method and the site are as follows: (1) Because of the low permeability of shale and high viscosity of the injection grout, pore pressure in the rock, does not increase due to the injections; the injected grout enters newly formed bedding-plane fractures, (2) the injection site is free of local faults and of major interconnected joints and fractures; accordingly, injected grout in its liquid state is expected to be confined principally in the induced nearly horizontal bedding-plane fractures, and (3) due to low tensile strength along shale bedding planes, the energy needed to induce bedding-plane fractures is small. An increase of pore pressure in rock and association with potentially active fault(s) do not occur at the proposed site at ORNL and that the disposal method does not produce an increase in pore pressure, it can be concluded that neither the method nor the site has the characteristics for triggering earthquakes during and after grout injections