Mechanical Stresses Affecting Deep Borehole Disposal of High Level Nuclear Waste

While extensive stress field data are available from crystalline rock boreholes drilled in France (Soultz), Germany (KTB), and the USA (Cajon Pass, Monticello Reservoir), Canada and Sweden, the data and methods used to analyze them have yet to be applied to very deep geologic disposal facilities. Typically, to alleviate the stress fields that are intensified in a borehole, muds (mixtures of clay and water) are a critical component and are widely used in the drilling industry. In the first portion of this paper, we review the available mechanical data and analysis methods. Based on the most applicable measurements (in Canada and Germany), we propose values of stress fields and rock properties to be used for generic assessment of deep borehole disposal. The minimum horizontal stress can be approximated as S_h=23.2D, the maximum horizontal stress as S_H=43.1D, and the vertical stress as S_V=27.5D, where the stresses are in MPa and D is depth in km. This analysis also incorporates the effect of thermal stresses (relieved) by the cool drilling mud. Using an average uniaxial compressive strength results (C=212 MPa) and conservatively neglecting the increase in strength of crystalline rock under polyaxial conditions, a stable borehole can be drilled to 4.55 km with mud density of 1020 kg/m³. This is based on a stability limit such that at the bottom of the hole, a significant portion of the wall (180°/360°) reaches a critical state of stress (i.e., experiences spalling). Using relations developed for shallow mines (which may be overly conservative) the spalled zone is estimated to have a radius that is approximately twice that of the borehole. To reach 5 km, the mud density should be raised to 1420 kg/m³, or be actively cooled (90°C) below the ambient temperature of the rock (∼135°C) at that depth.