A Continuum-Scale Approach to Predicting Wellbore Stability in Ghareb Chalk for Safe Nuclear Waste Disposal

ABSTRACT: Safe nuclear waste storage in the kerogen-bearing Ghareb chalk in Israel requires accurate assessment of wellbore stability and excavation damage. We propose a continuum constitutive model to describe the mechanical response of Ghareb chalk during drilling and long-term storage. We assume inelastic deformation occurs through coupled volumetric and deviatoric creep related to visco-plastic deformation of the organic and clay mineral phases. Rate-independent mechanisms of damage and plasticity are related to non-clay mineral deformation. We incorporate nonlinear elasticity with elastic-plastic coupling and rate-independent strength. We also capture the effects of pressure- and Lode-angle dependence, porous compaction, brittle damage, strain hardening, and non-associative plastic flow during shear-enhanced compaction. The model allows for concurrent relaxation through volumetric creep and power-law shearing. We fit model results to hydrostatic and triaxial experimental data conducted under compression and creep loading conditions. This approach allows us to assess the importance of rate-independent (compression experiment) versus rate-dependent (creep experiment) mechanisms. We predict borehole stability under various in-situ stress conditions. This analysis allows us to more accurately predict the creep-collapse behavior of the Ghareb Formation, a critical step for assessing its suitability for safely storing nuclear waste.