The Benefits of Rock Creep in Underground Storage & Sequestration

ABSTRACT: The tendency of geological materials to undergo slow deformation under long-term persistent mechanical stress, i.e. creep, can be beneficial for various kinds of underground storage, including deep disposal of both nuclear waste and carbon dioxide. Rocks, like argillaceous claystone and shale that are often considered as geologic barrier material maybe more or less creep prone, typically depending on their clay mineral content. Over the long term, such creep deformation will tend to make the in-situ stress more isotropic, a process that will have a profound impact on host rock sealing and barrier functions. Here a few examples of numerical modeling are presented to demonstrate the benefit of creep deformation on barriers functions associated with both nuclear waste and carbon dioxide disposal. In the case of the nuclear waste disposal, the numerical modeling demonstrates that high temperature generated from hot nuclear waste effectively accelerates creep deformation to even out any deviatoric (shear) stress that would otherwise occur as result of thermally-induced stresses. This will have the benefit of sealing the host rock, including the excavation disturbed zone around nuclear waste emplacement tunnels. In the case of caprocks, numerical modeling demonstrates how historic long-term creep and creep during a 30-year carbon sequestration operation will be beneficial for caprock integrity. During active injection, creep deformation will tend to maintain an isotropic caprock stress that can prevent fracturing and shear activation of existing fractures and faults. It is concluded that the potential impact of creep should be considered when assessing the long-term performance of underground storage and sequestration, such nuclear waste and carbon dioxide disposal.