Experimental Poroviscoelasticity of Common Sedimentary Rocks

The success of geoenergy applications such as petroleum recovery or geological storage of CO₂ depends on properly addressing the physical coupling between the pore fluid diffusion and mechanical deformation of the subsurface rock. Constitutive models should include short-term hydromechanical interactions and long-term behavior and should incorporate the principles behind the mathematical models for poroelastic and poroviscoelastic responses. However, the viscous parameters in constitutive relationships still need to be validated and estimated. In this work, we experimentally quantify the time-dependent response of fluid-filled sedimentary rocks at room temperature and isotropic stress states. Drained, undrained, and unjacketed geomechanical tests are performed to measure the poroelastic parameters for Berea sandstone, Apulian limestone, clay-rich material, and Opalinus clay (shale). A poroviscous model parameter, the bulk viscosity, is included in the constitutive relationships. The bulk viscosity is estimated under constant isotropic stress conditions from time-dependent deformation of rock in the drained regime for timescales ~10⁵ s and from observations of the pore pressure growth under undrained conditions at timescales of ~10⁴ s. The bulk viscosity is on the order of 10¹⁵–10¹⁶ Pa s for sandstone, limestone, and shale and ~10¹³ Pa s for clay-rich material, and it decreases with an increase in pore pressure despite a corresponding decrease in the effective stress. In the long term, fluid pressure can asymptotically approach minimum principal stress, which in natural reservoirs may lead to liquefaction or rock embrittlement, causing slip instabilities and earthquakes and creating high-permeability channels in low-permeable rock.