Poroelasticity is a material concept that expresses the reversible, macroscale process interactions that occur in a porous material, such as rocks. These process interactions take place between the pore fluids and the rock framework (or “skeleton”) that contains the pores. The phenomenological basis of poro-elasticity is examined via a micromechanics analysis, using a simplified digital-rock model that consists of solid elements in a lattice arrangement, and which hosts a well-connected, lattice-like network of simply shaped pore elements. The quasistatic poromechanical bulk response of this model is defined fully by closed-form equations that provide a clear understanding of the process interactions and that allow key effects to be identified. Several external boundary conditions (nonisotropic strain and stress) are analyzed, with drained and undrained pore-fluid conditions, along with arbitrary pore pressure states. The calculated responses of the pore-scale model, when translated into continuum-scale equivalent behaviors, indicate significant problems with the existing theories of poroelasticity that are rooted in an enriched-continuum perspective. Specifically, the results indicate that the principle of effective stress (and the Biot coefficient alpha) is wrongly attributed to a deficiency in the role of pore pressure. Instead, the micromechanics-based phenomenological understanding identifies the change of effective stress, in a characteristically confined setting, as being the result of changes in the stress components, with a key dependency on the specifics of the far-field constraints. Thus, poroelasticity is not a material characteristic; instead, it is a description of a nonlinear system operating at the pore scale. The analysis reveals a discrepancy between the stress states within the model domain and the external stress state. This yet remains to be addressed, to translate the microscale behavior into an equivalent material law.
Investigation of effects of clay content on F-Φ relationship by Lattice gas automation using digital rock model
