In this study, we developed a new Numerical Manifold Method model for the analysis of coupled hydro-mechanical processes in fractured, porous rock. The fractures are considered as porous media with nonlinear behavior of hydraulic and mechanical properties involving both direct (poro-elastic) and indirect (property-change) couplings. Based on an energy-work model, we stringently established all components of the work related to fluid flow and mechanical processes in porous media in a unified form and their direct coupling. For indirect coupling, we derived a new formulation that implicitly considers the nonlinear fracture properties and deformation by directly assembling the corresponding strain energy. Compared with traditional methods with approximation of the nonlinear constitutive equations, this new formulation achieves more accurate representation of the nonlinear fracture deformation behavior. With the new model, we developed a new computer code in our Numerical Manifold Method package. We tested the code with very dense mesh for direct coupling on (1) a classical poroelastic column and (2) a semiinfinite media under strip load, respectively and compared the results with the analytical solutions, achieving very good agreement. Finally, we tested for direct and indirect coupling on a model with a single dominant fracture and obtained sensible results.
Hydro-mechanical (HM) coupling refers to the interaction between hydraulic and mechanical processes that may be triggered by mechanical loading/unloading or fluid injection/extraction. This interaction is significant in geological engineering, such as oil and gas extraction, geothermal energy, nuclear waste disposal, where the geological media usually consist of fractured rock . These fractured rock masses may contain fractures with complex geometry and fillings, thus could be modeled as a fractured porous media. Basically, the mechanisms of HM coupling in fractured porous media are: (1) Direct coupling, i.e. the poro-elastic instantaneous undrained coupling between mechanical and hydraulic fields. Specifically, the fluid pressure affects deformation while the volume change affects the pressure distribution. (2) Indirect coupling, i.e. interaction between mechanical and hydraulic fields indirectly, through their properties. Specifically, the effective stress change, affected by fluid pressure change, would change the stiffness of fractures, while the deformation of fractures change their hydraulic conductivities [1- 2].