An analysis of the role of fluids in fracture initiation and propagation using direct simulation of the coupled fluid-solid system

It is well known that pore fluid pressure fundamentally influences a rock's mechanical response to stress. However, most measures of the mechanical behavior of rock (e.g. shear strength, Young's modulus) do not incorporate, either explicitly or implicitly, pore fluid pressure or transport properties of rock. Current empirical and theoretical criteria that define the amount of stress a given body of rock can support before fracturing also lack a direct connection between fluid transport and mechanical properties. The research outlined below explores these issues both qualitatively and quantitatively using a novel application of discrete numerical models of coupled fluid and solid physics. In strongly coupled fluid-solid systems the evolution of the solid framework is influenced by the fluid and vice versa. These couplings often result in changes of the bulk material properties (i.e. permeability and failure strength) with respect to the fluid's ability to move through the solid and the solids ability to transmit momentum. Feedbacks between fluid and solid framework ultimately play key roles in understanding the spatial and temporal evolution of the coupled fluid-solid system. Continuum numerical models of these systems attempt to capture these changes by using complicated constitutive relations, simple rules, or choosing to ignore them altogether. This often results in less than acceptable comparisons with observed behaviors. To address these issues discrete numerical models of the coupled systems based on fundamental fluid and solid physics have been developed. These models use direct simulation of fluids and solids to capture the evolving behavior of the system of interest. Our formulation couples the popular discrete element method (DEM) for solid mechanics and the lattice-Boltzmann method (LB) for solving the Navier-Stokes solutions of incompressible fluid flow. To examine the role of fluids in strongly coupled systems we developed 2-dimensional models of fracture initiation and propagation using the coupled DEM-LB model. Models exploring the rates of propagation and the initiation of fractures were developed to help isolate the role of mechanical and hydrologic heterogeneities on the overall system behavior. Preliminary results indicate that the model captures linear poro-elastic behavior. Comparisons of model results to simple laboratory experiments indicate that the general elastic and in-elastic behavior of the model is quite realistic. Finally, more complicated models of fracture initiation and propagation in saturated media point towards a hydrologic control on fracture development and behavior.