This paper presents a new triple-continuum conceptual model for simulating flow and transport processes in fractured rock. Field data collected from the unsaturated zone of Yucca Mountain, a potential repository site of high-level nuclear waste, show that there are significant numbers of small-scale fractures. Although these small fractures may not contribute to the global flow and transport within the fracture networks, they may have a considerable effect on solute transport and liquid flow between the fractures and the matrix. The effect of these small fractures has not been considered in previous modeling investigations within the context of a continuum approach. A new triple-continuum model (consisting of matrix, small-fracture and large-fracture continua) has been developed to investigate the effect of these small fractures. This paper derives the model formulation and discusses the basic triple-continuum behavior of flow and transport processes under different conditions, using both analytical solutions and numerical approaches. The simulation results from the site-scale model of the unsaturated zone of Yucca Mountain indicate that these small fractures may have an important effect on radionuclide transport within the mountain.Key Words: Naturally fractured reservoir, fractured porous media, double-porosity model, dual-permeability model, triple-continuum model, numerical reservoir simulation, and fractured unsaturated rock. 2
IntroductionThe study of flow and transport processes in fractured rock has recently received increased attention because of its importance to underground natural-resource recovery, waste storage, and environmental remediation. Since the 1960s, significant progress has been made towards the understanding and modeling of flow and transport processes in fractured rock (Barenblatt et al., 1960;Warren and Root, 1963;Kazemi, 1969;Pruess and Narasimhan, 1985). Despite these advances, modeling the coupled processes of multiphase fluid flow, heat transfer, and chemical migration in a fractured porous medium remains a conceptual and mathematical challenge. The difficulty stems primarily from (1) the nature of inherent heterogeneity, (2) the uncertainties associated with the characterization of a fracture-matrix system for any field-scale problem, and (3) the difficulties in conceptualizing, understanding, and describing flow and transport processes in such a system.Mathematical modeling using a continuum approach involves developing conceptual models, incorporating the geometrical information of a given fracture-matrix system, and setting up the general mass and energy conservation equations for overlapping fracturematrix domains. The majority of the computational effort is used to solve the governing equations that couple fluid and heat flow with chemical migration either analytically or numerically. The key issue for simulating flow and transport in fractured rock is how fracture-matrix interactions under different conditions involving multiple processes are handled. The commonly us...