MODELING GAS TRANSPORT AND DETECTION IN FRACTURED ROCK

The detection of gas-component release from the subsurface depends on the gas-phase transport processes in the geological system. The geological structure, as well as many physical and chemical properties, controls the concentration profile of gas components at and near ground surface. The detectability of gas components is characterized by (1) arrival time; (2) detection window width; and (3) detection window height (above the detection threshold). The arrival time is defined as the time when the concentration of a gas component reaches its detection threshold. The detection window width is the time duration between the arrival time and the time when the concentration drops below the threshold by radioactive decay. The detection window height is measured as the order of magnitude of the peak concentration higher than the threshold. The physical model is conceptualized as a dual permeability presentation of overlapping fracture and matrix continua. The fracture system is characterized by its frequency, aperture size, permeability, and orientation. In this study, we use numerical simulation of non-isothermal multi-phase and multi-component transport to investigate how gas components release and transport in fractured rock system. Taking those four fracture parameters as uncertain parameters in the model of gas transport, we describe the use of uncertainty quantification software called PSUADE to investigate the role of uncertain parameters in the detectability of gas components. As demonstrated in the applications, sensitivities of detectability characteristics in terms of fracture parameters are provided.