Papers by Author: Cheng Yuan Zhang

Abstract: Coupled THM simulator on FEBEX case is a highly nonlinear system including several nonlinear sub-models. To better understand the coupled THM processes and their influence on the prediction of disposal system behavior, this paper presents an uncertainty study and model optimization on coupled THM simulation under DECOVALEX-THMC framework. It focus on the effect of complexity on model uncertainty which is based on the hypothesis stating that as a model becomes more complex in terms of increased number of processes involved and parameters, the error between simulations and measurements decreases and the overall model sensitivity increases. An uncertainty function is defined which minimizes both error and sensitivity. The result of comparative study basing on different complexity level models of FEBEX case study verifies this hypothesis and indicates that method presented can address uncertainty of numerical model and give us a criterion to choose most suitable model in a relative manner.

Abstract: Task_D of the DECOVALEX_THMC project focus on predictive analysis of the long-term coupled processes(up to 10,000 years) in two generic repositories, FEBEX type and Yucca Mountain Project type for comparison. To better understand the coupled THM processes and their influence on the system behavior, we have introduced a set of generic coupled THM governing equations. Basing on these equations, we develop simplified models according to given Task_D model inception phase request. Boiling model and empirical bentonite swelling model are introduced into general simulation which makes model more practical. Our numerical code FRT-THM upon this practical models is developed and used in two BMT case study of Task_D. Simulation results are shown and verified in the 3rd and 4th workshop of DECOVALEX_THMC in 2005. There is a good agreement with results of different participant teams which enhances confidence in prediction of coupled THM processes.

Abstract: Most rocks are saturated or partly saturated with different fluids under different depth, temperature and pressure conditions. It is generally acknowledged that fluids have the most important effect on the attenuation and dispersion of seismic waves. There exists a relation between frequency- and temperature- dependence on rock’s seismic properties. It is not yet clear in literature whether there exist other equally important attenuation mechanisms as that in Biot’s model, since there are other sources of dissipation, also related to fluids, that are not considered in Biot theory but that may also contribute to the overall dissipation of seismic energy. Identifying the precise relaxation mechanisms is still the subject of experimental and theoretical research. In this article, a series of experiments are conducted on dry and saturated rocks (sandstone, marble, granite) at different temperatures and frequencies to find the attenuation mechanism of interaction between rock skeleton and pore-fluid. Fluid viscosity generally depends on temperature, so the effect of pore fluid on attenuation is confirmed in terms of apparent viscosity variation of rock caused by the change of pore-fluid conditions (such as frequency or temperature). Based on our experimental data, we develop a new model of macroscopic apparent viscosity in saturated rock which is consistent with the nonlinear relaxation law. It helps to derive the analytical expressions to compute velocity dispersion and attenuation as functions of frequency and temperature.

Abstract: Stochastic simulation method considering the heterogeneity of hydraulic conductivity is suggested in this paper. Hydraulic field is simulated by using our FEM code for THM coupling processes. The results show that hydraulic gradient at different location around the cavern is differently dependent on water-curtain pressure in deterministic model. A probability range of hydraulic gradient at each point with 10m space between each was examined by means of stochastic realization of spacial variability of hydraulic conductivity. These analyses are contributed to determine the values interval of water-pressure and groundwater flux into the cavern with a given security for LPG containment design and evaluation considering the spacial variability of hydraulic conductivity.