Characterization of a Geothermal Reservoir Crack Based on the Analysis of Dynamic Response of a Crack Filled with Fluid

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The objective of the present project is the establishment of a method for characterizing the geometrical and physical characteristics of a fluid-filled geothermal reservoir crack. Firstly, we have proposed a model for a fluid-filled crack within an elastic medium with permeation of fluid. The upper and lower surfaces are in partial contact to each other, thus there exists interface stiffness along the crack, and also the fluid viscosity is taken into account. It has been revealed that, in the case that the partial contact is absent, the eigen angular frequencies are highly dependent on the aspect ratio (the ratio between the crack size and aperture), when the viscosity and permeability are small. However, in the case that the partial contact is present, the eigen angular frequencies are highly dependent on the interfacial stiffness. When the permeability is large as that of a formation in a usual geothermal field, then the effects of the permeability and viscosity become stronger. In order to get clear understanding of the effects of permeability and fluid viscosity, we have analyzed the case that the rock is rigid and have found that there exists an optimum value of fluid viscosity which gives the largest pressure response. The optimum value of fluid viscosity is mostly determined by the permeability of the formation. Inversely speaking, it is possible to get the maximum pressure response by choosing the fluid with the optimum value of viscosity depending on the formation permeability. Finally, on the basis of the results of the analysis, we have set up the inverse problem to estimate the aspect ratio of the crack, the initial crack aperture and the interfacial stiffness by using a newly introduced objective function.