New Seismic-Hydraulic Approach to Modeling Flow in Fractured Rocks

摘要

Modeling flow of ground water in fractured rock is a major problem. This paper summarizes a program of investigations currently underway in this laboratory to characterize the geometry of fractured rocks and develop methods of handling flow in such systems. Numerical models have been developed to investigate flow behavior in two- and three-dimensional fracture networks. Two-dimensional studies of fracture interconnection indicate that systems with shorter fracture lengths have smaller permeabilities and behave less like porous media than those with longer lengths. Studies of transport indicate that the effects of increasing the coefficient of variation of aperture are opposite to those of increasing the correlation between length and aperture. Studies of scale and boundary effects indicate that the size of the representative elementary volume for homogeneously fractured systems may only be a function of fracture frequency. These are some examples of the insights to be gained from modeling studies of fractured rocks. A key problem is gathering the necessary data on fracture geometry. Investigations have been started to determine how vertical seismic profiling (VSP) might be improved and applied to this problem. According to theory, a fracture can have a significant effect on shear wave propagation in rock systems. Laboratory measurements show a significant attenuation of shear waves as fracture stiffness decreases and frequency increases. A VSP experiment in The Geysers geothermal field where fracture orientation is known, produced shear wave splitting and velocity anisotropy in agreement with theory. The results suggest the potential application of 3-component, multi-source VSP data in determining fracture orientation and average spacing. A downhole seismic wave generator would greatly improve VSP, and we are developing such a device. (ERA citation 12:021263)