Hydraulic pathways in the crystalline rock of the KTB

Fracture systems and fluid pathways must be analysed in order to understand the dynamical processes in the upper crust. Various deterministic as well as stochastic fracture networks in the depth section of the Franconian Line ament (6900 to 7140 m), which appears as a brittle ductile shear zone and p rominent seismic reflector, were modelled to simulate the hydraulic situati on at the two boreholes of the Continental Deep Drilling Program (KTB). The y led to estimations of the hydraulic permeability in crystalline rock. The geometrical parameters of the fractures, such as fracture locations and or ientations, were determined from structural borehole measurements, which cr eate an image of the borehole wall. The selection of potentially open fract ures was decided according to the stress field. Only fractures with the dip direction (azimuth) of the fracture plane perpendicular to the maximum hor izontal stress field were assumed to be open. The motivation for this assum ption is the fact that the maximum horizontal stress is higher than the ver tical stress from the formation, indicating that the state of stress is a s trike-slip faulting. Therefore, the probability of open fractures due to th is particular stress field at the KTB sites is enhanced. Length scales for fracture apertures and extensions were stochastically var ied and calibrated by hydraulic experiments. The mean fracture aperture was estimated to be 25 mu m, assuming an exponential distribution, with corres ponding permeability in the range of 10(-16) m(2). Similar results were als o obtained for log-normal and normal distributions, with a variation of per meability of the order of a factor of 2. The influence of the fracture leng th on permeability of the stochastic networks was also studied. Decreasing the fracture length beyond a specific threshold of 10 m led to networks wit h vanishing connectivity and hence vanishing permeability. Therefore, we as sume a mean fracture length exceeding the threshold of 10 m as a necessary assumption for a macroscopic hydraulically active fracture system at the KT B site. The calculated porosity due to the fracture network is of the order of 10(-3) per cent, which at first sight contradicts the estimated matrix porosity of 1 to 2 per cent from borehole measurements and core measurement s. It can be concluded from these results, however, that if the fluid trans port is due to a macroscopic fracture system, only very low porosity is nee ded for hydraulic flow with permeabilities up to several 10(-16) m(2), and hence the contribution of matrix porosity to the hydraulic transport is of a subordinate nature.