Stochastic hydrogeological modelling of fractured rocks: a generic case study in the Mórágy Granite Formation (South Hungary)

Abstract

Abstract:In connection with the Hungarian radioactive waste disposal program a detailed study of the mass properties of the potential host rock (granite) has been carried out. Using the results of this study the various parameters (orientation, length, intensity, transmissivity, etc.) describing a fracture set were estimated on the basis of statistical considerations. These estimates served as basic input parameters for stochastic hydrogeological modelling of discrete fracture networks (DFN), which is a strongly developing area of hydrology, providing geologically realistic geometry for site investigations. The synthetic fracture systems generated were tested against some (but not all) field observations. The models built up on the basis of the statistical descriptions showed the same equivalent hydraulic conductivity for the modelled region as the field measurements. In addition, the models reproduce the observed hydraulic head-scattering along vertical boreholes. On the basis of the stochastic simulations of the fracture system some input parameters for the performance assessment of the planned repository were investigated. Calculation of flows into a planned disposal tunnel indicated that if the hydraulic conductivity of the material in the tunnel is the only variable parameter then there are two thresholds: under 1 × 10 -9 m/s and above 1 × 10 -5 m/s further change of the hydraulic conductivity does not dramatically affect the inflow.

Fracture network characterization using 1D and 2D data of the Mórágy Granite body, southern Hungary

“…Calculations on the wall prove that fracture lengths in the studied granitoid body follow power law distribution with length exponent values that are very similar for a wide range of scales. This result is in agreement with Benedek and Dankó (2009), who did not find any fundamental difference between the trace lengths of fractures with different orientations and sizes.…”

“…When studying fracture networks from a geometric aspect, fracture size appears to be related to the distance from major fault zones; larger fractures appear to cluster preferentially around them (Benedek and Molnár, 2013). Nevertheless, length exponents were found varying within a very narrow range (2.15-2.44) at different scales (outcrop scale: 0.4-7 m trace length, vertical seismic profile measurements: 6-40 m trace length, seismic line measurements: 100-400 m trace length, Benedek and Dankó, 2009).…”

“…4. Using an essentially different methodology, Benedek and Dankó (2009) found that fractal dimensions of the fracture networks are very close to 1.0 along boreholes, suggesting a random fracture pattern in space without any significant change. A definite advantage of the present approach compared to that used previously is the ability to sensitively follow variation in fracture geometry parameters along wells and so simulate fracture networks much more reliably.…”

“…Although hydraulically active fracture zones are frequent in the study area, fracture clusters are not entirely interconnected. As a consequence, there is no hydraulic connectivity between all points of the studied region (Benedek and Dankó, 2009). According to such a compartmentalization is not exclusively the result of fracture geometry, but in part the consequence of intensive secondary mineralization of certain fracture zones.…”

Groundwater flow into underground openings in fractured crystalline rocks: an interpretation based on long channels

“…Notwithstanding this difficulty, two projects around the world seeking sites for deep repositories for nuclear waste have begun to describe fractured crystalline rocks in terms of compartments (e.g. Kamaishi research mine in Japan (Sawada et al 2000) and Bátaapáti possible repository site in Hungary (Benedek and Dankó 2009). Other repository investigations have recorded abrupt head changes in deep borehole profiles but possibly only the UK Sellafield investigation had sufficient density to warrant a compartment-based interpretation (Black and Barker 2016).…”