Transient hydraulic tomography approach to characterize main flowpaths and their connectivity in fractured media

Abstract

In sparse fracture systems flow and transport patterns are often dominated by main flowpaths and characterized by a strong complexity induced by the fractures’ surface roughness, hierarchical arrangement in networks, and from the interaction of the fractures with the surrounding rocks. The accurate characterization of the hydraulic properties and connectivity of major fracture zones is essential to model flow, solute and heat transport and fluid pressure propagation in fractured media. In this study, we present a novel deterministic inverse modeling method for imaging the connectivity and spatial variability of transmissivity and storativity of a network of fractures. The method is based on a numerical model that simulates fluid flow in a simple three-dimensional (3-D) discrete fracture network (DFN) with a fixed fracture network structure. The forward model is coupled to an inverse algorithm to match observed pressure transients obtained from sequential hydraulic cross-hole tests. This method has been successfully tested for constant rate injection tests carried out at the Grimsel Test Site (GTS) in Switzerland. Cross-hole injection tests were conducted in a tomographic configuration, with hydraulic responses monitored at four observation intervals at various depths in two different boreholes. A discrete fracture network approach with a simple parametrization is developed to estimate log-transformed transmissivity (T) and storativity (S) values of hydraulically active fractures between boreholes by inverting the pressure transients, under the hypothesis that T and S are independent variables. We identified several permeable fractures and their connectivity without attempting to represent explicitly the true fracture network geometrical properties (length, orientation, dip), focusing instead on the calibration of a simple model capturing the observed pressure main behavior. The identified inter-borehole connectivity structure agrees well with independent information, including additional data from a cross-borehole step rate injection and hydrogeophysical data. Hence, the proposed tomography approach appears to be a promising approach for characterizing the connectivity structure and hydraulic properties of the main flowpaths in sparsely fractured rock.