Period Dependence in Flow Properties of a Fractured Bedrock Aquifer: Investigating Heterogeneity and Fluid Exchange as Potential Sources

Abstract:

Fractures in sedimentary bedrock aquifers act as preferential flow paths that serve to enhance mass and energy transport. Given this, extensive research efforts have been dedicated to developing methods that characterize and model fracture flow and transport in an accurate manner. Recent studies have implemented periodic pressure signals to characterize fracture flow properties; however, the estimated flow parameters are found to be dependent on the input period. To date, there has been no systematic investigation into the source of this period dependence. Our work investigates a plausible range of scenarios that may explain this observed period dependence.

We conducted more than 100 cross-well periodic pumping tests – across a range of signal frequencies and distances – at a fractured sedimentary bedrock aquifer near Madison, WI. Through the use of inflatable packers, we isolated and tested a single fracture across a range of periods from 4 to 360 seconds and inter-well spacings from 5 to 15 meters to estimate effective flow properties of the fracture using established analytical solutions. Our analysis indicates that estimated diffusivity decreases and estimated storativity increases with increasing input period, consistent with previous studies, if homogeneous fracture properties are assumed. Further, we find that for a given input period estimated diffusivity increases and estimated storativity decreases with increasing radial distance from the stimulation well, which has not been previously been reported. Through the use of analytical and numerical modeling, we test multiple conceptual frameworks to understand the extent to which fracture aperture heterogeneity, fluid exchange between the fracture and host rock or a combination of the two can explain the observed frequency dependence in these estimated fracture flow parameters.