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Do Simple Analytical Models Capture Complex Fractured Bedrock Hydraulics? Oscillatory Flow Tests Suggest Not.
Jeremy R Patterson, Michael Cardiff1
1Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin, USA.
Ground Water
|February 6, 2023
Summary
Characterizing fractured bedrock aquifers is challenging. Oscillatory flow testing reveals period-dependent parameters, suggesting complex flow dynamics beyond simple models for fractured sedimentary rock.
Area of Science:
- Hydrogeology
- Geosciences
- Environmental Science
Background:
- Fractured sedimentary bedrock aquifers exhibit complex flow, combining fast fracture and slow porous media pathways.
- Characterizing flow and transport in these systems is a significant hydrogeologic challenge.
- Previous studies utilized oscillatory flow testing for single bedrock fractures in low-porosity settings.
Purpose of the Study:
- To apply oscillatory flow testing across various frequencies and spacings to a fracture in poorly cemented sedimentary bedrock.
- To investigate the period-dependent flow parameters observed in previous studies.
- To explore potential mechanisms contributing to observed hydraulic responses.
Main Methods:
- Field application of oscillatory flow testing at a range of frequencies and inter-well spacings.
- Data interpretation using an idealized analytical model assuming Darcian flow.
- Analysis of non-Darcian flow, borehole storage, and fluid leakage to host rock.
Main Results:
- Apparent period-dependence in flow parameters was confirmed: hydraulic diffusivity decreased and storativity increased with longer oscillation periods.
- Idealized fracture models and analyses of potential artifacts (non-Darcian flow, leakage) did not fully explain the observed responses.
- The study highlights the limitations of current models in representing pressure propagation in fractured sedimentary bedrock.
Conclusions:
- The period-dependent parameters observed during oscillatory flow testing in fractured sedimentary bedrock are not adequately explained by idealized fracture models or simple alternative mechanisms.
- Complex processes such as aperture heterogeneity and fracture hydromechanics are likely crucial for accurately modeling pressure propagation.
- Further research is needed to develop more comprehensive conceptual models for fractured bedrock aquifer systems.
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