Stiff, Smooth, and Solid? Complex Fracture Hydraulics' Imprint on Oscillatory Hydraulic Testing

• Published in: Water resources research Vol. 59; no. 11
• Main Authors: Patterson, Jeremy R., Cardiff, Michael
• Format: Journal Article
• Language: English
• Published: 01.11.2023
• Subjects: coupled processes; fracture characterization; fractured rock; numerical modeling; oscillatory flow; period‐dependence
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2023WR034621

Fractured bedrock aquifers, especially deep aquifers, represent increasingly common targets for waste storage and alternative energy development, necessitating detailed quantitative descriptions of fracture hydraulic properties, geometry, and connectivity. Yet, multi‐scale characterization of the physical properties that govern fluid flow through and storage in fractured bedrock remains a fundamental hydrogeologic challenge. Oscillatory hydraulic testing, a novel hydraulic characterization technique, has been showing promise in field experiments to characterize the effective hydraulic properties of bedrock fractures. To date, these characterization efforts utilize simplified diffusive analytical models that conceptualize a non‐deforming, parallel‐plate fracture embedded within impermeable host rock, and have found that the returned fracture hydraulic parameter estimates exhibit an apparent period‐dependence. We conduct synthetic experiments using three different numerical models to examine proposed mechanisms that might contribute to the observed period‐dependence including heterogeneous flow and storage within the fracture (i.e., aperture heterogeneity), fracture‐host rock fluid exchange, and fracture hydromechanics. This work represents the first systematic analysis that seeks to understand the process(es) occurring within a bedrock fracture that might be contributing to this apparent period‐dependence. Our analysis demonstrates that all investigated mechanisms generate period‐dependent effective hydraulic parameter estimates, each with their own potentially diagnostic trends; however, fracture hydromechanics is the only explored mechanism that consistently reproduces period‐dependent trends in parameter estimates that are consistent with existing field investigations. These results highlight the need to develop more complex numerical modeling approaches that account for this hydromechanical behavior when characterizing fractured bedrock aquifers. Plain Language Summary Deep aquifers frequently contain multiple fractures that provide channels for rapid transport and storage of water, heat, and contaminants through the subsurface, making them increasingly popular resources for alternative energy and waste storage. Building computer models that create accurate predictive simulations of flow and transport through fractured bedrock requires detailed knowledge and description of the physical properties that govern flow and transport through and the hydraulic processes occurring within these fractures. Periodic pressure testing has recently been used to determine these hydraulic properties; however, they have been found to be dependent on the period of the pressure signal, which indicates the presence of hydraulic processes occurring within the fracture that are not considered during analysis. We use three computer models to investigate potential mechanisms that might contribute to this apparent period‐dependence, and we find that fracture displacement due to pressure changes along the fracture leads to inaccurate averaged fracture flow parameter estimates when simple analytical expressions are used during data analysis. Key Points Heterogeneous fracture storage, fracture‐host rock fluid exchange, and hydromechanics all produce period‐dependent effective parameters Each explored mechanism produces distinct—potentially diagnostic—period‐dependent hydraulic parameter estimates Fracture hydromechanics is the only mechanism that reproduces period‐dependent parameter trends described in previous field experiments