Describing Connected Fracture Flow with Pressure Waves – Oscillating Flow Interference Testing

The state of Wisconsin is blessed with abundant groundwater resources resulting from the thick, high-porosity and high-permeability sedimentary aquifers that underlie the state, as well as the humid climate it experiences. In managing the state’s resources, a common modeling assumption is that these aquifers behave as traditional porous media, in which flow occurs dominantly through pores between individual sediment grains. Investigations in the state of Wisconsin by the WGNHS and other researchers (e.g., Runkel et al. 2006) have demonstrated that both laterally-continuous horizontal fractures and high-angle (near-vertical) fractures in these sedimentary units heavily influences their hydrogeologic behavior. For example, very fast travel times – between the surficial aquifer in Madison, WI and the deep sandstone aquifer tapped for water supply – that are not representative of porous media flow have been demonstrated recently by Bradbury et al. (2013) through analysis of virus transport. Understanding the hydraulic behavior of these fractures will thus be crucial for future modeling efforts that simulate flow and transport in the fractured rock aquifers underlying much of Wisconsin.

The research project for this grant performed field characterization of fractures at two sites in Dane County and Iowa County, Wisconsin where multiple observation wells were available. To characterize these fractures, we utilized a novel technology, multi-frequency oscillatory testing (in which a pumping well alternatingly injects and extracts fluid from the fractured interval).

Specific objectives of the project were as follows:

1. To test the ability of cross-well Multi-frequency Oscillatory Hydraulic Testing (M-OHT) to produce measurable and analyzable signals at fractured rock sites where fracture connectivity has been previously inferred;

2. To use data from M-OHT testing to characterize the spatial “flow dimension” and hydraulic properties of fractures at each of the research sites; and

3. To develop an apparatus for performing single-well M-OHT testing, such that vertical connection between different fractured intervals could be assessed.