Measuring Fracture Flow Changes in a Bedrock Aquifer Due to Open Hole and Pumped Conditions Using Active Distributed Temperature Sensing

Abstract

Efficiently measuring groundwater flow in bedrock aquifers is inherently challenging due to the irregular distribution and fine scale of fractures. Recent advances in Active Distributed Temperature Sensing (A-DTS) in boreholes temporarily sealed with liners have made it possible to quantify flow rates in such aquifers at many different depths using heat as a tracer, but until now only data collected under a single hydraulic condition have been published. This paper presents the first field data from multiple A-DTS field tests conducted under different hydraulic conditions to quantify groundwater flow redistribution within a bedrock aquifer. Three separate quasi steady state A-DTS tests were collected in a sealed borehole: (1) natural gradient condition where all boreholes were sealed with flexible and impermeable liners, (2) cross-connected condition where a nearby borehole was open allowing vertical flow within the borehole, and (3) forced gradient condition where the nearby open borehole was pumped at a constant rate of 54 L/min. The depth-discrete hydraulic head responses were also measured during the three tests using a string of transducers in a sealed borehole. Results provide quantifiable insights as to how the bedrock aquifer responds, including A-DTS-derived measurements of flow changes in fractures at multiple depths driven by changes in gradients. The results confirm that a single open borehole or long-screened well can significantly alter the site hydraulics and demonstrate that not all large or transmissive fractures show evidence of active flow and thus, transmissivity and aperture should not be used alone to infer active flow zones. Plain Language Summary Measuring groundwater flow in fractured bedrock aquifers is difficult because flow is primarily controlled by small and irregularly spaced fractures. Very few tools exist to measure the natural flow through fractures in these aquifers, which is essential for understanding contaminant transport flow paths. One emerging technique, called Active Distributed Temperature Sensing (A-DTS), uses a type of fiber optic sensor that can measure temperature at many different intervals along a fiber optic cable. This cable is lowered into a borehole, and a flexible inflatable liner is installed to prevent vertical flow within the borehole. The cable is then heated using integrated heating wires for an extended period, and the temperature response can be used to locate and estimate groundwater flow rates. This study collects field data under three different flow conditions at a site to demonstrate how the flow in a bedrock aquifer responds when it is stressed and the sensitivity of the A-DTS technique. Results demonstrate highly variable flow with depth and that having a single open borehole on a site can strongly affect the natural flow system. A-DTS allows efficient measurement of this variable flow with depth and provides a better understanding of these complex bedrock groundwater systems.

Numerical and Experimental Validation of the Applicability of Active‐DTS Experiments to Estimate Thermal Conductivity and Groundwater Flux in Porous Media

“…Then, considering the need to characterize flow under a natural gradient without the effect of the borehole (Pehme et al., 2010), further developments focused on deploying active‐DTS methods in direct contact with the rock matrix. Active‐DTS experiments have thus been conducted in sealed boreholes (Coleman et al., 2015; Maldaner et al., 2019; Munn et al., 2020; F. Selker & J. S. Selker, 2018) and other promising studies proposed the direct deployment of FO cables vertically into unconsolidated sedimentary aquifers using direct‐push equipment (Bakker et al., 2015; des Tombe et al., 2019). Concurrently, active‐DTS methods were largely developed and applied in unsaturated soils, offering the possibility of estimating the soil water content and thermal properties (Benitez‐Buelga et al., 2014; He, Dyck, Horton, Li, et al., 2018; He, Dyck, Horton, Ren, et al., 2018; Sayde et al, 2010, 2014; Weiss, 2003; Wu et al., 2019) or conducting distributed thermal response test for geothermal energy applications (Vélez Márquez et al., 2018; Zhang et al., 2020).…”

“…Then, considering the need to characterize flow under a natural gradient without the effect of the borehole (Pehme et al, 2010), further developments focused on deploying active-DTS methods in direct contact with the rock matrix. Active-DTS experiments have thus been conducted in sealed boreholes (Coleman et al, 2015;Maldaner et al, 2019;Munn et al, 2020;F. Selker & J. S. Selker, 2018) and other promising studies proposed the direct deployment of FO cables vertically into unconsolidated sedimentary aquifers using direct-push equipment (Bakker et al, 2015;des Tombe et al, 2019).…”

Constraining the lithostratigraphic architecture of a buried bedrock valley using surface electrical resistivity and seismic refraction tomography

“…The Eramosa and Cabot Head formations are generally considered regional aquitards, but the former has been demonstrated to be discontinuous and is absent in the study area (e.g., Golder Associates, 2013;Matrix Solutions Inc., 2017). While the bedrock units can be highly fractured, flow typically occurs along dissolution-enhanced sequence stratigraphic breaks, bedding plane partings, or lithologic contrasts (Priebe et al, 2018, Munn et al 2020). The regional Quaternary geology consists of glacial and non-glacial sediments deposited since the late Pleistocene through multiple cycles of glaciations across Ontario (Karrow, 1967;Chapman and Putnam, 1984;Burt, 2018).…”

“…In rock core, the dolostones of the Ancaster member are notably harder and more resistant to scratching than the overlying Eramosa Formation and underlying Niagara Falls member. Moreover, hydrogeological studies utilizing multilevel systems with many depth-discrete ports and/or deployment of numerous pressure sensors with depth in and around the Guelph area (e.g., Nunes, 2015;Capes, 2017;Skinner, 2019;Johnson, 2020;Munn et al, 2020;Munn et al, submitted;Nunes et al, 2021) show hydraulic head profiles with a significant hydraulic head loss across the Ancaster member, indicating that it acts as a competent aquitard in many locations. Based on this evidence, the Guelph Formation could have supported valley incision until the thalweg reached the mechanically resistive Ancaster member.…”

From Fluid Flow to Coupled Processes in Fractured Rock: Recent Advances and New Frontiers

“…Munn et al. (2020) used DTS fibers to isolate non‐connected, naturally connected, and “forced” connected (i.e., during a pumping test) fractures within a fractured aquifer, which is valuable information for many types of subsurface uses. Recently, Hopp et al.…”