Investigating Water Movement Within and Near Wells Using Active Point Heating and Fiber Optic Distributed Temperature Sensing

Abstract

There are few methods to provide high-resolution in-situ characterization of flow in aquifers and reservoirs. We present a method that has the potential to quantify lateral and vertical (magnitude and direction) components of flow with spatial resolution of about one meter and temporal resolution of about one day. A fiber optic distributed temperature sensor is used with a novel heating system. Temperatures before heating may be used to evaluate background geothermal gradient and vertical profile of thermal diffusivity. The innovation presented is the use of variable energy application along the well, in this case concentrated heating at equally-spaced (2 m) localized areas (0.5 m). Relative to uniform warming this offers greater opportunity to estimate water movement, reduces required heating power, and increases practical length that can be heated. Numerical simulations are presented which illustrate expected behaviors. We estimate relative advection rates near the well using the times at which various locations diverge from a heating trajectory expected for pure conduction in the absence of advection. The concept is demonstrated in a grouted 600 m borehole with 300 heated patches, though evidence of vertical water movement was not seen.

Determining the Relation between Groundwater Flow Velocities and Measured Temperature Differences Using Active Heating-Distributed Temperature Sensing

“…The calculated velocities are in line with the measured velocities. The modeled groundwater flow velocity using Equation (13) results in a good fit to the measurement data for low ∆ (∆ < 7 °C) and accordingly higher flow velocities (Figure 8). For higher ∆T, the flow velocity is overestimated by the equation compared to the measurement data.…”

“…The improvements to the cable setup, the use of two heating wires directly connected to the fiber-optic cable, made the results from the 2018 dataset more reliable. The distance between the heating wire and fiber-optic cable in the numerical modeling efforts and solving Equations (6) and (13), was now known and constant. The heating cycles were increased up to 90-120 min duration in the 2018 experiments (2017: 30-45 min) resulting in more reliable fits for the equilibrium temperatures.…”

“…Using the estimated constants C and m, Equation (13) is applied to estimate the groundwater flow velocity based on the measured ∆T. The result is plotted together with the experimental results (mean values per groundwater flow velocity) for the 2018 data ( The modeled groundwater flow velocity using Equation (13) results in a good fit to the measurement data for low ∆T (∆T < 7 • C) and accordingly higher flow velocities (Figure 8). For higher ∆T, the flow velocity is overestimated by the equation compared to the measurement data.…”

“…The promise is that using the experimentally determined values of C and m and assuming these to be applicable for the current cable setup, Equations (7), (9) and (12)- (14) can be used to determine groundwater flow velocities from observations of ∆T, from a similarly configured cable under field conditions as authors intend to test. The influence of the cable presence on the groundwater flow is incorporated in the FlexPDE modeling but is not specifically considered in Equation (13). The constants C and m are however fitted based on the laboratory data results, including this effect in the fit.…”

“…The relation between the transient temperature response and groundwater flow will depend on the thermal inertia of the passive cable used to detect this response, the thermal properties of the lithology and the rate and direction of groundwater flow close to the cable [6,8,9]. Several studies applied the AH-DTS setup in boreholes and well screens to estimate the inflow of groundwater and the background groundwater flow itself, where DTS cables are placed in the bore of uncased boreholes, mostly in fractured rock settings [6,[10][11][12][13]. The potential of the AH-DTS methodology using cables in direct contact with unconsolidated sediments was first shown by [7].…”