Thermal Attenuation and Lag Time in Fractured Rock: Theory and Field Measurements From Joint Heat and Solute Tracer Tests

Abstract

The modeling and prediction of heat transfer in fractured media is particularly challenging as hydraulic and transport properties depend on a multiscale structure that is difficult to resolve. In addition to advection and dispersion, heat transfer is also impacted by thermal attenuation and lag time, which results from fracture-matrix thermal exchanges. Here we derive analytical expressions for thermal lag time and attenuation coefficient in fractured media, which quantify the effect of fracture geometry on these key factors. We use the developed expressions to interpret the results of single-well thermal and solute tracer tests performed in a crystalline rock aquifer at the experimental site of Ploemeur (H+ observatory network). Thermal breakthrough was monitored with fiber-optic distributed temperature sensing (FO-DTS), which allows temperature monitoring at high spatial and temporal resolution. The observed thermal response departs from the conventional parallel plate fracture model but is consistent with a channel model representing highly channelized fracture flow. These findings, which point to a strong reduction of fracture-matrix exchange by flow channeling, show the impact of fracture geometry on heat recovery in geothermal systems. This study also highlights the advantages to conduct both thermal and solute tracer tests to infer fracture aperture and geometry. Key Points:• We present expressions for thermal lag time and attenuation coefficient, quantifying the impact of flow channeling on heat transfer in fractured media • Joint solute and thermal tracer tests support analytical results and provide new constraints on fracture aperture and flow topology • Single-well thermal tracer tests offer an alternative to cross-borehole thermal tracer tests for characterizing thermal transport in the field

Tracking Fluid Flow in Shallow Crustal Fault Zones: 2. Insights From Cross‐Hole Forced Flow Experiments in Damage Zones

“…The storage and transport of fluids in fault zones is generally assumed to be highly partitioned owing to the structural complexity of fault‐related fracture systems (Bonnet et al, ; Kim et al, ; Sibson, ; Tchalenko, ) and sharp contrasts in hydraulic properties (Bense et al, ; Brace, , ; Caine et al, ; Evans & Goddard, ; Roques et al, ). This affects processes in the Earth's crust operating on a broad range of scales, from the emergence of flow pathways (Tsang & Neretnieks, , and references therein) channeling solute (Kang et al, ) and heat transport (De La Bernardie et al, ; Fox et al, ; Geiger & Emmanuel, ; Klepikova et al, ) to convecting instabilities (Murphy, ; Patterson et al, ), frictional heating (Mase & Smith, ; Rice, ; Vredevoogd et al, ), dynamic fault weakening (Byerlee, ), and earthquake sequences (Jansen et al, ; Miller et al, ; Noir et al, ; Nur & Booker, ; Ross et al, ; Shapiro et al, ; Wang et al, ). Tracking how fluids migrate in faults in situ can therefore provide insights on key geological and physical processes.…”

Dipole and Convergent Single-Well Thermal Tracer Tests for Characterizing the Effect of Flow Configuration on Thermal Recovery

“…Heat has been widely used as active and passive tracers for the characterization of thermal transport and hydrological processes in aquifers [1][2][3][4][5][6][7]. For instance, it has been shown in many examples during the last 20 years that the use of heat is very efficient for characterizing surface-groundwater exchanges [6,8].…”

“…Some other studies have shown that borehole temperature profiles under induced fluid flow conditions by pumping in an adjacent borehole could be used to estimate fracture interconnections [20] and hydraulic properties [21]. Very recently, thermal tracer tests have also shown their efficiency to demonstrate the impact of flow channeling on heat transport [5,22,23]. In particular, it has been shown that flow channeling may highly reduce thermal transit times and improve thermal recovery during thermal tracer tests [5,22].…”

“…Very recently, thermal tracer tests have also shown their efficiency to demonstrate the impact of flow channeling on heat transport [5,22,23]. In particular, it has been shown that flow channeling may highly reduce thermal transit times and improve thermal recovery during thermal tracer tests [5,22]. Nevertheless, it remains very challenging to achieve thermal tracer tests in fractured crystalline media due to both the relatively low permeability of fractured rocks in general [24][25][26] and the important thermal lag time and attenuation induced by conduction of heat within the matrix [5,21,27].…”

“…In particular, it has been shown that flow channeling may highly reduce thermal transit times and improve thermal recovery during thermal tracer tests [5,22]. Nevertheless, it remains very challenging to achieve thermal tracer tests in fractured crystalline media due to both the relatively low permeability of fractured rocks in general [24][25][26] and the important thermal lag time and attenuation induced by conduction of heat within the matrix [5,21,27]. As a consequence, although heat tracer tests in fractured media are crucial for predicting and optimizing geothermal storage and extraction in crystalline rock, only few thermal tracer tests have been achieved in fractured media to our knowledge, and little is known about thermal injection and recovery in such complex media.…”