| Authors: |
R. Tung, T. Poulet, M. Peters, M. Veveakis, K. Regenauer-Lieb |
| Keywords: |
THMC couplings, fault zone permeability, hydrothermal convection, chemical reactions, mechanical instabilities |
| Conference: |
New Zealand Geothermal Workshop |
Session: |
Session 1.1 Reservoir Modelling |
| Year: |
2018 |
Language: |
English |
| Abstract: |
Geothermal systems portray a rich style of coupled Thermal, Hydraulic, Mechanical, and Chemical (THMC) processes that underpin the heat transfer in a variety of geothermal manifestations. A particularly interesting phenomenon is the fault-bounded transfer of hot fluid in settings such as the Soultz-sous-Forêts graben-hosted geothermal resource. This data-rich environment allows detailed insights into the processes and serves as a benchmark for numerical modelling of such systems. Conceptual models can be devised that duplicate the anomalous thermal gradient by assigning arbitrarily high permeabilities to fault zones. However, the models do not match geological/geophysical observables of these fault zones, which in particular have a much lower permeability than the best-fit values of the models. Here we show - in a systematic numerical analysis - the potential contribution of TH, THM, and THMC feedback processes that can reconcile geophysical/geological observations with the observed heat transfer. The classical TH models without additional feedbacks require unrealistically high permeabilities (10−14 − 10−15m2), and too wide fault zones. THM solutions require extreme deformation rates (10−13s−1) to match the heat profile with the observed permeability. We suggest that a THMC solution could reproduce the observations with realistic values of permeability (10−16m2), fault zone widths (≪ 100m), and strain rates (10−16s−1). Our analysis shows that fault zone permeability can be a dynamic process significantly enhanced by chemical reactions. In a fully-coupled system, these reactions are oscillating at relatively short timescales and allows us to couple geodynamic timescales with shorter engineering timescales. The implications of this analysis also have bearings on borehole stability and reservoir integrity, and may open new pathways for reservoir stimulation. |
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