Several studies have explored the feasibility of Reservoir Thermal Energy Storage (RTES) in the Portland Basin, Oregon (Burns et al, 2018, 2020). Thermal-hydrological models by the latter authors have shown the effectiveness of open system heat and cold fluid storage within permeable interflow zones in the Columbia River basalts. The potential for induced or triggered seismicity and surface deformation associated with RTES has not been evaluated quantitatively in the Portland Basin, and as well as in most other RTES projects worldwide. In this study we developed a Thermal-Hydrological-Mechanical (THM) model of an idealized Portland Basin site and the simulation of a scenario similar to that explored by Burns et al. (2016). The simulation showed that thermal changes on the confined aquifer, particularly during the cooling cycle, caused near-wellbore thermal contraction and fracturing. Surface subsidence reached a maximum of about 8mm at the producing well, however alternation of injection and production in the wells could limit this effect. Given that the injection and production zones were placed at deeper levels than many areas in the Portland Basin, there is the potential for greater surface deformation locally. Changes in horizontal stresses of about 0.5 MPa extend almost a km from the wells, so the potential for triggered seismicity should be evaluated with more detailed structural models.

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