THM Modeling of Cold Front Migration and Microseismic Response in a Fractured Geothermal Reservoir

Abstract

Reinjection of cold fluid is essential for sustaining deep geothermal systems but inevitably perturbs the reservoir stress state through coupled thermo-hydro-mechanical (THM) processes, triggering microseismicity. A critical challenge in reservoir management is distinguishing between hydraulic and thermal triggering mechanisms, which tend to exhibit distinct spatiotemporal characteristics. In this study, we investigate the spatiotemporal evolution of induced microseismicity using a fully coupled 2D THM model in conjunction with the discrete fracture network (DFN) approach. By comparing isothermal and non-isothermal injection scenarios, we isolate the specific contributions of poroelastic pressure diffusion and thermoelastic contraction to fracture slip. We find that while pressure perturbations propagate rapidly and induce diffuse, widespread seismicity at early times, the dominant triggering mechanism progressively shifts toward thermoelastic stressing. Analysis of Coulomb failure stress changes (ΔCFS) reveals that cooling-induced instability forms a localized band that co-migrates with the advancing thermal front. Unlike the pressure front, which quickly decouples from the microseismic cloud, the late-time seismic front aligns closely with the cold domain. These findings demonstrate that induced microseismicity is not merely a hydraulic byproduct but can serve as a reliable tracer for cold-front migration, offering a potential tool for forecasting thermal breakthrough in fractured reservoirs.