Coupled THM Modelling of Induced Seismicity Associated with Geothermal Fluids Re-Injection: The Role of Transient Cooling-Induced Permeability Enhancement

Abstract

ABSTRACT: Seismicity induced by injection of energy depleted geothermal fluid, as well as field injectivity, has been observed to be inversely correlated with the temperature of re-injected fluids at the Hellisheiöi geothermal field, Iceland. Using the Hellisheiöi field injection data, transient cooling-induced permeability enhancement as a distinctive mechanism for induced seismicity in geothermal reservoirs was investigated. A 3D coupled THM model was developed to model the colder fluids re-injection process over a 1-year period and evaluate the potential for induced seismicity at Hellisheiöi. Three modelling scenarios considering respectively the poroelastic effect, thermoporoelastic effect, and thermoporoelastic effect with permeability enhancement, were examined and compared to identify the dominant mechanism for induced seismicity and examine the contribution from each individual mechanism. Results have shown that, under normal fluid re-injection pressure and temperature conditions, the permeability enhancement effect is the dominant mechanism for induced seismicity at Hellisheiöi. In addition, by reducing temperature of re-injected fluids from 120 °C to 20 °C, the amount of heat transferred can be increased by 8.4 times, which explains the high sensitivity of induced seismicity to temperature. The findings of this work suggest that temperature control of injected fluids can be a feasible regulation method to mitigate injection-induced seismic risk. 1. INTRODUCTION Induced seismicity has long been a major challenge faced by the geothermal industry, as well as other industries concerned with fluid injection into or resources extraction from the subsurface. In addition to adverse physical effects on buildings and facilities in the surrounding communities, the magnitude and intensity of injection-induced seismicity associated with current and future geothermal exploitation remain a great public concern. Typical mechanisms identified for induced seismicity in geothermal environments include fluid pore pressure increase, poroelastic stressing, thermoelastic stressing, and chemical alteration of frictional properties of fractures (Majer et al. 2007; Rathnaweera et al. 2020). As a specific mechanism relevant to geothermal systems, the thermoelastic effect has been of particular research interest in recent years (e.g., Jeanne et al. 2014a). The thermoelastic stress change, analogous to the poroelastic stress change, influences the occurrence of induced seismicity by changing the prevailing stress states.