Seismogenic potential of withdrawal-reinjection cycles: Numerical modelling and implication on induced seismicity

Induced seismicity can be associated to the activity of fluid withdrawal and injection from/into the shallow crust (fracking, wastewater disposal into the deep crust, Enhanced Geothermal Systems technology, fluid extraction in oil fields and geothermal power plants). Long-term injection of large volumes of fluids is normally associated with induced seismicity, but the effect of withdrawal-reinjection in the same reservoir is less known, at least regarding its relation to simple injection. However, it is common experience worldwide that small (i.e. 10 MW or less) geothermal plants with withdrawal and re-injection of fluids in the same reservoir are mostly not associated with significant induced/triggered seismicity. This paper aims at understanding how to discriminate, on a numerical modelling basis, the seismogenic potential of withdrawal-reinjection with respect to injection only. With this aim, we analysed the induced pressure changes, the perturbed volumes of rocks and the potential for induced seismicity due to these operations. A set of simulations of injection or withdrawal-reinjection cycles, obtained by using the numerical code TOUGH2®, is applied to simple models of geothermal reservoirs, with varying permeability and lateral boundary constraints. For each permeability model, we then compare the time growth of perturbed volumes obtained with withdrawal-reinjection cycles to those obtained during simple injection, using the same flow rates. The size of perturbed volumes is then related to the maximum magnitude of induced/triggered seismicity, using models accredited in recent literature. Our results show that, for all models, withdrawal-reinjection is by far less critical than simple injection, because the perturbed volumes are remarkably smaller and remain constant over the simulated time, so minimizing the likelihood of interference with seismogenic faults. These results have significant implications for geothermal projects, and in the assessment of the potential risk related to fluid stimulation and induced seismicity.