A discrete fracture hybrid model for forecasting diffusion-induced seismicity and power generation in enhanced geothermal systems

Abstract

SUMMARY We describe a 3-D discrete fracture hybrid model (DFHM) that returns forecasts of both induced seismicity and of power generation in an Enhanced Geothermal System (EGS). Our model considers pore-pressure increase as the mechanism driving induced seismicity, similarly to other hybrid models, but it uses discrete fracture modelling for flow and heat that allows accurate and realistic transient solutions of pore pressure and temperature in fractured reservoirs. Earthquakes and flow are thus considered as closely coupled processes. In the DFHM model, the creation phase of an EGS is described as a Markovian process with a transitional probability that encapsulates the irreducible uncertainty with regards to induced seismicity. We conditioned this transitional probability on field observations from the 2006 EGS project in Basel, achieving a good match with observations of seismicity evolution. Specifically, our model effectively reproduces and explains the observed long-term exponential decay of seismicity after the well was shut in, suggesting that pore pressure diffusion in a critically stressed fractured reservoir is sufficient to explain long-lasting post-injection seismic activity as observed in Basel. We then investigate alternative injection scenarios, using Monte Carlo simulations to capture the uncertainties in fault locations and stressing conditions. We show that the number of induced events depends not only on the total injected volume but also on the injection strategy. We demonstrate that multistage injection schemes are superior to single-stage ones, since the former are associated with less seismic risk and can generate at least the same revenue in the long term.