Cumulative Controls on Intermediate-Field Seismic Migration: Comparative Evidence from Three Geothermal Stimulation Campaigns

Injection-induced seismicity in Enhanced Geothermal Systems (EGS) can migrate hundreds of meters from the well and often persists after shut-in, raising operational and hazard concerns. Here we present a cross-site comparative analysis of three stimulation campaigns—Soultz-sous-Forêts (France, 1993), Basel (Switzerland, 2006), and Utah FORGE Stage 3 (USA, 2022)—to identify the dominant controls on intermediate-field seismic migration.

Using a unified dynamic time-windowing framework, we track seismic front evolution via three complementary distance metrics and evaluate their relationships with injection rate, wellhead pressure, cumulative injected volume, hydraulic energy, seismicity rate, and modeled pore pressure at the migration front. Across all sites, cumulative variables—particularly injected volume, hydraulic energy, and injection duration—show the strongest and most consistent correlations with seismic front expansion, whereas instantaneous parameters exhibit weaker or site-specific influence.

Post-injection behaviors distinguish three migration regimes: (i) a pressure-limited regime at Soultz, where the front halts immediately after shut-in; (ii) a diffusion-dominated regime at Basel, with continued post-shut-in propagation; and (iii) a stress-sensitive, limited-diffusion regime at Utah FORGE, characterized by rapid early migration followed by stagnation. Building on these contrasts, we introduce a six-indicator radar classification that quantitatively distinguishes the three regimes.

Our results show that cumulative hydraulic forcing provides transferable, physically interpretable predictors of intermediate-field migration and that distinct post-shut-in signatures reflect underlying connectivity and stress conditions. This comparative framework supports improved seismic hazard assessment and operational planning for geothermal reservoir stimulation.