Influence of pore‐pressure on the event‐size distribution of induced earthquakes

TLDR

High‑pressure fluid injection into crystalline rock during an Enhanced Geothermal System experiment enhances permeability but induces fracturing that generates microseismicity. The study aims to link pore‑pressure to the seismic frequency–magnitude distribution, specifically the b‑value, to understand how injection rates affect event magnitudes. Using Basel EGS data, the authors compare the observed event‑size distribution with a minimalistic pore‑pressure evolution model that relates event sizes to differential stress, finding that b‑values decline with distance as pore‑pressure decreases. Consequently, the probability of large‑magnitude events rises with distance from the injection point and over time.

Abstract

During an Enhanced Geothermal System (EGS) experiment, fluid is injected at high pressure into crystalline rock, to enhance its permeability and thus create a reservoir from which geothermal heat can be extracted. The fracturing of the basement caused by these high pore‐pressures is associated with microseismicity. However, the relationship between the magnitudes of these induced seismic events and the applied fluid injection rates, and thus pore‐pressure, is unknown. Here we show how pore‐pressure can be linked to the seismic frequency–magnitude distribution, described by its slope, the b ‐value. We evaluate the dataset of an EGS in Basel, Switzerland and compare the observed event‐size distribution with the outcome of a minimalistic model of pore‐pressure evolution that relates event‐sizes to the differential stress σ D . We observe that the decrease of b ‐values with increasing distance of the injection point is likely caused by a decrease in pore‐pressure. This leads to an increase of the probability of a large magnitude event with distance and time.