Limited Earthquake Interaction During a Geothermal Hydraulic Stimulation in Helsinki, Finland

Abstract

We investigate induced seismicity associated with a hydraulic stimulation campaign performed in 2020 in the 5.8 km deep geothermal OTN‐2 well near Helsinki, Finland as part of the St1 Deep Heat project. A total of 2,875 m3 of fresh water was injected during 16 days at well‐head pressures <70 MPa and with flow rates between 400 and 1,000 L/min. The seismicity was monitored using a high‐resolution seismic network composed of 10 borehole geophones surrounding the project site and a borehole array of 10 geophones located in adjacent OTN‐3 well. A total of 6,121 induced earthquakes with local magnitudes MLnormalHnormalenormall>−1.9 ${M}_{\mathrm{L}}^{\mathrm{H}\mathrm{e}\mathrm{l}} > -1.9$ were recorded during and after the stimulation campaign. The analyzed statistical parameters include magnitude‐frequency b‐value, interevent time and interevent time ratio, as well as magnitude correlations. We find that the b‐value remained stationary for the entire injection period suggesting limited stress build‐up or limited fracture network coalescence in the reservoir. The seismicity during the stimulation neither shows signatures of magnitude correlations, nor temporal clustering or anticlustering beyond those arising from varying injection rates. The interevent time statistics are characterized by a Poissonian time‐varying distribution. The calculated parameters indicate no earthquake interaction. Focal mechanisms suggest that the injection activated a spatially distributed network of similarly oriented fractures. The seismicity displays stable behavior with no signatures pointing toward a runaway event. The cumulative seismic moment is proportional to the cumulative hydraulic energy and the maximum magnitude is controlled by injection rate. The performed study provides a base for implementation of time‐dependent probabilistic seismic hazard assessment for the project site.

Months-long preparation of the 2023 MW 7.8 Kahramanmaraş earthquake, Türkiye

“…Following [53] the Probability Density Function (PDF) of empirical magnitude difference data, 𝑝(𝜟𝑴) is correlated, if it significantly deviates from the distribution of magnitude differences containing uncorrelated magnitudes, 𝑝(𝜟𝑴 ⋆ ). Such a distribution can be constructed multiple times by considering 𝛥𝑀 𝑖 ⋆ = 𝑀 𝑖 * − 𝑀 𝑖 , where 𝑀 𝑖 * is a magnitude randomly drawn from the original catalogue.…”

Crustal permeability generated through microearthquakes is constrained by seismic moment

“…This metric has been instrumental in estimating the size of the activated reservoir volume, leading to the strategic placement of a new production well 21 , 44 . Moreover, a variety of field studies on seismicity triggered by fluid injection have shown that the total release of seismic energy (or seismic moment) is directly correlated with hydraulic energy 45 , 46 . The cumulative logarithmic seismic moment, , is defined as the cumulative sum of seismic moment during the cumulative time interval of , expressed as: …”

On the Potential Role of Viscoelasticity in Fluid‐Induced Seismicity

“…Specifically, with an increasing injection rate, the time intervals between background events becomes consistently shorter (Figure 7), such that the increase in triggering rate due to a large event is no longer detectable (Figures 6b and 7d), and any attempt to estimate p and α leads to significantly biased values (Figures S8d and S10d in Supporting Information S1). It is conceivable that these different scenarios occur in the field and might lead to the (incorrect) conclusion of aftershock triggering being absent in a given catalog, e.g., Geysers geothermal fields in California, USA (Martinez‐Garzon et al., 2018), Soultz geothermal field in France (Langenbruch et al., 2011), and the Helsinki geothermal site in Finland (Fischer et al., 2023; Kwiatek et al., 2022). At the Geysers geothermal fields in California, the fluid injection significantly modulates the rate of background events (Martinez‐Garzon et al., 2018), suggesting that higher fluid injection leads to more background events making the identification of aftershock triggering challenging.…”

“…Some studies, such as those on the Salton Sea and Coso geothermal fields in California, USA (Martinez‐Garzon et al., 2018), the Hoadley and Horn river gas fields of Alberta, Canada (Maghsoudi et al., 2018), the gas fields in the Groningen, Netherlands (Post et al., 2021), the WD Rongchang gas reservoir, China (Wang et al., 2020), and the WD fields in southern Kansas and Oklahoma, USA (Karimi & Davidsen, 2021), have found evidence of aftershocks. However, in other cases, such as the Soultz geothermal field in France (Langenbruch et al., 2011) and geothermal hydraulic stimulations in Finland (Fischer et al., 2023; Kwiatek et al., 2022), aftershocks have not been observed.…”