Discrete element modeling of fluid injection–induced seismicity and activation of nearby fault

Abstract

Enhanced geothermal systems, shale gas, and geological carbon sequestration all require underground fluid injection in high-pressure conditions. Fluid injection creates fractures, induces seismicity, and has the potential to reactivate nearby faults that can generate a large magnitude earthquake. Mechanisms of fluid injection-induced seismicity and fault reactivation should be better understood to be able to mitigate larger events triggered by fluid injection. This study investigates fluid injection, induced seismicity, and triggering of fault rupture using hydromechanical-coupled discrete element models. Results show that a small amount of fluid pressure perturbation can trigger fault ruptures that are critically oriented and stressed. Induced seismicity by rock failure shows in general higher b-values (slope of magnitude-frequency relation) compared to seismicity triggered by the fault fracture slip. Numerical results closely resemble observations from geothermal and shale-gas fields and demonstrate that discrete element modeling has the potential to be applied in the field as a tool for predicting induced seismicity prior to in situ injection.Key words: induced seismicity, fault reactivation, large-magnitude events, triggered seismicity, magnitude-frequency relation b-value.Résumé : Les systèmes géothermiques améliorés, l'extraction du gaz de schiste et la séquestration du carbone géologique nécessitent l'injection de fluides à haute pression dans le sous sol. Ce type d'injection génère des fractures, cause des secousses sismiques et peut réactiver des failles avoisinantes, lesquelles peuvent provoquer des tremblements de terre de forte magnitude. Il serait utile de mieux comprendre les mécanismes des séismes induits par l'injection de fluides et ceux de la réactivation de failles, et ce afin d'être en mesure de prévenir les événements graves causés par l'injection de fluide. Dans le présent article, on étudie l'injection de fluides, la sismicité induite et le déclenchement de ruptures de failles à l'aide de modèles couplés hydromécaniques basés sur la méthode des éléments discrets. Les résultats montrent qu'une légère perturbation de la pression du fluide injecté peut déclencher des ruptures d'orientation critique soumises à des contraintes. La sismicité induite par la rupture de roches est généralement caractérisée par des valeurs de b (pente de la fonction de relation entre la magnitude et la fréquence) par rapport à celles observées dans le cas de la sismicité déclenchée par le cisaillement de rupture des failles. Les résultats numériques se rapprochent beaucoup des observations faites sur les sites géothermiques ou d'extraction de gaz de schiste et montrent que la modélisation basée sur la méthode des éléments discrets pourrait être utilisée sur ces sites comme outil servant à prédire la sismicité induite avant l'injection de fluides in situ. [Traduit par la Rédaction] Mots-clés : sismicité induite, réactivation de failles, incidents de forte magnitude, sismicité déclenchée, valeur de b: pente ...

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