Abstract
Temporal changes in groundwater chemistry can reveal information about the evolution of flow path connectivity during crustal deformation. Here, we report transient helium and argon concentration anomalies monitored during a series of hydraulic reservoir stimulation experiments measured with an in situ gas equilibrium membrane inlet mass spectrometer. Geodetic and seismic analyses revealed that the applied stimulation treatments led to the formation of new fractures (hydraulic fracturing) and the reactivation of natural fractures (hydraulic shearing), both of which remobilized (He, Ar)-enriched fluids trapped in the rock mass. Our results demonstrate that integrating geochemical information with geodetic and seismic data provides critical insights to understanding dynamic changes in fracture network connectivity during reservoir stimulation. The results of this study also shed light on the linkages between fluid migration, rock deformation and seismicity at the decameter scale.
Highlights
Temporal changes in groundwater chemistry can reveal information about the evolution of flow path connectivity during crustal deformation
While the identification of the source of the mobilized fluids is still speculative, two complementing hypotheses on its origin are proposed: (1) radiogenic He accumulated in the rock matrix is released by the formation of new fracture areas; and/or (2) the shearing of pre-existing fractures induces the remobilization of stagnant fluids, with higher residence times enriched in radiogenic He and Ar, trapped in lower transmissivity zones of the fracture network
We provide in situ evidence of He and Ar anomalies occurring during the deformation of a crystalline fractured system at the decameter scale
Summary
Temporal changes in groundwater chemistry can reveal information about the evolution of flow path connectivity during crustal deformation. Characterizing the timing and spatial extent of crustal deformation is critical for both industrial applications, including geothermal, oil and gas production, and improving our mechanistic understanding of earthquakes From this perspective, seismic and geodetic monitoring systems generally provide the vast majority of data collected. By analyzing specific dissolved chemical tracers, one can reconstruct the origin of fluids and gain insights into their recharge, percolation and storage conditions[9,10] From this perspective, noble gases such as radon (Rn), helium (He) and argon (Ar) have received particular attention in recent decades because of their widespread occurrence in the subsurface and their low chemical reactivity, proving to be ideal tracers to track the origin of fluids, analyze fluid-rock interactions and determine residence times[7,8,11,12,13,14].
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