Abstract
AbstractHydraulic fracturing is performed to enhance rock permeability, for example, in the frame of geothermal energy production or shale gas exploitation, and can potentially trigger induced seismicity. The tracking of increased permeabilities and the fracturing extent is often based on the microseismic event distribution within the stimulated rock volume, but it is debated whether the microseismic activity adequately depicts the fracture formation. We are able to record tilt signals that appear as long‐period transients (180 s) on two broadband seismometers installed close (17–72 m) to newly formed, meter‐scale hydraulic fractures. With this observation, we can overcome the limitations of the microseismic monitoring alone and verify the fracture mapping. Our analysis for the first time combines a catalog of previously analyzed acoustic emissions ([AEs] durations of 20 ms), indirectly mapping the fractures, with unique tilt signals, that provide independent, direct insights into the deformation of the rock. The analysis allows to identify different phases of the fracturing process including the (re)opening, growth, and aftergrowth of fractures. Further, it helps to differentiate between the formation of complex fracture networks and single macrofractures, and it validates the AE fracture mapping. Our findings contribute to a better understanding of the fracturing processes, which may help to reduce fluid‐injection‐induced seismicity and validate efficient fracture formation.
Highlights
Monitoring the movement of fluids in rocks remains one of the major challenges in geosciences
The two observations depict different aspects of the fracturing process. We link these observations by modeling the theoretical deformation, the tilt at the position of the seismometers, caused by an opening fracture based on the fracture extent obtained in a previous study of the acoustic emissions (AEs) activity
The models do provide evidence for the interpretation of the tilt signal to be induced by the opening and closing of hydraulic fractures, and show that the AE activity successfully mapped the fracture extent in most experiments
Summary
Monitoring the movement of fluids in rocks remains one of the major challenges in geosciences. A broad variety of research topics relies on the detailed knowledge about the increase of permeabilities and the fracture evolution. This includes the triggering of natural or induced earthquakes, the study of magmatic systems beneath volcanoes, and the exploitation of the subsurface for example, for oil/gas extraction or geothermal energy production in enhanced geothermal systems. We focus on the analysis of hydraulic fracturing, which can be used to develop enhanced geothermal systems by increasing the contact surface between the injected cold fluid and the hot rock via the creation of new fluid pathways. In the case of hydraulic fractures of a few meters in length and some tens
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