@article{NiemzDahmMilkereitetal.2021,
  author    = {Niemz, Peter and Dahm, Torsten and Milkereit, Claus and Cesca, Simone and Petersen, Gesa Maria and Zang, Arno},
  title     = {Insights into hydraulic fracture growth gained from a joint analysis of seismometer-derived tilt signals and scoustic emissions},
  series = {Journal of geophysical research : Solid earth},
  volume    = {126},
  journal   = {Journal of geophysical research : Solid earth},
  number    = {12},
  publisher = {American Geophysical Union},
  address   = {Washington},
  issn      = {2169-9313},
  doi       = {10.1029/2021JB023057},
  pages     = {14},
  year      = {2021},
  abstract  = {Hydraulic 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. <br /> Plain Language Summary Hydraulic fracturing (HF) describes the opening of fractures in rocks by injecting fluids under high pressure. The new fractures not only can facilitate the extraction of shale gas but can also be used to heat up water in the subsurface in enhanced geothermal systems, a corner stone of renewable energy production. The fracture formation is inherently accompanied by small, nonfelt earthquakes (microseismic events). Occasionally, larger events felt by the population can be induced by the subsurface operations. Avoiding such events is important for the acceptance of HF operations and requires a detailed knowledge about the fracture formation. We jointly analyze two very different data sets recorded during mine-scale HF experiments: (a) the tilting of the ground caused by the opening of the fractures, as recorded by broadband seismometers-usually deployed for earthquake monitoring-installed close to the experiments and (b) a catalog of acoustic emissions, seismic signals of few milliseconds emitted by tiny cracks around the forming hydraulic fracture. The novel joint analysis allows to characterize the fracturing processes in greater detail, contributing to the understanding of the physical processes, which may help to understand fluid-injection-induced seismicity and validate the formation of hydraulic fractures.},
  language  = {en}
}
@article{ZangStephanssonStenbergetal.2017,
  author    = {Zang, Arno and Stephansson, Ove and Stenberg, Leif and Plenkers, Katrin and von Specht, Sebastian and Milkereit, Claus and Schill, Eva and Kwiatek, Grzegorz and Dresen, Georg and Zimmermann, G{\"u}nter and Dahm, Torsten and Weber, Michael},
  title     = {Hydraulic fracture monitoring in hard rock at 410 m depth with an advanced fluid-injection protocol and extensive sensor array},
  series = {Geophysical journal international},
  volume    = {208},
  journal   = {Geophysical journal international},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0956-540X},
  pages     = {790 -- 813},
  year      = {2017},
  abstract  = {In this paper, an underground experiment at the Aspo Hard Rock Laboratory (HRL) is described. Main goal is optimizing geothermal heat exchange in crystalline rock mass at depth by multistage hydraulic fracturing with minimal impact on the environment, that is, seismic events. For this, three arrays with acoustic emission, microseismicity and electromagnetic sensors are installed mapping hydraulic fracture initiation and growth. Fractures are driven by three different water injection schemes (continuous, progressive and pulse pressurization). After a brief review of hydraulic fracture operations in crystalline rock mass at mine scale, the site geology and the stress conditions at Aspo HRL are described. Then, the continuous, single-flow rate and alternative, multiple-flow rate fracture breakdown tests in a horizontal borehole at depth level 410 m are described together with the monitoring networks and sensitivity. Monitoring results include the primary catalogue of acoustic emission hypocentres obtained from four hydraulic fractures with the in situ trigger and localizing network. The continuous versus alternative water injection schemes are discussed in terms of the fracture breakdown pressure, the fracture pattern from impression packer result and the monitoring at the arrays. An example of multistage hydraulic fracturing with several phases of opening and closing of fracture walls is evaluated using data from acoustic emissions, seismic broad-band recordings and electromagnetic signal response. Based on our limited amount of in situ tests (six) and evaluation of three tests in Avro granodiorite, in the multiple-flow rate test with progressively increasing target pressure, the acoustic emission activity starts at a later stage in the fracturing process compared to the conventional fracturing case with continuous water injection. In tendency, also the total number and magnitude of acoustic events are found to be smaller in the progressive treatment with frequent phases of depressurization.},
  language  = {en}
}
@phdthesis{Milkereit1998,
  author    = {Milkereit, Claus},
  title     = {Modellrechnungen zur elastischen Deformation inhomogener anisotroper Gesteine},
  pages     = {93 S. : graph. Darst.},
  year      = {1998},
  language  = {de}
}