@article{FarkasYoonZangetal.2019,
  author    = {Farkas, M{\´a}rton P{\´a}l and Yoon, Jeoung Seok and Zang, Arno and Zimmermann, G{\"u}nter and Stephansson, Ove and Lemon, Michael and Danko, Gyula},
  title     = {Effect of foliation and fluid viscosity on hydraulic fracturing tests in mica schists investigated using distinct element modeling and field data},
  series = {Rock Mechanics and Rock Engineering},
  volume    = {52},
  journal   = {Rock Mechanics and Rock Engineering},
  number    = {2},
  publisher = {Springer},
  address   = {Wien},
  issn      = {0723-2632},
  doi       = {10.1007/s00603-018-1598-7},
  pages     = {555 -- 574},
  year      = {2019},
  abstract  = {Several hydraulic fracturing tests were performed in boreholes located in central Hungary in order to determine the in-situ stress for a geological site investigation. At a depth of about 540m, the observed pressure versus time curves in mica schist with low dip angle foliation shows atypical pressure versus time results. After each pressurization cycle, the fracture breakdown pressure in the first fracturing cycle is lower than the refracturing or reopening pressure in the subsequent pressurizations. It is assumed that the viscosity of the drilling mud and observed foliation of the mica schist have a significant influence on the pressure values. In order to study this problem, numerical modeling was performed using the distinct element code particle flow code, which has been proven to be a valuable tool to investigate rock engineering problems such as hydraulic fracturing. The two-dimensional version of the code applied in this study can simulate hydro-mechanically coupled fluid flow in crystalline rock with low porosity and pre-existing fractures. In this study, the effect of foliation angle and fluid viscosity on the peak pressure is tested. The atypical characteristics of the pressure behaviour are interpreted so that mud with higher viscosity penetrates the sub-horizontal foliation plane, blocks the plane of weakness and makes the partly opened fracture tight and increase the pore pressure which decreases slowly with time. We see this viscous blocking effect as one explanation for the observed increase in fracture reopening pressure in subsequent pressurization cycles.},
  language  = {en}
}
@book{ZangStephansson2010,
  author    = {Zang, Arno and Stephansson, Ove},
  title     = {Stress field of the earth's crust},
  publisher = {Springer Science+Business Media B.V},
  address   = {Dordrecht},
  isbn      = {978-1-402-08443-0},
  doi       = {10.1007/978-1-4020-8444-7},
  pages     = {240 S.},
  year      = {2010},
  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}
}