@article{HofmannZimmermannFarkasetal.2019,
  author    = {Hofmann, Hannes and Zimmermann, G{\"u}nter and Farkas, M{\´a}rton P{\´a}l and Huenges, Ernst and Zang, Arno and Leonhardt, Maria and Kwiatek, Grzegorz and Martinez-Garzon, Patricia and Bohnhoff, Marco and Min, Ki-Bok and Fokker, Peter and Westaway, Rob and Bethmann, Falko and Meier, Peter and Yoon, Kern Shin and Choi, JaiWon and Lee, Tae Jong and Kim, Kwang Yeom},
  title     = {First field application of cyclic soft stimulation at the Pohang Enhanced Geothermal System site in Korea},
  series = {Geophysical journal international},
  volume    = {217},
  journal   = {Geophysical journal international},
  number    = {2},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0956-540X},
  doi       = {10.1093/gji/ggz058},
  pages     = {926 -- 949},
  year      = {2019},
  abstract  = {Large-magnitude fluid-injection induced seismic events are a potential risk for geothermal energy developments worldwide. One potential risk mitigation measure is the application of cyclic injection schemes. After validation at small (laboratory) and meso (mine) scale, the concept has now been applied for the first time at field scale at the Pohang Enhanced Geothermal System (EGS) site in Korea. From 7 August until 14 August 2017 a total of 1756 m(3) of surface water was injected into Pohang well PX-1 at flow rates between 1 and 10 l s(-1), with a maximum wellhead pressure (WHP) of 22.8 MPa, according to a site-specific cyclic soft stimulation schedule and traffic light system. A total of 52 induced microearthquakes were detected in real-time during and shortly after the injection, the largest of M-w 1.9. After that event a total of 1771 m(3) of water was produced back from the well over roughly 1 month, during which time no larger-magnitude seismic event was observed. The hydraulic data set exhibits pressure-dependent injectivity increase with fracture opening between 15 and 17 MPa WHP, but no significant permanent transmissivity increase was observed. The maximum magnitude of the induced seismicity during the stimulation period was below the target threshold of M-w 2.0 and additional knowledge about the stimulated reservoir was gained. Additionally, the technical feasibility of cyclic injection at field scale was evaluated. The major factors that limited the maximum earthquake magnitude are believed to be: limiting the injected net fluid volume, flowback after the occurrence of the largest induced seismic event, using a cyclic injection scheme, the application of a traffic light system, and including a priori information from previous investigations and operations in the treatment design.},
  language  = {en}
}
@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}
}