@article{WinterleitnerSchuetzWenzlaffetal.2018,
  author    = {Winterleitner, Gerd and Sch{\"u}tz, F. and Wenzlaff, Christian and Huenges, Ernst},
  title     = {The impact of reservoir heterogeneities on High-Temperature aquifer thermal energy storage systems},
  series = {Geothermics : an international journal of geothermal research and its applications},
  volume    = {74},
  journal   = {Geothermics : an international journal of geothermal research and its applications},
  publisher = {Elsevier},
  address   = {Oxford},
  issn      = {0375-6505},
  doi       = {10.1016/j.geothermics.2018.02.005},
  pages     = {150 -- 162},
  year      = {2018},
  abstract  = {We conducted a geoscientific feasibility study for the development of a high-temperature thermal aquifer energy storage system (HT-ATES) outside the capital of Muscat, northern Oman. The aquifer storage is part of a solar geothermal cooling project for the sustainable and continuous cooling of office buildings. The main concept is that excess solar energy will be stored in the subsurface through hot water injection and subsequently utilised as auxiliary energy source during peak demand times. The characterisation of aquifer heterogeneities is thus essential to predict subsurface thermal heat plume development and recovery efficiency of the storage system. We considered two aquifer systems as potential storage horizons, (i) a clastic-dominated alluvial fan system where individual channel systems in combination with diagenetic alterations constitute the main heterogeneities and (ii) a carbonate-dominated system represented by a homogenous layer-cake architecture. The feasibility study included a multidisciplinary approach from initial field work, geocellular reservoir modelling to finite element fluid flow and thermal modelling. Our results show that for the HT-ATES system, with a high frequency of injection and production cycles, heat loss mainly occurs due to heterogeneities in the permeability field of the aquifer in combination with buoyancy driven vertical fluid flow. An impermeable cap-rock is needed to keep the heat plume in place. Conductive heat loss is a minor issue. Highly complex heat plume geometries are apparent in the clastic channel system and ATES well planning is challenging due to the complex and interconnected high permeable channels. The carbonate sequence shows uniform plume geometries due to the layer cake architecture of the system and is tentatively more suitable for ATES development. Based on our findings we propose the general concept of HT-ATES traps, incorporating and building on expertise and knowledge from petroleum and reservoir geology regarding reservoir rocks and suitable trap\&seal geometries. The concept can be used as guideline for future high-temperature aquifer storage exploration and development.},
  language  = {en}
}
@article{SchuetzWinterleitnerHuenges2018,
  author    = {Sch{\"u}tz, Felina and Winterleitner, Gerd and Huenges, Ernst},
  title     = {Geothermal exploration in a sedimentary basin},
  series = {Geothermal Energy},
  volume    = {6},
  journal   = {Geothermal Energy},
  number    = {1},
  publisher = {Springer},
  address   = {London},
  issn      = {2195-9706},
  doi       = {10.1186/s40517-018-0091-6},
  pages     = {23},
  year      = {2018},
  abstract  = {The lateral and vertical temperature distribution in Oman is so far only poorly understood, particularly in the area between Muscat and the Batinah coast, which is the area of this study and which is composed of Cenozoic sediments developed as part of a foreland basin of the Makran Thrust Zone. Temperature logs (T-logs) were run and physical rock properties of the sediments were analyzed to understand the temperature distribution, thermal and hydraulic properties, and heat-transport processes within the sedimentary cover of northern Oman. An advective component is evident in the otherwise conduction-dominated geothermal play system, and is caused by both topography and density driven flow. Calculated temperature gradients (T-gradients) in two wells that represent conductive conditions are 18.7 and 19.5 °C km-1, corresponding to about 70-90 °C at 2000-3000 m depth. This indicates a geothermal potential that can be used for energy intensive applications like cooling or water desalinization. Sedimentation in the foreland basin was initiated after the obduction of the Semail Ophiolite in the late Campanian, and reflects the complex history of alternating periods of transgressive and regressive sequences with erosion of the Oman Mountains. Thermal and hydraulic parameters were analyzed of the basin's heterogeneous clastic and carbonate sedimentary sequence. Surface heat-flow values of 46.4 and 47.9 mW m-2 were calculated from the T-logs and calculated thermal conductivity values in two wells. The results of this study serve as a starting point for assessing different geothermal applications that may be suitable for northern Oman.},
  language  = {en}
}
@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}
}
@misc{SchuetzWinterleitnerHuenges2018,
  author    = {Sch{\"u}tz, Felina and Winterleitner, Gerd and Huenges, Ernst},
  title     = {Geothermal exploration in a sedimentary basin},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  number    = {934},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-45931},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-459317},
  pages     = {25},
  year      = {2018},
  abstract  = {The lateral and vertical temperature distribution in Oman is so far only poorly understood, particularly in the area between Muscat and the Batinah coast, which is the area of this study and which is composed of Cenozoic sediments developed as part of a foreland basin of the Makran Thrust Zone. Temperature logs (T-logs) were run and physical rock properties of the sediments were analyzed to understand the temperature distribution, thermal and hydraulic properties, and heat-transport processes within the sedimentary cover of northern Oman. An advective component is evident in the otherwise conduction-dominated geothermal play system, and is caused by both topography and density driven flow. Calculated temperature gradients (T-gradients) in two wells that represent conductive conditions are 18.7 and 19.5 degrees C km(-1), corresponding to about 70-90 degrees C at 2000-3000 m depth. This indicates a geothermal potential that can be used for energy intensive applications like cooling or water desalinization. Sedimentation in the foreland basin was initiated after the obduction of the Semail Ophiolite in the late Campanian, and reflects the complex history of alternating periods of transgressive and regressive sequences with erosion of the Oman Mountains. Thermal and hydraulic parameters were analyzed of the basin's heterogeneous clastic and carbonate sedimentary sequence. Surface heat-flow values of 46.4 and 47.9 mW m(-2) were calculated from the T-logs and calculated thermal conductivity values in two wells. The results of this study serve as a starting point for assessing different geothermal applications that may be suitable for northern Oman.},
  language  = {en}
}
@article{WeberZetscheRybergetal.2005,
  author    = {Weber, Michael H. and Zetsche, F. and Ryberg, Trond and Schulze, A. and Spangenberg, Erik and Huenges, Ernst},
  title     = {Seismic detection limits of small, deep, man-made reflectors : a test at a geothermal site in northern Germany},
  issn      = {0037-1106},
  year      = {2005},
  abstract  = {A key question for the development of geothermal plants is the seismic detection and monitoring of fluid injections at several kilometers depth. The detection and monitoring limits are controlled by several parameters, for example, the strength of seismic sources, number of receivers, vertical stacking, and noise conditions. For a known reference reflector at 2.66 km depth at a geothermal site in northern Germany the results of a simple surface seismic experiment were therefore combined with numerical forward modeling for different injection scenarios at 3.8 km depth. The underlying idea is that changes of reflectivity from the injection at 3.8 km must be larger than the variance of the measurements to be observable. Assuming that the injection at 3.8 km depth would produce a subhorizontal disklike target with a fracture porosity of 2\% or 5\% (the critical porosity) the water injection volume has to be at least 443 and 115 m(3), respectively, to be detectable from the surface. If the injection on the other hand does not create subhorizontal but subvertical pathways or only reduces the seismic velocities via the increased pore pressure in the immediate vicinity of the bore hole, the injection is undetectable from the surface. The most promising approach is therefore to move sources and/or receivers closer to the target, that is, the use of borehole instrumentation},
  language  = {en}
}