THEMATIC ISSUE Investigation of a new HDR system with horizontal wells and multiple fractures using the coupled wellbore–reservoir simulator TOUGH2MP-WELL/EOS3 Mengting Li • Yang Gou • Zhengmeng Hou • Patrick Were Received: 30 September 2014 / Accepted: 25 February 2015 Ó Springer-Verlag Berlin Heidelberg 2015 Abstract Geothermal energy is renewable, sustainable and available in large amounts. The hot dry rock (HDR) systems, in particular, have the largest potential for long- term sustainability and therefore draw a lot of attention. The combination of horizontal wells and the technology of multiple transverse fractures technology is an attractive approach of such HDRs, which is called in this paper as the specific EGS (enhanced geothermal system). The main objective of this paper is to study the heat extraction over a period of 20 years by water circulating in a deep geother- mal reservoir using this specific EGS. The wellbore flow module, T2WELL/ECO2N, is implemented in the parallelized simulator TOUGH2MP as a new code TOUGH2MP-WELL/EOS3, which enables coupled well- bore–reservoir simulations. Using this newly developed code, the sensitivity of heat extraction against various pa- rameters of the formation and fractures is assessed. The influences of the fracture geometry and the space of two neighborhood fractures on the geothermal energy perfor- mance are analyzed. According to the results in this paper, the injection rate, the wellbore radius and the fracture permeability are three main influence factors for the dis- tribution of total flow through individual fractures. A large injection rate, a small wellbore radius and a large fracture width will cause the short-circuit effect, which reduces the performance of this specific EGS. Compared with the classic HDR system (doublet or triplet vertical wells ? single fracture), this specific EGS has a much higher per- formance and a longer duration of the economic produc- tion. Furthermore, the horizontal wells can also be drilled in both directions of the minimum horizontal stress instead of in just one direction, so that the energy performance is doubled and the investment cost per kW energy production is decreased and the advantage of this specific EGS is significantly enlarged. Keywords Thermal hydraulic modeling  Coupled wellbore–reservoir simulation  Hot dry rock  Horizontal well  Multiple fractures List of symbols c Specific heat capacity [J/(kg K)] F Mass flow rate (kg/s) k Rock permeability (m 2 ) k(Dz l ) Permeability-thickness product (m 3 ) PI Productivity index (m 3 ) p well Fluid pressure in wellbore cell (Pa) p rock Fluid pressure in the adjacent reservoir formation cell (Pa) r e Grid block radius (m) r w Wellbore radius (m) s Skin factor (-) T Temperature (°C) w Fracture width (m) a Correction factor for permeability-thickness product (-) k Thermal conductivity [W/(m K)] l Fluid viscosity (Pa s) M. Li  Y. Gou  Z. Hou Sino-German Energy Research Center, Sichuan University, Chengdu, China M. Li  Y. Gou (&)  Z. Hou  P. Were Energy Research Center of lower Saxony, Clausthal University of Technology, Goslar, Germany e-mail: yang.gou@tu-clausthal.de Z. Hou Institute of Petroleum Engineering, Clausthal University of Technology, Clausthal-Zellerfeld, Germany 123 Environ Earth Sci DOI 10.1007/s12665-015-4242-9