Observations of hydraulic stimulations in seven enhanced geothermal system projects Linmao Xie a , Ki-Bok Min a, * , Yoonho Song b a Department of Energy Resources Engineering, Seoul National University, Seoul, Korea b KIGAM (Korea Institute of Geoscience and Mineral Resources), Daejeon, Korea article info Article history: Received 19 March 2014 Accepted 22 July 2014 Available online 20 August 2014 Keywords: Enhanced geothermal system (EGS) Hydraulic stimulation In situ stress Injectivity Induced seismicity (IS) Large magnitude event (LME) abstract Numerous stimulation tests have been performed on Enhanced Geothermal System (EGS) or Hot Dry Rock (HDR) projects during the past three decades, however, there is much room for improvement in our knowledge and understanding of the mechanisms of stimulation. This paper investigated the hydraulic stimulation tests carried out on seven EGS or HDR projects where massive volume of fluid was injected into the long open section of the well with interval of tens to hundreds of meters in the crystalline formation. The key characteristic test and performance parameters were defined and collected through extensive survey of stimulation results. Attempts were made to carry out comparative analysis on reservoir conditions, test parameters and test observations. The analysis and discussion suggest that 1) the reservoir stress regime impacts the growth of stimulated region and the reverse faulting stress regime can be favorable for the layout of multiple well system as it may lead to a horizontally or sub- horizontally oriented stimulated zone; 2) the injection pressure for activating shear slip and the asso- ciated onset of seismicity is mainly field stress controlled; 3) there is strong dependency of injectivity on injection pressure and a high pressure makes a better hydraulic injectivity during stimulation and consequently afterwards for circulation; 4) the stimulated region and number of induced seismic events are mainly injection volume controlled and the potential strategy to reduce seismic risks is either to extend stimulation in time or to separate stimulation in space; and 5) the differential stress condition is one of the necessary factors to raise a large magnitude event (LME) and the difference of maximum injection pressure achieved over that at onset of seismicity is an important additional factor to induce LMEs. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Ever since the first geothermal electricity production in Lar- derello, Italy, the humans have the experience of harnessing the earth heat to generate electricity for more than one century. It is increasingly accepted that power generation from earth heat has become an attractive option to serve as a CO 2 -free, base-load renewable energy source [1]. However, geothermal power ac- counts for only 0.3% of the global electricity supply due to the limited geologically viable locations where the natural heat, water and rock permeability is sufficient for economical heat resource extraction [2]. It is known that most hydrothermal resources are within the volcanic regions near tectonic plate boundaries that form the Ring of Fire [3] and those used for geothermal power generation are just pinpoints on a map of global scale [4]. The huge amounts of geothermal resources within the drillable depth are stored in the formations that are deficient in water or permeability. For example in the US, only 2% of the total thermal energy stored between 3 km and 10 km reservoir, which is considered to be conservative recoverable Enhanced Geothermal System (EGS) resource, is sufficient to provide the US primary energy for 2800 years [5]. EGS or previously named Hot Dry Rock (HDR) are the technologies being developed to exploit the vast earth heat resource in the non-volcanic regions where the natural perme- ability of host rocks is very low [5]. It involves artificially enhancing or creating the permeability of the reservoir mainly by hydraulic stimulation, then circulating the water through injection and pro- duction wells to extract heat. Eventually, the high temperature water or vapor is transferred to the power generation facilities. In this study, we chose a narrower definition of EGS which excluded the case with hydraulic stimulation applied to existing hydrother- mal reservoir for additional permeability increase. * Corrensponding author. Tel.:þ82 2 880 9074; fax: þ82 2 877 0925. E-mail address: kbmin@snu.ac.kr (K.-B. Min). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene http://dx.doi.org/10.1016/j.renene.2014.07.044 0960-1481/© 2014 Elsevier Ltd. All rights reserved. Renewable Energy 79 (2015) 56e65