PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 11-13, 2013 SGP-TR-198 A PIECE OF ‘GEOTHERMAL ART’ IN THE N-GERMAN BASIN: HORSTBERG 2004, REVISITED I. Ghergut 1 , H. Behrens 1 , R. Jung 2 , M. Sauter 1 , T. Tischner 3 1 University of Göttingen, Applied Geology Dept. Goldschmidtstr. 3, Göttingen 37077, Germany e-mail: ighergu@gwdg.de 2 JUNG-GEOTHERM, Isernhagen, Germany e-mail: Jung.Geotherm@googlemail.com 3 The Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany e-mail: Torsten.Tischner@BGR.de ABSTRACT The memorable waterfrac + tracer test conducted, almost a decade ago, at Horstberg (Jung et al. 2004, 2005, 2006; Orzol et al. 2004, 2005; Schulz and Tischner 2008; Wessling et al. 2009; Tischner et al. 2003, 2004, 2010; Sulzbacher and Jung 2010; Behrens et al. 2006) is still inspiring some new answers to „old‟ questions like: why single-well? why fracturing? why tracers? does this only work at Horstberg, or can it work almost anywhere else in the N-European sedimentary basin? Heat and tracer transport within waterfrac and matrix turn out to fit into a surprisingly simple description, as the plain arithmetic sum of certain „petrothermal‟- and „aquifer‟-type contributions, whose relative weighting can vary from site to site, depending upon stratigraphy and wellbore geometry. At Horstberg, within the particular formations tested (“Solling”, “Detfurth”, “Volpriehausen”, comprising mainly claystone and sandstone layers), thermal lifetime results to be petrothermally dominated, while tracer residence times appear to be „aquifer‟-dominated. Despite this incongruence, thermal lifetime can reliably be predicted from tracer test results. What cannot be determined from “waterfrac flow-path tracing”, is precisely the waterfrac aperture; aperture uncertainty, however, does not impede upon thermal lifetime predictability. Results of a semi-analytical approach are confirmed by numerical simulations using a FE model that includes more details of hydrogeological heterogeneity for the Horstberg site. Various names, acronyms and abbreviations used in the text are explained within the final section (“Projects and people involved”). WHAT IS SO ‘UNIQUE’ ABOUT HORSTBERG? To begin with, „piece of art‟ is not meant ironically. It does not refer to the packer blockage that occurred in the first GENESYS well, nor to the salt plug that has developed in the second GENESYS well. It refers to a truly successful experiment, a waterfrac + tracer test that has generated new prospects for geothermal heat extraction from tight rock by means of single- well techniques, and has substantiated a new view on hydraulic fracturing in deep sedimentary formations. Since one of the most interesting experiences made at Horstberg is the ability to unite (so to say „in one well‟) the advantages of „petrothermal‟-type to those of „aquifer‟-based EGS, let us first briefly recall what this typology is about (Ghergut et al. 2013). In liquid- phase hydrothermal systems, the formation‟s permeability, times its thickness over which the geothermal wells are screened, is sufficient for ensuring a thermally viable reservoir (with long enough thermal lifetime, whose correlation with fluid residence time is roughly linear, the relative retardation of the cooling front against the tracer front keeping constant in time). Also, the naturally- given permeability is good enough to prevent excessive pressure buildup/drawdown from developing at injection/production wells, in spite of the strong hydraulic gradients (sharp velocity divergence/convergence) in narrow vicinity of the wells. The latter is not the case in „aquifer‟-based EGS: here, artificial fractures are needed in order to prevent such hydraulic gradients from occurring, as well as to increase the effective cross section for heat transport; whereas the correlation between thermal and tracer fronts remains linear, like for hydrothermal systems. In contrast, in „petrothermal‟-type EGS, the geologic formation‟s permeability is „bad‟ enough