H21L-01
Time-dependent permeability anisotropy at the Soultz-sous-Forêts geothermal site, France
Tuesday, 15 December 2015: 08:00
3016 (Moscone West)
Michael John Heap1, Luke Griffiths1, Fei Wang1, Damien Daval2, H. Albert Gilg3, Patrick Baud1, Albert Genter4 and Jean Schmittbuhl5, (1)EOST École et Observatoire des Sciences de la Terre, Strasbourg Cedex, France, (2)Lawrence Berkeley National Laboratory, Berkeley, CA, United States, (3)Technische Universität, Munich, Germany, (4)ES Géothermie, Strasbourg, France, (5)University of Strasbourg, Strasbourg Cedex, France
Abstract:
Fluid circulation in geothermal reservoirs depends on the density, geometry, and hydraulic properties of fractures. The Soultz-sous-Forêts site located in the Upper Rhine Graben in Alsace, France, consists of a granitic reservoir overlain by a 1.4 km-thick sedimentary succession. Core analysis and borehole wall imagery collected from reconnaissance well EPS1 (2230 m depth) revealed an extensive fracture network throughout the granite and overlying sediments, including both open fractures and fractures filled through mineral precipitation (primarily quartz, barite, calcite, and galena). Here we present an experimental study that aims to quantify the impact of healed or partially-healed fractures on permeability anisotropy in the Triassic Buntsandstein sandstone (1000-1400 m). We targeted borehole samples that best represent the variability of these fractures. Forty cylindrical core samples (40 mm length and 20 mm diameter) were prepared such that they contain fractures either parallel or perpendicular to their axis. We also prepared samples of the intact host rock. Porosity and permeability were measured for each core, and thin sections were made to characterise the nature of the fractures and the precipitated minerals. The low permeability of the host rock (10-15 to 10-17 m2) can be explained by prevalent pore-filling clays. We found that fractures may present a conduit for or a barrier to flow, depending on the extent of healing and the nature of the filling. Crystal precipitation models, using borehole fluid composition and temperature data from Soultz, provide a timescale for crack healing. Our modelling shows that fractures that once represented effective conduits for flow may heal rapidly (on the order of months) and switch from providing conduits for flow to presenting barriers to flow, modifying the permeability anisotropy within these units. These data are important for understanding fluid flow and heat transfer within the reservoir.
