Characterisation of the Structural Control on Fluid Flow Paths in Fractured Granites

PROCEEDINGS, Thirty-Sixth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 31 - February 2, 2011 SGP-TR-191 CHARACTERISATION OF THE STRUCTURAL CONTROL ON FLUID FLOW PATHS IN FRACTURED GRANITES Joachim PLACE(1&2), Edouard LE GARZIC(1), Yves GERAUD(1), Marc DIRAISON(1), Judith SAUSSE(3) (1) IPGS - UMR 7516, Université de Strasbourg/EOST, France, (2) Now at: EIfER - European Institute for Energy Research, Deep Geothermal Energy, Karlsruhe, Germany e-mail: [email protected] (3) G2R, Nancy-Université, Vandœuvre lès Nancy, France ABSTRACT INTRODUCTION The characterisation of fluid flow in fractured media Drilling costs represent the major part of the total is complex due to the fact that the access to the amount of a deep geothermal project (Heidinger, reservoirs is restricted to the boreholes, and the 2010). Thus, optimising the ratio between the length resolution of geophysical methods decreases with of the boreholes and their hydraulic and thermal depth. In addition, the structures in a sedimentary performances is a key point to address for future cover of a granite mass targeted for geothermal development of deep geothermal energy (Petty et al., exploitation reflect only a partial amount of the 2009). Especially in the case of fractured reservoirs, structures affecting the hot basement. In this respect, clever locations and trajectories of boreholes can two fractured variscan granites have been selected to answer this need, for instance by intersecting as much investigate the fluid flow paths; both of them are permeable structures as possible (Luthi, 2005; located within the European Cenozoic Rift System Schutter, 2003). Nevertheless, this requires a good (ECRIS). knowledge of the reservoir, facing some major The first case study is the experimental geothermal problems: site at Soultz-sous-Forêts (Rhine Graben). Borehole - Surface geophysics allow to investigate the seismic data (VSP) have been used to map some largest structures of a reservoir, but their generally permeable structures developed at the restricted resolution prevent their efficiency hectometre scale in the vicinity of the wells. The at lower scales (Al-Ali et al., 2009). pattern of structures revealed by this way suggests a - Surface seismic reflection methods are not clear expression of the variscan inheritance rather efficient to delineate dipping faults in than a faulting linked to the opening of the graben. crystalline basements, due to the lack of The activation of some of these structures can be due subhorizontal reflectors (for instance: to the current stress field controlled by the Alpine sedimentary horizons) and due to the push, and explains the major hydraulic connections acquisition and processing parameters which between the wells. are not adapted to such targets. The second case study is a batholith outcropping in - Coring and borehole imaging provide local the Catalan Coastal Ranges, allowing a continuous and non exhaustive structural data. structural analysis from centimetre to kilometre Extrapolation of these small scale fractures scales. A network of carbonate veins has been at reservoir scale is tricky. considered as a witness of past circulations. Both the extent and the conditions of precipitations of these In view of this, we present our recent works in the carbonates allow to consider the outcrop as a fossil characterisation of fluid pathways in fractured geothermal reservoir. The veins studied in this granites. The above-mentioned problems are analogue outcrop and at Soultz-sous-Forêts allowed addressed in two case studies, the Soultz-sous-Forêts to identify different drainage patterns developed in a geothermal site (Rhine Graben, France) and an fractured granite. At the kilometre scale, the drainage analogue outcrop located in Tamariu (Catalan may either be localised in the major structures, or be Coastal Ranges, Spain). These two sites are more homogeneously or randomly distributed, composed by fractured granites that are very similar implying an important contribution of the protolithe. due to their age (variscan origin) and their tectonic history (especially the opening of the European Cenozoic Rift System). LIMITS TO THE CHARACTERISATION OF A Lubine-Baden-Baden fault zone and the Tepla Suture FRACTURED RESERVOIR: (Fig. 1). THE SOULTZ-SOUS-FORÊTS EXAMPLE The Soultz-sous-Forêts granites emplaced in Carboniferous times (Cocherie et al., 2004) during In the framework of the industrial exploration of the the transition between collision and collapse tectonics Upper Rhine Graben (URG) for geothermal energy, of the variscan orogen. The collapse occurred until an Enhanced Geothermal System (EGS) is being Permian times, forming basins trending NE-SW to developed at Soultz-sous-Forêts since decades ENE-WSW (Ziegler et al., 2006). Large brittle and (Gérard and Kappelmeyer, 1987) (Fig. 1). At present ductile strike-slip faults mainly oriented NNE-SSW time, three deep geothermal wells reach 5000 m to ENE-WSW and NW-SE were activated more or depth in the Soultz-sous-Forêts basement and allow less contemporaneously with the development of to access to temperature of 200°C (Genter et al., these basins (Arthaud and Matte, 1975). 2010). A set of older and less deep wells is also An other intensive deformation phase occurred in the available, like GPK1 and EPS1 which are studied Tertiary, with compression tectonics forming Alpine later in this paper. orogenes and extension developed through Europe The boreholes reach the crystalline basement at a (European Cenozoic Rift System) (Bourgeois et al., depth of about 1400 m, below a sedimentary cover 2007; Dèzes et al., 2004; Ziegler, 1992). The deposited from Permian to Quaternary. The basement formation of the Rhine Graben is attributed to a of the Rhine Graben area formed by the collision of polyphased history, with a main E-W extension phase large variscan domains (Édel and Schulmann, 2009; in Rupelian followed by transpressional and Franke, 2006). Major thrusts and suture zones trend transtensional tectonics in Chattian and Miocene NE-SW to ENE-WSW, such as the Vittel-Lalaye- (Schumacher, 2002; Villemin and Bergerat, 1987). Figure 1: a) Position map of the Soultz-sous-Forêts EGS within the Upper Rhine Graben, illustrating variscan units and structures (modified after Édel and Schulmann (2009) and Édel et al. (2007)). b) E-W oriented reflection seismic profile, allowing to delineate much more structures in the sedimentary cover than in the crystalline basement. The arrow at the bottom of the profile indicates the position of a major fault zone indentified by other methods in the basement; the expression of this structure in reflection seismic is only a discreet flower-like structure in the sediments. Major Oligocene normal faults controlling the These problems are added to the previous ones cited deposition of sediments strike N-S to NNE-SSW (see above. To address them, we worked in one hand in orange traces in Fig. 1a and green traces in Fig. 1b). the direct investigation of structures by a new The NNE-SSW orientation of some fault segments seismic methodology applied to the Soultz-sous- suggest a reactivation of Paleozoic structures (Édel et Forêts site, and in another hand, we tried to identify al., 2007; Schumacher, 2002, see Fig. 1). general behaviours in the circulation of fluids in fracture networks with the help of an analogue Thus, from this introduction, one can observe that: massif. (1) As the sedimentary cover is younger than the basement, the fault network of the DIRECT INVESTIGATION OF A FRACTURED sedimentary cover is probably a subset of RESERVOIR BY UNCONVENTIONAL reactivated and newly formed structures of SEISMIC METHOD (VSP) the underlying basement (Fig. 2). In In the Soultz-sous-Forêts site, seismic reflection and addition, decollements such as represented conventional processing of walkaway and Vertical in Fig. 2 may contribute to differentiate the Seismic Profiling (VSP) have not been able to structural networks of the basement and its delineate steep structures (Beauce et al., 1991; cover (see the Hermerswiller fault in Fig. 1b Cautru, 1989; Dylikowski, 1985, and see Fig. 1b). for a field example). In other words, some Even the contact between the sediments and the important populations of faults within the basement is discontinuously imaged. P and S wave basement may not affect the cover as long velocities and hodograms have been used to track as they are not reactivated during or after velocity variations and S-mode wave splitting; only a the deposition of sediments. heterogeneity in the deep part of the GPK1 borehole (2) The sedimentary cover may be efficiently has been detected, but without any orientation investigated with typical geophysical information (Le Bégat et al., 1994). methods like reflection seismic (Fig. 1b). However, no structure can be clearly Nevertheless, such borehole seismic data are acquired observed in the so-called 'seismic fog' of the with a source at a fix surface position and sensors underlying crystalline rocks (Fig. 1b). Other located in a well at successive depth levels (Mari et geophysical investigation like gravimetric al., 2003). By this way, seismic events such as and magnetic anomalies which produced reflections or refractions generated by structures that Fig. 1a do not have a resolution fine enough may present a high dip value can be recorded and to detect faults at the scale of the reservoir; analyzed for structural characterization. Last bu not only large trends may be delineated ( del et É least, thanks to the geometry of the equipment, VSP al., 2007). have the potential to bridge the scale gap between surface and boreholes methods (see the introduction and Al-Ali et al., 2009; Cosma et al., 2001; Emsley et al., 2007; Martí et al., 2006). In this paper, we resume the processing, analysis and interpretation of a three-component (3C) VSP data set acquired at the Soultz-sous-Forêts EGS in 1993 in GPK1 and EPS1 wells. The aim is the 3D mapping of steep faults affecting the granitic basement at the geothermal reservoir scale.

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