Microseismicity and permeability enhancement of hydrogeologic structures during massive fluid injections into granite at 3 km depth at the Soultz HDR site

Abstract

S U M M A R YA high-rate injection of 20 000 m 3 of water into granite between 2.8 and 3.4 km depth at the Soultz hot dry rock (HDR) test site in France in 1993 September led to a 200-fold increase in borehole transmissivity and produced a subvertical cloud of microseismicity of dimensions 0.5 km wide, 1.2 km long, 1.5 km high and oriented 25 • NW. The resulting data set is unusually complete and well suited to studying permeability creation/enhancement processes in crystalline rock and the utility of microseismic data for revealing them. Although the microseismic cloud defined using joint hypocentre determination (JHD) locations was diffuse and showed little structure, application of the collapsing method showed it to be composed largely of discrete tubes and planes that propagated coherently. One prominent structure that extended 350 m downwards from the vicinity of a flow inlet early in the injection and that appears to contain a major flow path was subjected to detailed investigation to establish its hydrogeologic nature and the mechanisms underpinning its inferred permeability enhancement. High-resolution microseismic mapping techniques (i.e. multiplets and clustering) showed it to be a subvertical, NNW-SSE striking, fracture zone of width 10-20 m. The strike and scale of the structure identifies it as a member of a family of hydrothermally altered, cataclastic shear structures that constitute the primary permeable paths for fluid migration within the rock mass, both under ambient and forced fluid flow conditions. The microseismicity occurred on subvertical, smallscale fractures within the cataclastic shear zone whose azimuths scatter within 22 • of parallel to the parent structure. Although the structure is likely to have been naturally permeable to some degree, its permeability appears to have been significantly enhanced as a consequence of the injection. The most likely mechanism of permeability enhancement, which is in accord with the strong preference for the microseismicity to grow downwards, involves strike-slip shearing, which produced the opening of vertical tubes at along-strike jogs in the fault (the so-called Hill mesh). Seismic moment release averaged over the structure suggests shear displacements of at least 0.3 mm occurred, which are sufficient to generate aperture changes that are hydraulically significant. The preponderance of discrete structures within the microseismic cloud after collapsing suggests that significant flow and permeability enhancement (i.e. stimulation) within the rock mass is largely confined to the interiors of shear zones that appear to have a spacing of approximately 100 m.

Analysis of induced seismicity in geothermal reservoirs – An overview

“…All of these sites have been investigated within the framework of the European Union project GEISER. In Europe, locations include the Lower Rhine Graben site in Soultz-sous-Forêts, France (Evans et al, 2005), the Upper Rhine Graben site in Basel, Switzerland (Häring et al, 2008), Icelandic geothermal test sites , Latera (Italy) and Groß Schönebeck in the North German Basin (Kwiatek et al, 2010). Non-European sites investigated include Berlin, El Salvador (Bommer et al, 2006;Kwiatek et al, 2014), The Geysers in California, USA (Oppenheimer, 1986;Majer et al, 2007), Cooper Basin (Asanuma et al, 2005;Baisch et al, 2006) and Paralana (Hasting et al, 2011;Albaric et al, 2014), both in Australia and Bouillante in Guadeloupe (Sanjuan et al, 2010;Calcagno et al, 2012).…”

On the link between stress field and small-scale hydraulic fracture growth in anisotropic rock derived from microseismicity

“…At near isotropic stress conditions the fractures branch more strongly and without a preferred propagation direction, a phenomenom often referred to as high fracture complexity (e.g., Katsaga et al, 2015). During large-scale stimulations, there is a tendency for seismic clouds to develop perpendicular to the minimum principal stress direction σ 3 (Häring et al, 2008;Evans et al, 2005), particularly for HF operations (e.g., Rutledge et al, 2004), although for HS stimulations in crystalline rocks there are many examples in which the seismicity cloud is oblique to the σ 3 direction (e.g., Block et al, 2015;Murphy and Fehler, 1986;Pine and Batchelor, 1984), presumably reflecting the complex interplay between stress and the pre-existing fracture population that is suitably oriented for slip reactivation. Furthermore, individual seismicity clusters within the overall seismicity cloud often strike oblique to the maximum principal stress (Eaton and Caffagni, 2015;Deichmann et al, 2014).…”

“…40 V. S. Gischig et al: Stress field, hydrofractures and microseismicity tinuity sets present in the reservoir and is invariably less than required to drive new hydrofractures (Pine and Batcherlor, 1984;Kaiser et al, 2013). HS is often exploited in enhanced geothermal systems (e.g., Häring et al, 2008;Evans et al, 2005). Small-volume HF is also utilized in stress measurement (e.g., Haimson and Cornet, 2003;Hubbert and Willis, 1972) and is routinely used in many geological engineering projects for which a detailed understanding of the stress state is needed to optimize the design of underground facilities (e.g., nuclear waste storage, gas storage, mining, tunnelling, hydropower facilities, etc.…”

Finite element modeling of hydraulic fracturing in 3D

“…We are interested in the deformations caused by the difference in effective stress from the initial state. Poro-elasticity relates these deformations to the Lamé-equations τ (1) ij = − λǫ kk δ ij + 2Gǫ ij (10) where ǫ ij is the strain…”

“…Hydraulic fracturing is necessary in the production of the gas from low-permeable shales [6]. Another example is the use of hydraulic fracturing to create conductivity between injection and production wells in geothermal systems [10,27]. Hydraulic fracturing can also have unwanted effects, like for instance leakage of natural gas into the groundwater from shale gas operations [20].…”