February 13-15, 2006 Hydromechanical modeling of fractured crystalline reservoirs hydraulically stimulated S. Gentier*, X. Rachez**, A. Blaisonneau*, *BRGM ** Itasca Consultants->BRGM BRGM/Geo-Energy unit

February 13-15, 2006 Engine > 2 In situ hydraulic stimulation tests at Soultz- sous-Forêts > Irreversible increase of the permeability around the wells but not in the same proportions for the all the wells (1) Stim. GPK (2) Stim. GPK (1) (2) (3) (4) (4) Injec. GPK (3) Injec. GPK > Micro-seismic events associated to the hydraulic stimulation tests Stimulation curves (GPK1/GPK2) Micro-seismic events (GPK2/GPK3) Gérard et al., 1997 Gérard et al., 2004

February 13-15, 2006 Engine > 3 Objectives of our modeling work and of the talk... > Objective of our work at BRGM is: to understand which physical mechanisms are involved in the hydraulic stimulation of the well in crystalline rocks to extract the main parameters playing a role in the hydraulic stimulation to establish the link with the micro-seismic activity observed during the hydraulic stimulation tests > Objective of my talk is much less ambitious : to give you an idea of the first results obtained up to now by means of some examples extracted from the various hydraulic stimulation tests performed at Soultz-sous-Forêts

February 13-15, 2006 Engine > 4 Thermal effect is neglected in a first step for two reasons : – we consider very short duration test – we are interested in what it could happen at some distance of the well (the Thermo-Hydro- Mechanical behavior of the near well is in progress with another and more appropriated numerical tool) Hydro-mechanical modeling approach > Conceptual model : The rock mass is considered as a blocks assembly which are separated by discontinuities Blocks are deformable and impermeable 400m 1000m F Flow takes place in the fractures exclusively > Numerical tool : 3DEC code integrating a real HM coupling based on : Distinct Element method for the mechanical part Finite difference schema for the hydraulic part of the model in the discontinuities > Aim : to simulate the interaction between mechanical process (deformations, stresses,…) and hydraulic process (pressures, apertures,…)

February 13-15, 2006 Engine > 5 What kind of data do have we to construct the model ? > hydraulic stimulation tests : solicitation in the well > Stress regime (?): mechanical boundary conditions Klee and Rummel (1993) Cornet et al. (to be published) > Fracture network mobilized during the hydraulic stimulation : identification of this network from : – flow logs – temperature logs – geological analysis (cutting analysis) – bore-hole imagery sHsH shsh vv North East P i = r g z y = 0 zz xx x = z = 0 Injection under P = P i +  P well

February 13-15, 2006 Engine > 6 What it could happen during the hydraulic stimulation of a well (if we exclude thermal effect...) hh HH VV In continuous homogeneous and isotropic medium HH VV hh But in general, the granite is already fractured

February 13-15, 2006 Engine > 7 More in details... UnUn UsUs VV HH HH hh Evolution of the hydraulic aperture is linked to the normal displacement (Un) and the tangential displacement (Us) closure of the fracture UnUn UsUs initial state opening : reduction of the normal component release of the shearing Increase of the aperture Well ToTo T1T1 T2T2 TfTf

February 13-15, 2006 Engine > 8 Four examples... To illustrate our Hydro-Mechanical modeling approach, we are going to consider the influence of the following parameters : > number of fractures involved in the stimulated network (GPK1) > orientation and dip for a given fracture network (GPK2) > heterogeneity of the hydro-mechanical properties of fractures (GPK3) > stress regime (GPK4)

February 13-15, 2006 Engine > 9 Influence of the number of fractures (GPK1) Hydraulic apertures in the fracture zones #1 : the most permeable in situ F Model with 7 fractures #1? Model with 8 fractures #1#8 Extra fracture (depth 2884 m, dip 80°, dip-dir 230°) connecting two fractures in the upper part of the open hole No significant change in the global behavior but significant change in the fracture #1 : better fitting with the in situ flow log data # 4, 5, 6

February 13-15, 2006 Engine > 10 Model with 7 fractures Maximum Aperture = amax = 0.25mm Connection with other fractures GPK1 Model with 8 fractures Maximum Aperture # 0.20mm Few meters from well GPK1 View in plane of Fracture #1 - Overpressure  P=10.0 MPa Influence of the number of fractures (GPK1) Extra fracture

February 13-15, 2006 Engine > 11 Influence of the geometry (GPK2) Tangential displacements  P = 14 MPa Shearing propagates from the top to the bottom of the open hole Regular network Us max  5 cm Statistical network Shearing is concentrated in the upper part of the open hole Us max  6 cm Us max  2.5 cm Shearing is concentrated in the lower part of the open hole N 250° -> N 290°

February 13-15, 2006 Engine > 12 75% of fluid flow Heterogeneity of the hydro-mechanical properties (GPK3) 4905m 4930m 4960m 5015m 4980m 4750m 4860m 4% of fluid flow Dezayes et al. (2004)

February 13-15, 2006 Engine > 13  P = 10.5 MPa Influence of the heterogeneity in the hydro- mechanical properties (GPK3)

February 13-15, 2006 Engine > 14 Heterogeneity of the hydro-mechanical properties (GPK3) Shear displacements 2D/cross section (EW) Us max  1 cm  P = 15 MPa Slip : points of rupture  Micro-seismicity ? Existence of a very permeable fracture limited extension of shear displacements for this range of overpressures Increase of the permeability remains moderated W E

February 13-15, 2006 Engine > 15 Stress regime ? hh P hyd HH VV ? 1. Klee and Rummel (1993)  H : N170° 2. Cornet et al. (2006?)  H : N 175° Strike slip regime Normal fault stress regime

February 13-15, 2006 Engine > 16  P = 18,3 MPa Influence of the stress regime (GPK4) Normal fault stress regime Strike slip regime Us max  6 cm Us max  12 cm x 2 Tangential displacements more concentrated in some fractures Tangential displacements more spread

February 13-15, 2006 Engine > 17 Conclusions > Increase of the permeability could be explained by : shear mechanisms which are developed only in some fracture zones depending of : – geometry and connectivity of the fracture network / stress field – heterogeneity in the hydro-mechanical properties of the fracture in the network This modeling approach can help to understand better a geothermal site but it must be based on a good geological and structural knowledge of the site > Difficulties in relationship with the site : definition of the in situ stress regime definition of the fracture network. The model is very sensitive and requires good structural data how this main stimulated fracture network is connected to the global fracture network constituting the real volume of the exchanger? > Difficulties in relationship with the model : which law of behavior to consider for the main fracture zone and how to define the associated hydro-mechanical parameters ?