The understanding of the main mechanisms taking place in a hydraulic stimulation of a
fracture network is a fundamental point to improve our ability to define stimulation
strategy in fractured geothermal reservoir. Since several years, various modeling
studies at various scales have been undertaken on the base of the results of the
hydraulic stimulation tests performed on the site of Soultz-sous-Forêts (France).
A study, started several years ago, aimed to construct a 3D hydro-mechanical model of
the stimulation of the fracture network around each well at Soultz-sous-Forêts. A
numerical approach is developed in order to understand and to explain the physical
mechanisms which are at the origin of the hydraulic behavior observed during the
stimulation tests conducted in the various wells. The model is designed to reproduce
the observed behavior in terms of pressure-flow curves as well as in terms of flow
rate flowing into each fracture constituting the network.
The numerical model used for the simulations is the 3DEC software (Itasca, 2003). It
is employed to make the coupled hydro-mechanical calculations that allow us to
simulate interactions between the mechanical process (deformations, stresses,…) and
hydraulic process (pressures, apertures, …) in a solid cut by discrete
discontinuities corresponding to a realistic geometry of the fracture network
intersecting the blocks. These blocks are considered to be deformable and
impermeable. Fluid flow occurs exclusively through the fracture network and obeys a
cubic law. The modification of the pressure field results in a modification of the
actual stresses applied to the surrounding formations, which may themselves cause
changes in the openings of the fractures and hence of the pressure field.
Based on the various studies performed over several years by geologists, the
conceptual model of the site used for this modeling is the following: a homogeneous
impermeable granitic mass crossed by a network of discontinuities constituted by
hydrothermalized fractured zones formed by the tectonic and hydrothermal history of
the site (Dezayes et al., 2004). The location and the orientation of the fractures
are more or less constrained according to the quality of the basic data.
The modeling approach has been first developed on the upper part of the reservoir
where a maximum of data are available in the two wells constituting the geothermal
doublet at a depth of around 3,500m. Considering the satisfactory results of the
first application, the hydro-mechanical model has been extended to the lower part of
the reservoir at depth between 4,500m and 5,000m where the three deepest geothermal
wells have been drilled.
The various modeled cases show the role played by some main parameters on the results
of the modeling such as orientation of the main permeable fractures, the intersection
between them and their hydro-mechanical properties. The results obtained by the
3-dimensional models show that the permeability increase is highly associated with
shearing on some fracture planes. These results also show a significant correlation
between the orientations of the permeable fractures in relationship with the
orientation of the in situ stresses. In conclusion, these models point out the
importance of the knowledge of the geological structural and tectonic context of the
site and the role of shearing mechanisms in the stimulated fracture planes.
References
Dezayes Ch., Genter A. and Gentier S. (2004) - “Fracture network of the EGS
Geothermal Reservoir at Soultz-sous-Forêts (Rhine Graben, France).” Geothermal
Resources Council Transactions, v. 28, p. 213-218.
ITASCA (2003) - “3DEC Version 3.0, 3 Dimensional Distinct Element Code.” User’s
Manual. Itasca Consulting Group Inc., Minneapolis, MN., June 2003.
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