Fracture networks dominate the permeability of
crystalline geothermal reservoir rocks. Insitu stress
conditions have a significant impact on the flow,
transport and exchange characteristics of fracture
networks. Here a geomechanical model is presented
which describes fracture closure under effective
stress and the change in parameters such as storage,
permeability, porosity and aperture. The model uses
geometrical considerations based on a fractal
distribution of apertures on the fracture surfaces, and
applies analytical elastic deformation solutions to
calculate the strain response to increases in effective
stress. The model is first applied to fit laboratory
scale experimental data gained on the compressive
closure of a fractured sample (Durham 1997)
recovered from a depth of 3800m from the KTB pilot
borehole (Emmermann and Lauterjung 1997). The
elastic constants for these fits were established
externally, the fitting parameters applied included the
initial aperture of the fracture, the minimum contact
area between the surfaces and the number of
allowable contacts. After accurate fitting of the
laboratory scale experimental data, the
geomechanical model was applied at a field scale to
aid in the modelling of a long term pump test in the
KTB pilot hole, the open hole section being 3850 to
4000m. Effective hydraulic parameters determined
by a finite element model of the fracture systems
connected to the KTB pilot borehole were analysed
on hand of the geomechanical model to allow the
determination of the discrete fracture geometry
operating within the fracture zone. This
geomechanical model takes account of the changes in
the flow parameters within the fracture systems due
to changes in local effective stress as a result of the
groundwater extraction. Applying the geomechanical
model and an iterative procedure allowed the number
of fractures in the fracture zones comprising the
hydraulic signal, and their average aperture to be
estimated. The number of fractures predicted to be
hydraulically active in the fracture zone is of the
same order as in-situ field measurements and the
original fracture logs.

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