Hydraulic fracturing at the European HDR/HFR test site Soultz-sous-For

Abstract

The HDR/HFR (Hot-Dry-Rock, Hot-Fractured-Rock) technique allows the extraction of heat
from deep crystalline rock by circulating fluids through an underground heat exchanger which
is formed by a hydraulically stimulated fracture system. During the last 10 years massive
hydraulic fracturing tests have been performed successfully at the European test site Soultz.
By injection of water at high rates and pressure, large fracture systems could be generated in
the crystalline basement between 3 and 5 km depth. A clear understanding of the hydro-frac
process is necessary for the development of more efficient stimulation techniques in order to
achieve higher flow rates and larger fracture surfaces.
Thus, a conceptual model for hydraulic fracturing is proposed which has been developed
based on hydraulic and seismic observations. It is assumed that the hydro-frac process creates
tensile artificial fractures within or between natural faults and major fracture zones. The
opening of artificial fractures during hydraulic stimulation leads to irreversible shearing along
the natural faults. The stimulation experiments at Soultz revealed that a self-propping
mechanism kept the fractures open. This observation is attributed to the shearing mechanism.
Preliminary results from analytical and numerical model calculations confirm the validity of
the conceptual model.