Three 5 km depth wells have been drilled into the Soultz geothermal reservoir, which is made of highly fractured granite. The bottom hole zone of the 2 last boreholes were targeted using the development of microseismic clouds induced during hydraulic stimulation tests. This was done under the assumption that the more you get seismic events the higher the density of fractures should be. This method was successful for drilling GPK3, as it was proved that a good hydraulic connection with GPK3 does exist, but not so good for GPK4, which showed a low productivity index. The difference is due to the fact that GPK3 crosses a large, permeable fracture zone at 4.3 km depth and this seems not to be the case for GPK4. As it appears that, for EGS reservoirs, the underground water circulation is mainly driven by several large, permeable fractures, the challenge is to obtain the best reliable characterization of the geometry of these faults.
The VSP experiment achieved last April in Soultz could strongly help to get an optimized view of a fractured reservoir. Based on the reprocessing of older VSP data recorded in GPK1, which showed the ability of the method to define the extension of a main fracture intersecting the well at 3.5 km depth, it was decided to apply a similar technique at greater depth in GPK3 and GPK4. Preliminary results show at least two fractures or fracture zones near GPK4 that are known at borehole’s wall.
Thus for future exploration projects, a methodology for targeting borehole trajectories through a fractured reservoir and reducing the risks of low productive wells could be the following:
- Compilation at regional scale and re-interpretation of geological and geophysical data (wells, 2D seismics);
- Integration of these datasets in a 3D geo-modelling tool in order to produce a geometrically coherent model;
- Perform at local scale (25 km²) a surface 3D seismic survey in order to get a first characterization of the geometry of the fractured medium,
- Drill a first exploratory borehole, which trajectory would be defined by the 3D seismics and analyze the logging data to get the main fractures intersecting the well,
- Perform a VSP survey in order to:
Define the extension of the fractures intersecting the first well;
Locate other main fractures that are not crossed by the well and characterize their geometry;
Optimize the trajectories of future wells, i.e., try to intersect the observed large fractures zones, using the technique of deviated drilling.
Applying this large-scale faults driven procedure could allow to minimize the risks of getting a low productive well, to have a better characterization of the reservoir and consequently to reduce a part of the uncertainty related to the underground.