Between 2000 and 2005 each of the three 5000 m deep wells (GPK2,GPK3, and GPK4) of the European HDR/EGS project at Soultz (Upper Rhine Graben, France) was hydraulically stimulated through massive water injections. A microseismic monitoring through a network of up to 6 downhole sensors accompanied the stimulation operations and allowed to locate the hypocenters. The purpose of this paper is to review all hydraulic stimulations with focus on the seismic events reflecting the spatial reservoir development. By this, we deduce probable hydraulically active structures and investigate, as far as possible, the fracturing process.
The stimulation of GPK2 triggered 31,511 events of which about 14,000 events could be localised. The spatial distribution is rather narrow with a strike direction around N145°E and reveals a high concentration of events along a plane which intersects the well at about 4760 m MD (measured depth). This depth matches with a fracture zone observed in both geological study of drill cuttings and temperature profiles. It appears that this zone has for a great part governed the stimulation.
A similar flow rate (50 l/s) was applied to stimulate GPK3, but this well was more productive prior to the stimulation. Therefore, an effective stimulation could only be performed in the last days of the operation and it seems that probably a great amount of the injected water penetrated into the formation without a significant stimulation effect. The 21,600 localised events out of 90,000 triggered events show a similar strike direction as for GPK2 but are spatially widely dispersed. The flow zone at 4760 m MD, which was already permeable before, might explain this behaviour. In the course of the operation, the concept of “dual injection” into GPK2 and GPK3 was applied and will be discussed in the presentation.
The stimulation of GPK4 was performed in two stages. During the first operation in 2004, a lower flow rate (30 l/s) was injected in order to reduce the risk of higher magnitude seismicity. The spatial density of the corresponding seismic events is comparable to the stimulation of GPK2, again indicating an effective stimulation and a significant productivity enhancement. The overall strike direction of the seismic cloud (5700 localised events) is in agreement with a shearing process although the stimulation is hydraulically characterised, at least in the early stimulation phase, by a tensile fracturing process. This contradiction might be solved since flow logs also indicate a tensile fracture which is operative only in the vicinity of the borehole.
In the second stage performed in 2005, a slightly higher volume of water than before was injected. The seismic activity was comparable low (3,000 events) and spatially much more dispersed than in the first step. Consequently, the productivity of the well was not improved much.
Two important conclusions can be drawn based on the seismic event distributions :
• Mainly planar structures have been activated by the massive hydraulic stimulations and single features carry the dominant part of the flow.
• The effect of the stimulation in terms of productivity enhancement can qualitatively be derived from the seismic density distribution. A high seismic density correlates with a significant productivity enhancement whereas a more dispersed distribution results from a less effective stimulation.

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