The European HDR site is located near Soultz-sous-ForÍt around 50 km north of Strasbourg in France. During the present phase (2001 to 2004) the drilling of two additional 5000 km deep wells in to a crystalline basement to form a three well HDR module has been completed. These two wells are named GPK3 and GPK4. The existing well GPK2 and the new well GPK4 will be the producers and the central well GPK3 will be the injector. The first well GPK2 was drilled in 1999 and stimulated in 2000. Microseismic monitoring was carried out during this stimulation. A second well GPK3 (injector) was targeted using microseismic and other data and drilled in 2003.

The bottom hole temperature was 200?C and separation between the two wells (GPK2 & GPK3) at the bottom was around 650 m. This well was then subsequently stimulated to enhance the permeability between the wells (Baria et al. 2004). Microseismic monitoring, flow logging and other diagnostic methods were used during these injections. A short two weeks circulation test was performed which showed encouraging result.

Based on the microseismic data the trajectory of GPK4 was planned whereby the bottom of GPK4 was around 1200 m away from the top of the well forming a deviated and complex targeting exercise in to cylinder shaped target volume of around 100 m diameter and 300 m deep at a depth of 5000 m. Prior to the completion of the well, various geophysical logs were performed in the well to get scientific data.

As GPK4 was the production well, it was necessary to stimulate it in such a way so that the near wellbore impedance is as low as possible but also has a good connection to GPK3 (injector). It was also desirable to access the deeper part of the reservoir below the well to obtain higher temperature for production. Around 850 m3 of brine of density 1.9 g/cm3 was used to initiate the opening of the bottom of the well to the formation. This was followed by injection of 9150 m3 freshwater at 30 l/s with occasional bursts of 45 l/s. It was planned to go up to 45 l/s but one of the pumps kept overheating and therefore it had to be shut down. During the injection the wellhead pressure reached ~17 MPa and remained just below this level for the remainder of the test.

The flow log carried out during the fresh water injection showed that around 60% of the fluid was going near the bottom of the well GPK4 with 15% at around 4775 m and 4825 m measured depth with 10% dispersed along the well.

Microseismic monitoring was carried out of the stimulation using the sparse microseismic network at the Soultz site that consists of a number of seismic sensors (Calidus) deployed in wells ~1500 m deep with the bottom hole temperature of ~ 130?C. Additionally a 3 component sensor was deployed in GPK1 at a depth of 3600 m and ~160?C. A 48 channel, 22 bit data digitising unit (Perseids, IFP) was used for data acquisition. The data acquisition was set up in conjunction with a proprietary software (DEVINE, Semore Seismic) to carry out automatic timing and location in real time. This gave a real time decision-making possibility and was very vital to the stimulation strategy and control of the reservoir.

Around 16,000 micro-earthquakes were triggered during the injections and around 4,000 events were automatically timed and located in real time. The microseismic data shows that the initial growth of the reservoir started at the bottom of the well (influenced by heavy brine) and subsequently grew around the open hole of the well as fresh water replaced the brine. The new microseismic data was compared with that from the stimulation of the central well GPK3 and this showed that the new stimulation had not broken through in to the previous stimulation. This would explain why there was no appreciable pressure connection to the adjacent well (GPK3) from the well being injected (GPK4).

Based on the observed hydraulic and microseismic data, a new stimulation strategy is being devised to improve the hydraulic injectivity of the well and to further develop the hydraulic connection between the two wells GPK3 and GPK4.

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