Details on recent seismic events at St. Gallen project in Switzerland
In late July, the geothermal project in St. Gallen, Switzerland reported details on the seismic events that took place near the well that has been drilled.
The project drilled up to a depth of 4,000 meters and showed successful first tests. On July 19th though there was a sudden pressure increase which changed the situation.
The seismic events of that day required a temporary stop of the project. Today, the well is stable and there remain several options for it. But if and how the geothermal project can continue is being subject of stringent discussions.
The first well was drilled to a depth of 4,002 meters according to plan. All technical goals were achieved, the same can be said about the time plan and the estimated cost structure. The results of pre-drilling 3D studies were conformed through the drilling.
On July 14, the developer initiated first injection tests. There was a quick pressure decrease on the well head after water was pumped down, this confirmed the permeability of the reached target zone of the well.
On July 17, the team conducted some acidity work to increase permeability, which were concluded successfully followed by preparation for production test and the installation of an airlift string at a depth of 684 meters.
Just ahead of production tests, on July 19, the project team was suddenly faced with appearing water and gas, which immediately led to a great pressure increase. As usual in situations like this, the team right away targeted the pressure increase by pumping in additional fluids. But a great loss of drilling fluids, also showing the permeability of the zone, did make things difficult. The gas was able to be pushed back according to plan. In normal situations no negative impact would have to be expected.
The following day though, there was a sudden seismic event in the magnitude of 3.5 on the Richter scale, followed by further successively smaller micro earthquakes.
If the pumped in fluids created unexpected strong reaction of the subsurface or this was the result of tension there is unclear. The relatively strong after shocks and other signs though point in that direction.
Overall, the project has shown great transparency and an information campaign to keep all stakeholders and the public involved. Pretty much a model approach that should be considered by other developers, not only in unfortunate events like now in St. Gallen.
To stop the gas flow into the well, the last part of the well was closed. The well though is still intact and is cased to a depth of 4,002 meters and in operating conditions. There currently is no loss in drilling fluids and the drilling rig is ready to continue operation. This allows to be prepared to react in flexible manner on decisions on the future of the project.
There are seen three options for the project:
- Information gathering and continuation: geophysical logging, casing between 4,002 and 4,450 meters, production tests
- information gathering and conservation: logging, casing (see above) with or without production tests, conservation of the well.
- Stop of the project, definitive closing of the well.
Which of these options will be chose cannot be said at this point in time.
Source: Project information newsletter of July 30, 2013 (in German)