GEOTHERMOVAL project

Initiated in 1988, the GEOTHERMOVAL program was designed to research, evaluate and develop potential geothermal resources in the canton of Valais as an alternative energy source, in particular for space heating and domestic hot water.

This geothermal prospecting program was carried out by the CRSFA (Centre de Recherches Scientifiques Fondamentales et Appliquées de Sion, formerly CREALP) in collaboration with several university institutes. It received financial support from public bodies and organizations under the aegis of the Swiss Federal Office of Energy (SFOE) and the Department of Energy of the Canton of Valais.

There are several reasons why the CRSFA decided to launch a global geothermal research project in Valais:

• Locally high thermal potential, which can be assessed thanks to the presence of springs, structures and tunnels.
• The absence of an exhaustive study of geothermal resources throughout the canton, despite a high concentration of thermal springs.
• The deep circulation flow system linked to the morphology of mountain regions.
• The geological and structural context of the Alps.
• Favourable conditions for the search for alternative energies.

The primary objective of the GEOTHERMOVAL program was to verify the hypothesis that the hot-water outcrops in Valais are merely superficial manifestations of a deep circulation system with very significant geothermal potential (temperature and flow).

To achieve this, it was first necessary to improve our knowledge of the thermal, hydraulic and geological characteristics of the subsoil. This very important work constituted the preliminary phase of the project and lasted two years. The project then entered a new phase (Phase I), which involved medium-depth drilling. This work enabled us to clarify the hydrogeological processes governing deep water circulation in the crystalline bedrock and its sedimentary cover.

Then, based on the results of the studies carried out, a final phase (Phase II) devoted to deep drilling began. Its main aim was to test the hydrodynamic and thermal qualities of the aquifers by means of pumping tests. It was completed in 1996 with the JAFE deep borehole at Saillon.

PHASE I – EXPLORATION AND PROSPECTING

A number of methods were used to study the surface:

• Geology: predictive cross-sections, study of fracturing and geological structures, examination of rocks likely to serve as geothermal aquifers.
• In geophysics: electrical methods, gravimetry, seismic-refraction and seismic-reflection.
• Hydrogeology: measurement of flow, temperature and electrical conductivity in springs, boreholes and observation piezometers, drawing up temperature and conductivity anomaly maps, pumping tests.
• Hydrochemistry: geothermometry, isotope geochemistry, chemical equilibrium modeling.

It enabled us to locate priority sites for study and to define, sometimes with the help of shallow boreholes, for each of them :

• Hydrogeological characteristics of the thermal water resource.
• Current thermal potential.
• How the geothermal-hydrogeological system works.
• The maximum temperature reached at depth.
• The type of rock through which the thermal water flows.
• Thermal potential and flow rate exploitable by drilling.

In addition, the technical and economic studies undertaken during this phase demonstrated that the use of geothermal resources could be economically viable. After four years of research, during which more than 1 million Swiss francs were invested, the GEOTHERMOVAL program made it possible to carry out an exhaustive assessment of the geothermal resources in the canton of Valais, and to come up with concrete proposals for the use of deep geothermal energy. Taking into account estimated temperatures at depth and known flow rates, Phase I identified a thermal potential of around 54 MWt for the whole of Valais.

SOME SIGNIFICANT RESULTS

In the Simplon region, four boreholes between 65 and 150 m were drilled from the tunnel. Two encountered artesian water between 50 and 100 m, with temperatures ranging from 42.5 to 45.5°C depending on depth. In the other two boreholes, rock temperatures ranged from 42 to 55°C. These measurements led to the conclusion that the geothermal gradient in the massif was 5°C/100 m, and enabled us to model temperature variations in the tunnel over the course of this century. A cooling radius of the rock around the tunnel of 150-200 m was also demonstrated, while beyond this radius, temperatures are of the order of those encountered during the tunnel boring work (>50°C).
Various seismic-reflection profiles (BESSON et al. 1992 and 1993) were carried out in collaboration with PNR 20 at Sion, Vétroz, Saillon, Martigny, St-Maurice and Villette-Montagnier (Vallée de Bagnes). Investigations have shown that the main hot-water reservoirs are closely related to the Triassic evaporites, the crystalline basement of the Mont-Blanc, Aiguilles Rouges and Aar massifs, and the Quaternary sediments at the base of the alluvial fill of the Rhône valley. In areas corresponding to thermal springs, deep circulations probably reach temperatures of between 30 and 110°C. In most cases, the total flow rates known per discharge zone are in excess of 50^m3/h. Water chemistry is mainly [Ca>Mg; SO4>HCO3] for Triassic aquifers or [Na; SO4>Cl] for crystalline aquifers.
In addition, a detailed interpretation of these seismic-reflection profiles has revealed a “canyon” at the base of the alluvium along the axis of the Rhône valley, which could act as a “geothermal trap”.

PHASE II – EXPLORATION

Based on the results of the first phase, three sites were proposed for deep drilling (St-Maurice, Saillon and Sion). Only the Saillon borehole was finally selected and completed in 1996. If successful, the plan was to use the hot water to heat premises and homes in the vicinity by means of a heat distribution network.

Jafe de Saillon borehole

The purpose of this drilling was to capture the deep thermal fluids of the Saillon area through several targets. After a long delay due to an administrative opposition procedure by a local citizen, work was finally able to start in March 1996. The project was financed by the NEFF (Fonds national pour la recherche énergétique, 50%), the SFOE (25%), the canton of Valais (13%) and the commune of Saillon (12%).
The JAFE borehole explored a structural compartment in which the lithology and fracturing should have offered a permeability favorable to the circulation of thermal waters. Unfortunately, the “autochthonous” geological model used as a scientific basis was only partially verified. While the drilling did indeed intersect the Saillon fault, the crystalline basement expected beneath the Saillon hill series was not reached. Drilling revealed that the latter series straddles a thick liasic marl-limestone formation, the base of which could not be reached within the contractual length of the project (900 m), despite the drilling being diverted. The absence of fracture permeability in the vicinity of the fault is certainly due to the plastic behavior of the marl. The Saillon fault therefore does not act as a drain where it separates the Liassic sedimentary cover series.
The calculated geothermal gradient of 2.7°C/100 m shows that the borehole did not cross a sector of deeper water upwelling. The chemism of the water tapped is unknown in the region, and is probably an integral part of the rocks in which it was found. As a result, the borehole did not touch any deep fluid corresponding to that captured by a piezometer located at the foot of the Saillon hill.
The geothermal potential of the JAFE borehole is much lower than the expected 2 MWth, coming in at 0.35 MWth. This borehole was therefore a failure according to the criteria set by the SFOE for federal coverage of the risk of negative geothermal drilling.
However, following an operation to acidify the aquifer in the spring of 97, the flow rate increased to 5.6 l/s, making the geothermal potential of the structure more attractive, and raising it to 0.6MWth. As a result, the commune of Saillon decided to use part of this resource to heat the school. The remainder was awarded to the Bains de Saillon company.