Servicio de Información Comunitario sobre Investigación y Desarrollo - CORDIS

Development of capabilities in geothermal reservoir modelling

The modelling of geothermal wells: The evaluation of the behaviour during experimentation of a geothermal reservoir requires knowledge from the surface at least of the temperatures and pressures at the top of the well�reservoir intersection.

On the contrary, the forecasting of the performances that can be used on the surface of a deep reservoir requires that we be able, based on characteristics of this reservoir at depth (productivity, temperature), to estimate the flow rates and temperatures on the surface of the fluid produced according to the operating conditions set by the user.

Measurements taken at the bottom of the well are often hazardous (high risk of equipment breaking down), costly, sometimes doubtful and in some cases even impossible with conventional equipments (case of production by submersible pump).

It is therefore particularly interesting to be able to calculate the data for the bottom of the well using measurements made on the surface. It is with this aim in mind that a model of a well has been developed, an example of whose application at Soultz during the stimulation of well GPK3 Modelling of the impact of the variations in fluid density Virtually all traditional numerical models fail to take account (or do so in a very rudimentary way) of the variations in the density of the fluids circulating underground.

Such variations have two causes (often combined): - mixtures of fresh water/geothermal brine In the case of Soultz, this is a major point due to the abundance of extremely salty thermal water, which is mixed with fresh water for injection in the hydraulic tests and therefore complicates interpretation considerably. - water/rock heat transfers

These will cause considerable variations in temperature and therefore in the density of the fluid and may be intense enough to generate in the reservoir zone exploited (by injection of cold water and production of hot water) some real convection currents.

Such currents will be capable, on top of the risks involved in interpreting hydraulic tests without taking these phenomena into account, of modifying long-term the distribution of the internal permeabilities in the zone exploited by dissolution�scaling reactions.

They may also have a considerable influence on the "thermal life" of the exchangers by developing their exchanges with the natural medium around them. Modelling of the hydromechanical impacts of the hydraulic tests (microseismicity): The aim of this type of modelling is to provide a contribution to the debate on the effectiveness of the hydraulic stimulation operations. The relevant models are still in the development stage.

The next stage (in progress) is essentially oriented towards the comparison observation vs. modelling of the "moment magnitudes"; however, they enable us now to simulate the microseismic activity generated by hydraulic stimulation operations at Soultz. Modelling of water-rock chemical reactions Reservoir conditions at Soultz (i.e. high temperature/pressure and concentrated brine) have involved the development of a new numerical code, FRACHEM, which couples thermal, hydraulic and chemical processes. FRACHEM is adapted from existing codes and its development was possible through the collaboration between ETH-Zürich, CHYN and CNRS teams.

These research works have been published as several PhD theses and articles. FRACHEM code, through the investigations of water-rock interactions, highlights the important reactivity of carbonates (calcite and dolomite) compared to other minerals: carbonated species, more soluble at low temperature, tend to dissolve near the injection well before re-precipitating farther in the fracture. Only a few silicates and aluminosilicates precipitate near the injection well but in low concentration. Chemical reactions induce changes on physical properties of the reservoir and in particular on the permeability.

Its evolution follows the same tendency than the one of carbonates with a permeability increase near the injection well and a low decrease in the fractured zone. This high reactivity of the carbonates is at the origin of "soft" acidification tests carried out at Soultz which results give encouraging prospects.

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EEIG Heat mining
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