Geothermal reservoirs

The geothermal reservoirs contain water - or at the very least an aqueous geological fluid - with very high enthalpy (i.e. at very high temperature and pressure). They are located typically starting from a depth about the kilometer.

It is this kind of reservoir which is required for the high enthalpy geothermy and hydrotherapy. If a thermal purpose is required, the high enthalpy is only one additional criterion, the key lies in the therapeutic virtues conferred by the mineralization of the geological fluid. When a geothermal reservoir can be exploited for the purpose of hydrotherapy or of balneotherapy, one speaks about hydrothermal reservoir (which is to be regarded as a particular type of geothermal reservoir).

The geothermal reservoirs are crossed by an underground water flow

The geothermal reservoirs - as the aquifers and phreatic - are not a stagnant water pocket; they are crossed by an underground water flow.

When a drilling reaches these reservoirs, no pumping is necessary to bring water surface. This One spouts out naturally with force because of very high pressure which reigns there.

If this reservoir were closed (without external feeding), overpressure would quickly be eliminated by the water taking away and water would cease spouting out in a very short period of time - according to overpressure, of the size of the reservoir and the diameter of the well. However such is not the case. Water continues to naturally spout out ground, over one long period (of tens of years, even much more…), according to the position of the collecting well compared to the reservoir. The positioning of this intake point is very important. It is not very to find a geothermal reservoir, it should also be punctured at the good place.

This permanent productive character (permanent compared to the duration of an industrial investment) of a geothermal reservoir implies two things:

• the geothermal reservoirs leak continuously: if they were fed without being able to leak, they would not finish exploding, which is not noted nowhere, in spite of the omnipresence of these geothermal reservoirs,
• the geothermal reservoirs are themselves fed continuously by water - or an aqueous geological fluid.

Efferent Flow (of exit)

The angiogeology could be redefined like the study of the hydrogeological, direct and indirect consequences, of the leakages which occur in the geothermal reservoirs. The hydrodynamics of these leakages produces stable structures of flow called water vessels (one speaks about water arteries the geothermal reservoirs in the case of). An important part of this website is devoted to the hydrogeological and thermodynamic characteristics of this efferent flow. This One results overall in an ascending underground water flow from the geothermal reservoirs.

Related Flow (of entry)

A steady state outgoing flow, resulting from a reservoir whose variables of state are steady state, supposes a steady state incoming flow. From which does the water come then which feeds the geothermal reservoirs uninterrupted?

With the present stage of its development, the angiogeology is not capable to answer this question. The angiogeoscopy measures 3 D structure and precisely maps outgoing flows of the geothermal reservoirs, but not their incoming flow (what it can do without problem for the aquifers and phreatic which are located at less depth, but not for the geothermal reservoirs). We are thus reduced by it to conjectures about the origin of the feeding flows of those.

A first working hypothesis can be quickly dismissed: this water feeding does not come from the infiltration. Beyond 50 to 100 meters deep (according to the permeability of the crossed layers), consolidated flows of infiltration become very weak whereas the geothermal reservoirs are located at depths which generally exceed the kilometer.

We assume that the geothermal reservoirs are fed by the water of the oceans which penetrates in the ground with depths of several thousands of meters via the oceanic subduction (i.e. the diving of a plate of oceanic lithosphere under another lithospheric plate, until a depth where this plate is recycled in the mantle).

This subduction is accompanied by a hiding of sediments saturated with water. This water is gradually released because of the conditions of pressure and temperature which reign with the depths where it is brought. The plates undergo a kind of drying by rolling. Because of the depths where she east flees and of her thermodynamic state, water circulates without meeting much resistance and can be transferred onto thousands or tens of thousands of kilometers of its place of hiding. It is qualified the unstable one, because it circulates freely and escapes any means from detection, even by angiogeoscopy.

The increase of water starting from the depths where it was flees and transported is accompanied by the formation of geothermal reservoirs which, because of this water feeding, leak and produce thus water arteries. The meeting of these water arteries on the geothermal reservoir is called the neck of the geothermal reservoir. It presents a lengthened form which justifies, amongst other things, that it is well on the level of this leakage point (also called not exurgence) that should be carried out the water collecting well. To Drill with a few tens of meters of this neck requires to dig more deeply. From There in particular the condition of 50 meters that we impose for competitor drillings on those that we let us recommend and which, if they were more productive than ours, would repeal our right to invoice our services according to the conditions that we auto--impose ourselves.

Omnipresence of the geothermal reservoirs

One could not insist too much on the omnipresence of the geothermal reservoirs. Those exist everywhere in world and not only in the areas where gradients of high temperature justify a coloring bright red on the geothermal charts. We do not think that the potential of exploitation of the geothermal reservoirs located in areas considered at high geothermal potential is significantly higher than that of the geothermal reservoirs located in areas deprived of this reputation. There can be a less density of geothermal reservoirs, but their individual potential is not overall lower.

If the angiogeoscopy does not make it possible to locate the incoming flow of the geothermal reservoirs, it makes it possible, on the other hand, to locate those thanks to their efferent water arteries. The omnipresence of the geothermal reservoirs is not a conjecture (like the east the nature of their related flow), but a fact checked by angiogeoscopy in a number of points statistically representative of the world situation. Without technology offering of the possibilities comparable with those of the angiogeoscopy and in the absence of thermal demonstrations of surface, the deep geothermal reservoirs (> 750 meters) are not detectable by the usual techniques of geophysical research. A just founded drilling on a beam of presumptions has only one weak chance to find them and a negligible chance to reach their optimal collecting location (which corresponds to their exurgence point). To Multiply exploratory drillings increases these chances well, but represents as an appreciable cost as the angiogeoscopy makes it possible to save.

This difficulty in locating the geothermal reservoirs brings the companies which want to exploit the high enthalpy geothermy to be folded back on the improved geothermal systems (English EGS). Those exploit a flow forced with important losses and not a natural flow. We support that it is a bad idea. We can only invite these companies interested by the high enthalpy geothermy to initially carry out a exploratory drilling (only one is enough) given by angiogeoscopy. An investment in the industrial exploitation of geothermal reservoirs is much more profitable than its equivalent in the improved geothermal systems.

The potential of the improved geothermal systems (English EGS) is determined by the geothermal charts. The potential of the geothermal reservoirs is given starting from estimates of underground water flows related to the subduction. An estimate of these flows on a world level concerns the macroscopic analysis of the cycle of water.