Variation of the pressure of natural underground water flows

The geothermal reservoirs produce a high enthalpy flow which they evacuate via the water arteries. The geological fluid which circulates within those is subjected to a high gradient of pressure which causes:

• to increase kinetic energy (i.e. to put the geological fluid moving and to increase its speed along its way -! it is of a gradient speed and not about an acceleration),
• to increase potential energy (i.e. to raise the geological fluid according to a profile of rise rising from the laws which governs its hydrodynamics),
• to fight the frictions undergone by the flow along its walls of contact.

Outline of the evolution of the pressure along ascending underground water flow

The figure below presents the schematic evolution of the pressure starting from the geothermal reservoir until immediate proximity of surface:

Evolution of the pressure along ascending underground water flow

The vertical axis represents the curvilinear X-coordinate, S, i.e. the way gone through by ascending underground water flow starting from its origin. This flow of the geological fluid is carried out between, on the one hand, the exurgence point of the geothermal reservoir and, on the other hand, the surface groundwaters and/or the sources formed on the surface.

The aquifers are concentrated near surface (typically with less than 100 meters deep), whereas the exurgence point of the geothermal reservoir is located at a depth about the kilometer. The key of the increase of underground water flow from the geothermal reservoir is thus carried out within water arteries (which come, by definition, of the geothermal reservoirs). Ascending underground water flow yields its high enthalpy (temperature and pressure) as it approaches surface. The pressure decrease takes place:

• on the whole via the tree structure of water vessels formed starting from the exurgence point;
• in a very weak way, during the transit of the water vessels by the aquifers and phreatic.

The horizontal axis represents the evolution of the pressure along water circulation. The gradient of pressure being almost constant within the same water vessel, the pressure decreases quasi linearly with the displacement of the geological fluid measured starting from the exurgence point.

Contrary to the change of the water temperature, the pressure decreases especially in the water vessels and little in the crossed water reservoirs.

Conclusion

As indicated for the exploitation of the thermal energy of natural underground water flows, it is advisable to carry out the water collecting well directly at the exurgence point of the water artery. The pressure falls in a definitely less way, according to the height, in a well of collecting well that in a water vessel.

ANGIOO recommends an exploitation of the pressure present within for the purpose of water vessels electrical production in accordance with the principles of the angiogeology.