2023-10-17 06:30:07
Research work carried out by IFPEN has shed light on the mechanisms at play in the problem of clogging in geothermal energy, which can lead, in the worst case, to the cessation of operation of a site. . The institute now recommends putting in place means of action on a case-by-case basis to remedy this, depending on the situation of each site.
Geothermal energy production consists of exploiting the heat present in the earth’s subsoil. Hot water is pumped from the depths, then the calories are extracted using a heat exchanger to supply heating or air conditioning installations in particular. To avoid depleting this natural resource, the cooled water is then reinjected into the well. However, on certain sites such as those in the Paris basin, this step can cause clogging problems and a progressive decline in the permeability of the underground environment linked to deposits of nano-particles. This problem dates back to the 1980s and to deal with it, manufacturers are using filters on a massive scale, but this technique is not completely effective. Research work1 carried out by IFPEN (IFP Energies nouvelles) has made it possible to highlight the mechanisms at play in this clogging and to foresee means of action to remedy it.
“The problem occurs in the presence of siliciclastic rocks whose structure is sandy and granular, rich in clay particles, and which behave a bit like sponges,” explains Eric Kohler, research engineer at IFPEN. Currently, before reinjecting the water, manufacturers filter it in order to retain the clay particles responsible for clogging and to preserve the installations where heat is extracted from the water. On an industrial level, the particles that pass through the filters are very small, because their size is less than a micrometer. Except that we have discovered that even very small particles measuring only a few tens of nanometers can aggregate and form aggregates whose size is larger than the pores of the rock. »
While water with very little salt has until now been considered to pose little risk of clogging, scientists have managed to demonstrate that the few mineral salts present inside can be a factor favoring the agglomeration of particles. The fluid flow speed is also one of the parameters to take into account and the IFPEN teams have observed that it is preferable to reinject the water immediately after having filtered it so as not to leave time for the particles to aggregate, because the phenomenon involved is dynamic.
Filters must always be added after the heat exchangers
Another factor to consider: the diameter of the pipe through which the fluid passes, which determines the mechanical stresses to which the water and particles are subjected. This diameter acts on the effect of shearing and turbulence inside the tube and can encourage the particles to come together, whereas naturally, they tend to repel each other, because they have similar surface properties. This shearing effect is even more significant at the level of the heat exchanger, inside which turbulators are placed, which contribute to bringing the particles together.
Faced with the risk of clogging, there has been a good practice guide published by the BRGM (Bureau de Recherches Géologique et Minières) for several years, but which only recommends using filters before heat exchangers. “Following our work, we recommend establishing remediation strategies on a case-by-case basis, depending on the nature of the fluid, its chemistry, and the type of particles in suspension,” advises Christine Souque, research engineer and project manager at IFPEN. We are already working with industrial groups, in partnership with the engineering company Geofluid and thanks to funding from ADEME, in order to implement adaptation solutions before drilling. In all cases, we recommend adding filters after the heat exchangers. Some countries like the Netherlands are already doing this empirically, without really understanding why, but not France. Given the shearing effect that we have highlighted, it is important to add more. »
Resolving the problem of clogging in geothermal energy is an important issue, because currently manufacturers are refraining from exploiting many natural resources underground in the face of this risk. You should know that when a site encounters this problem, it is very difficult to return to the initial permeability level of the underground environment. And in the worst case, the clogging can lead to the cessation of operation of the site. At the moment, it is mainly found in the Paris basin, the world’s leading producer of geothermal heat. “If we manage to eliminate this problem, it will free up potential for exploiting this considerable natural resource,” adds Christine Souque. Other European countries such as the Netherlands and Germany, which have embarked on major drilling campaigns to exploit this type of geothermal energy, could also be faced with this problem in the future. »
1 This research work comes from, among other things, two theses. The first: “Identification and understanding of colloidal destabilization mechanisms in geothermal processes” thesis defended on 12/20/2022 at CY Cergy Paris University by Ines Raies. The second: “Study of the mechanisms of formation damage during the reinjection of geothermal fluids”, thesis to be defended on 11/23/2023 by Anne-Sophie Esneu.
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