75 Proc. 7th NZ Geothermal Workshop 1985 REINJECTION: A FRENCH APPROACH JEAN-MICHEL COUDERT Gkothermal Public Service, BRGN, BP 6009, 45060 Orleans Cedex, France. The' Paris Basin ABSTRACT This large sedimentary basin, 3 km deep under the Brie, extends a5 far as England. It is distin- This paper gives a brief introduction to the guished by calm tectonics, with the geological geothermal resources of France. These are all of beds arranged regularly like a stack of plates, the low temperature (T < 100°C) type, initially and by the considerable development of porous identified in the sedimentary basins by petroleum formations. Hence the resource displays a exploration. "continuous" character. The geothermal "doublet" reinjection technique is The main aquifers appearing in the section (Fig. described. More than 40 doublets have been - 2) are designated by their standard geological drilled and are exploited in the Paris Basin. name. These geothermal developments have taken place only during the past 6 years, and so far no major A well drilled at the centre of the basin will problems have occurred. Mathematical modelling encounter the following sequence: has been carried out, and computer programs developed. Albian and Neocomian sands: the water is fresh and the temperature about 30°C under the Paris urban area. These aquifers are tapped for air- conditioning of buildings (the Albian for the Introduction National Radio Agency in 1956 and the Neocomian at Bruysres-le-Chhtel, south of Paris, in 1980). In 1973 France was importing 80% of its energy. Very stringent legislation now protects the Albian Among major occidental countries only Japan and aquifer, which was over-used at the turn of the Italy were more dependent on energy imports. By century. Nevertheless, operations tapping these 1985 this dependency has been lowered in France to aquifers do not (yet) need reinjection wells. 55%. This evolution has occurred for several reasons, among them increase in the use of nuclear The Dogger: this is Europe's most important energy, geothermal energy and energy savings. geothermal resource base. The high-productivity Even though its contribution is still marginal, area, where more than 300 m3/h of water at a the use of geothermal energy in France is of temperature exceeding 70°C can be obtained, covers interest because: about 15,000 km2 including the Paris urban area. An artesian flowrate of 400 m3/h was measured at - the temperature in French reservoirs is Bonneuil. The temperature is as high as 85°C at produced by normal terrestrial heat flow; Coulommiers at the centre of the Brie region. The saline pore water of the Dogger aquifers (20 to 35 - the doublet technique is used widely; g/R) must be reinjected. - development of geothermal energy has occurred Trias sandstones: These are well known in very quickly. Lorraine, where they supply swimming pools and breweries. Water composition is non-saline and does not require reinjection. However, in other areas Triassic sandstones are not tapped because Geothermal resources in France of reinjection problems. Geothermal resources in metropolitan France are The Aquitaine Basin nearly all of the low temperature (T < 100°C) type. They have been identified in sedimentary This basin contains several continuous aquifer basins such as the Paris Basin, the Aquitaine formations. Inframolassic sands are tapped at Basin and basins in the Alsace. Blngnac near Toulouse and at Lamazere (Gers); the Cenomanian of the Bordeaux region is intensely These deep aquifer formations were essentially tapped for geothermal energy (Bordeaux), and, to identified by petroleum exploration. The data the north, the Trias sandstones are tapped at obtained were used for geothermal developments, Jonzac and at Rochefort (Charente-Maritime). thanks to favourable legislation and close collaboration between oil companies and those However, most of the fields encountered are involved in geothermal energy. discontinuous. These include the very thick Jurassic and Cretaceous limestones tapped at Figure 1 shows the extension of known aquifer Mont-de-Marsan and at Dax (Landes). formations where the temperature exceeds 30°C. 76 COUDE RT \ RESOURCES Proved Pro ba ble Possible important PRODUCTIVITV averaae Crystal 1i ne 3, A Isoternperature curve of deepest known 0.. massifs aquifer Fig. 1: Extension of known aquifer formations in France where temperature exceeds 3OoC within the deepest known aquifer. 77 COIIDERT 0 (PARIS) E ANGERS TOURS ORLEANS MELUN MEAUX ' AElMS VEAOUN MET2 A b b b b b b b mar 0 -io00 - Zoo0 - -2000 -3000 - -3000 NIVEAUX STRATIGRAPHIQUES AQUIFERES 0 Toriiaire Q) Crdtacd supdrieur Sables de I'Albien Q) Crdtacd infdrieur Sables de la base du Cdtac6 @ si^^^ ~up6rieur Celcatre du "Lusitsnien" a Jurassiqua moyen Calcaire du Oogger I- N Lirntte sirattgraphique Fig. 2: Diagrammatic cross-section (W-E) of Paris Basin. Geothermal water in the Aquitaine Basin is This technique meets two objectives: normally fresh, making it possible to drill single wells and to discharge the water at the surface - to maintain the pressure of the field for all operations. At very great depths, the throughout the production period in order to Aquitaine also has aquifers in which very salty maintain the initial yield; water is at over ~OOOC. - to dispose of saline water which cannot be Several other areas also present interesting discharged into the surface environment. features, but up to now have not been exploited (Alsace, North Basin, Languedoc). Around the reinjection well a colder zone will be created which will spread gradually, and ulti- mately will reach the production well. The dis- Geothermal doublet technique tance between these two wells must therefore be calculated beforehand, at the level of the In France, the distinguishing feature of tapping aquifer, so that the decrease in temperature does systems in comparison with other' countries where' not affect the production well for a period at low enthalpy applications are .highly developed least as long as the amortization period of the (Iceland and Hungary, particularly) is the large installation (generally it is much longer). This number of operations using the geothermal period must be compatible with the accepted "doublet" technique. service life of a borehole. This interval is normally taken as about 30 years. The term "doublet" applies to systems which Rut it does not mean that when the cold zone include two boreholes, one for production and the arrives at the production well the operation must second one to reinject the water after extracting be closed down. In fact the geothermal doublet its heat. The first realization of such a doublet can continue to operate beyond this deadline as scheme took place in 1969 at Melun, a town of long as the production temperature remains high 50 50,000 inhabitants km south east of Paris. enough for the installation concerned and the state of the boreholes is satisfactory. The 3 4 Figs. and present schematically the geothermal decrease in temperature is estimated in normal doublet principle and a geothermal low enthalpy exploitation conditions as 2°C each five years. operation. 78 CO UDERT Dir tribu rim netuork Fig. 4: Diagram of a geothermal doublet scheme. Fig. 3: Principle of a geothermal doublet scheme. Rggion sud-ouesf Rggion parisimne Fig. 5: Geothermal exploitation in the Aquitaine and Paris Basins. 0) d ) Being developed 0) 0 Operational 79 COUDERT During the last ten years more than 40 doublets Fig. 6 shows the streamlines and chermal fronts have been drilled in the Paris Basin. All of them for a system of five doublets in the Paris Basin. have been exploited (Fig. 5). Other programmes have been developed based on Up to 1985, about 200,000 TOE (tonnes of oil equi- METERNIQ which allow prediction of the same valent) were saved with geothermal energy. The outputs but with multiple-layered aquifer, and target of the French Government is to save 800,000 with aquifers limited by faults. TOE per year by 1990, and more than one million per year by the year 2000. Other examoles Reinjection in Dogger limestones Triassic sandstones It is said in France: "Happy people do not have a Reinjection in this aquifer has not yet been history", and we could also say that happy successful. A lot of work on this subject is at reinjection does not have a history. No major present being done by the French Geothermal problem has occurred with reinjection wells in the Institute, IMRG (Institut Mixte de Recherche Dogger limestones. Tb-re are two main reasons for Geothermique). The main results of their work this: will be published soon. - the high porosity of the aquifer; Aquitaine Basin the fact that, in the geothermal loop, a In this part of France the very good quality of pressure (in general about 5 bars) higher than the tapped water has allowed it to be used with the "bubble point" pressure is maintained, single wells. But all the operations which have thus preventing a lot of scaling. It has been realized in the past few years include in therefore been possible to reinject at flow- their price a special cost for drilling reinjec- rates of 300 m3/h (83 els) without any tion wells as soon as this will be necessary. problem. Shallow aquifers Reinjection in the same aquifer means that sooner or later some cold water will arrive at the.. Reinjection in shallow aquifers is too complex a production well. The elapsed time between the problem to be discussed here; however, it is beginning of the operation and the declining of practised in France because heat pumps using temperature is a very important parameter both on shallow aquifers are increasingly being used. technical and economic grounds. Mathematical Reinjection problems are different in this case models allow us to predict when it will happen, for several reasons: and how.