Thermal water resources in Hungary, their utilisation and protection

NECESSITY AND POSSIBILITIES OF THERMAL WATER REINJECTION

To avoid the pollution by the disposal of used thermal waters in surface recipients and the pressure decrease as the consequence of thermal water production, the reinjection of thermal waters after utilisation appears as desirable solution.

In order to prevent the contamination of subsurface waters reinjection may be considered only when thermal water is utilised in closed energy production system, the possibility of pollution is excluded in a certified way and only into the subsurface space used for the thermal water production or for purpose identical with it.

In the case of balneological utilisation reinjection is allowed only when the reinjected water satisfies every environmental and public health prescription.

There are several examples of reinjection with wells in the world regularly in operation. Also the Hungarian oil industry applies water injection wells for the better production of hydrocarbons.

However examples connected to thermal water utilisation can be found mainly in the case of fissured reservoirs. International experiences are showing that the reinjection into fissured

rocks is a problemless issue, while on reinjection into porous clastic geothermal aquifers no information is available at the present time.

Nowadays in Hungary several systems are known where thermal water is being reinjected into the porous aquifers under lower pressure. Systems in regular operation have been constructed in Hódmezővásárhely, Szentes and Szeged. At the Szeged-Felsőváros Geothermal Heat Plant a doublet of diverted production and injection wells were drilled (Figure 7). At the Szentes and Hódmezővásárhely reinjection sites, production and injection is being realised with the use of the same well: the produced water is reinjected through the annular space (between the two sections of different diameters of the casing) into a layer in higher position (Figure 7). Data of the experimental and regular reinjections are summarized in Tables 2a and 2b.

Table 2a

Summary of the experiences of thermal water reinjection in Hungary

(porous formations)

Table 2b

Summary of the experiences of thermal water reinjection in Hungary

(fissured formations)

Note: In the case of injection wells temperature of outflwoing water is given during production.

Figure 7.
Types of reinjection wells

The Geothermal Public Utility System in Hódmezővásárhely is especially considerable where the favourable results experienced until now are the consequences of the precaution during planning and construction, the drilling technology fitting to the structure of the well, the minimization of pollution of the aquifer and the recipient layer, and the adequate filtering- and operating technology.

Although thermal water reinjection is still not a problemless issue in the case of porous clastic aquifers, results are considerable in a worldwide sense and the continuation of the process seems to be necessary.