Domestic geothermal potential

The increase of the proportion of renewable energy resources is a high priority task, both regarding the present energy policy situation and the energy directives of the European Union that keep in mind the protection of the environment. Geothermal energy deserves concentrated attention as our circumstances are outstanding even on a world scale. The solid crust underneath the Carpathian Basin has become thin therefore the hot magma located underneath heats up the rock mass better. This causes the considerable increase of heat flux of the earth; its average value is 90 mW/m² which is one and a half times greater than in the continent.  Due to the positive heat anomaly the average geothermal gradient in Hungary is also very high, 50°C/km, so the chances are high to find thermal water below 2000 m with a temperature above 100°C.

Deep geothermal exploration

There are only a few examples in the international geothermal energy practice of facing challenges like in the Pannon Basin where a certain part of the great number of thermal water occurrences are of positive pressure (even a 360 bar casing head pressure occurred in an extreme case); thermal water shows a high degree of salinity (a salt content of 20-40 g/l is frequent), or it can be found in sandstone reservoirs where the technology of pressing back water is a demanding task. Consequently geothermal energy exploration below 1500 m requires considerable technological preparedness and a significant ability to take risks.

For the utilisation of geothermal energy in power plants thermal water with a temperature higher than 120°C or hot steam is required. A number of presently available technologies are capable of generating power at much lower temperatures; however, the efficiency of these systems is normally unsatisfactory. Domestic deep geothermal exploration primarily aims preneogene reservoirs that are in an optimal depth range (2.5 – 5 km) in most part of the country. This geological medium normally has very low matrix porosity or permeability, respectively; the flow of the hot fluid is primarily tied to the mesh of cracks. Due to that the exploration and delimitation of deep, strongly tectonic zones plays an outstanding role. In case of low permeability the forced cracking of the reservoir in the surroundings of the borehole is a realistic option that may result in a significantly higher yield.

With the widely spread binary system power plants the opportunity to generate electric energy is given at numerous areas of Hungary. According to preliminary exploration and the calculations of CEGE it is realistic to achieve 60 MW_e electric and – in cascaded system – 240 MW_t total output by 2020, by installing geothermal power plants in Hungary.

The role of temperature

The precise determination of the high temperature demanded by power plant use has high priority when selecting the geothermal project. Generally we can rely upon measurement data of existing hydrocarbon exploration boreholes but in most cases these underestimate the actual temperature of the reservoir. Several methods are used to correct temperature of which the Horner approach and the approach performed with Semi log plot provide very reliable results. Their use depends on the fullness of the information available i.e. whether the period of circulating the mud is known, and whether temperature measurements were taken with adequate regularity in function of time elapsed since circulation.

A high earth heat flux is in fact an important aspect when delimiting potential geothermal areas; its value depends principally on the local temperature gradient and the heat conductivity of the rocks.

The role of permeability

Besides achieving the required temperature one of the most important conditions of the operation of deep geothermal systems is adequate permeability, as the permeability rate of the given reservoir substantially influences the thermal water quantity that can be exploited. This parameter can be expressed numerically through the laboratory testing of permeability if there is a core sample available from the preliminary drilling. By performing different borehole tests the average permeability of the reservoir can be calculated with good approach. Tests made by testers, swab tests, lifting tests performed by injecting gas, and pump tests lasting from a few hours to weeks can provide accurate information. Quite often it is not possible to express water providing permeability numerically. In such cases the different hole-geophysical sections can assist.

By the exact geological-structural interpretation of the seismic sections it is possible to isolate sedimental series, to identify and mark geological structures and tectonic borders. The comparison of seismic interpretations with the gravity and magnetic measurement results could provide further useful information. Electromagnetic methods, in particular the magnetotelluric method is a great assistance in exploring crust structures and delimiting zones having low resistance and good conductivity.

Besides surface exploration important information can be obtained by applying well logging geophysics in the borehole, and by using probes developed for different purposes the physical parameters of the reservoir can be determined with appropriate certainty.

 Direct heat supply

Geothermal energy stored by low enthalpy (<100°C) systems can be used for direct heat utilisation; in case of successful geological exploration suitable thermal water reservoirs can be found in 75% of Hungary’s territory.

The utilisation of geothermal energy is practical in a complex cascade system as the utilisation of all heat ranges of the thermal water further improves economic efficiency.  In case of closed system technological solutions a really environment-friendly and pollution-free operation can be expected. The thermal water delivers heat through heat exchangers and is pressed back to its own layer through boreholes.

The majority of the units presently operating in Hungary, supplying geothermal energy directly could not yet solve the pressing back of cooled down thermal water.  Maintenance of the boreholes is uncared-for at several locations and requires renewal. The geological and energy technology concept of operating systems requires re-consideration, update, and investments into the technology.

Many local governments and enterprises would like to become independent from conventional energy resources or at least reduce their dependency. They want to modernise their energy supply systems by using the existing, out of operation hydrocarbon boreholes in their surroundings, or even by greenfield investments, with earth's heat based technologies.

CEGE Zrt. has the solution for these situations. As a professional investor and with a technical-technological and financing concept up to the twenty first century we offer co-operation to municipalities, garden farms and other ventures, in order to implement economical energy technology systems with a long-term sustainability in Hungary and in the entire Pannon Basin as soon as possible.