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Bulgarian Geothermal Association

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Geothermal energy

in Bulgaria

Geothermal resource and potential

Bulgarian territory is characterized by a complex and diverse geological structures. It is built of rocks of different origin, various lithologic and petrologic compositions and of Quaternary to Archean and Proterozoic age.

The main hydrothermal deposits are grouped in three major hydrothermal units: Moesian plate (North Bulgarian Artesian basin), Sredna gora zone incl.Balkan (Intermediate zone) and Rila-Rhodope massif.

Three types of reservoirs are found out in the country - stratified, fractured and mixed (water from a fractured reservoir is secondary accumulated in a younger sediment reservoir).

Hydrothermal data come from prospecting carried out in hundreds of exploratory and production wells and from springs. Most temperature measurements are taken in wells drilled for oil, gas, coal and minerals. The depth of wells ranges from 100 m to 5000 m inNorthern Bulgaria and from 100 m to 1500 m inSouthern Bulgaria.

The Moesian plate has a Caledonian-Hercynian basement and a cover of Upper Paleozoic and Mesozoic sediments. Their thickness decreases from about 6-7 km in the west down to several hundred meters in the east. The main geothermal reservoirs in the platform area are situated in the carbonate strata of Malm-Valanginian, Middle Triassic and Upper Devonian age. They consist of up to 1000 m thick artesian aquifers built up of limestone and dolomite, very fractured and with high permeability. The Sredna gora zone is a rich and heterogeneous hydrothermal region where unstratifed (fault-fractured), stratified and mixed hydrothermal systems are present. Hydrothermal circulation takes place in the fractured massif of granite and metamorphic rocks and in the Upper Cretaceous volcano - sedimentary deposits. Thermal reservoirs are formed also in many postorogenic Neogene – Quaternary grabens filled up with terrigenious deposits.

The western Rila-Rhodopes massif is mainly built of Precambrian metamorphic and granite rocks, fractured by a dense system of seismically active faults. Unstratified hydrothermal systems with thermal waters of low salinity, meteoric origin and of highest measured temperature up to 100^oC are found in this area. The metamorphic basin contains some large bodies of marble that act as hydrothermal reservoirs. Permeable terrigenous-clastic materials in the deep Neogene and Paleogene grabens also contain thermal waters. The eastern part of the massif is not rich in thermal waters.

The most perspective regions for geothermal application are located in the central and eastern part of Moesian plate (J₃-K₁^V horizon) and in Rila-Rhodopes massif, Table1. Nowadays, thermal sources, situated on the northernBlack Sea coast, are mainly utilized. Still partially used are the reservoirs of high thermal potential located in the western part of the Rila-Rhodopes massif (Southern Bulgaria).

Resource Characteristics

The basic characteristics of geothermal water on the territory of the country have been reassessed and updated within the period 1994-1998 by extensive study carried out by the scientists from the Geological Institute under Bulgarian Academy of Sciences. The basis for resource reassessment was data taken from about 160 hydrothermal fields located all over the countries, of which 102 are state-owned. The water temperature of the discovered reservoirs is in the range from 20°C to 100°C. The total dynamic flow rate of sub thermal and thermal waters run up to 4600 l/s (Petrov et.al., 1998), of which 3000 l/s is the flow rate of the revealed thermal waters of T>25°C. About 43% of the total flow rate is water of temperature between 40^oC-60^oC. According to Petrov et al.(1998) a new drilling could discover about 2300 l/s of recoverable resource in addition.

Established chemical water content (TDS) varies respectively, in:

 - Southern Bulgaria - from 0.1g/l up to 1.0 g/l (only for a few sites it is between 1 g/l to 15g/l);

 - Northern Bulgaria - from 0.1g/l up to (100g/l - 150g/l).

About 70% of the thermal waters are slightly mineralized (<1g/l) with fluoride concentration ranging from 0.1 to 25mg/l, various metasilicic acid concentrations (up to 230mg/l) and of mostly low alkalinity. In comparison to most of the European mineral waters, the Bulgarian ones have a lot of advantages: low TDS close to the optimal one typical for potable water, high purity level especially in terms of anthropological pollution, microbiological purity and a variety of water types (Vladeva and Kostadinov, 1996).

The total hydrothermal potential is defined as the thermal energy contained in the discovered thermal waters and amounts to 9 957 TJ/year (Petrov et al, 1998). It has been calculated for output temperature of 15^oC.

The most perspective regions for geothermal application are located in the central and eastern part of Moesian plate (J₃-K₁^V horizon) and in the Rila-Rhodopes massif, (Bojadgieva et al., 2005).

Nowadays, the mostly utilized thermal waters are in five hydrothermal basins, (defined by Petrov et.al., 1998): North-east Bulgaria basin and those inSouth Bulgaria - Chepino, Sandanski, South Sredna Gora and Razlog. They account for 47% of the total thermal water flow rate in use in Bulgaria.

Geothermal catalogue

of Bulgaria