Proceedings World Geothermal Congress 2005 Antalya, Turkey, 24-29 April 2005 Possibilities of Production Enhancement of Natural Gas and Geothermal Energy Potential Utilization from Carbonate Reservoir Rocks in the Zavod Structure in the Slovak Part of the Vienna Basin Vladimir Drozd, Oldrich Vana, Miroslav Pereszlenyi SLOVGEOTERM, a.s., Palisady 39, 811 06 Bratislava, Slovak Republic [email protected] Keywords: Natural gas, geothermal energy, depressurization, thermal energy potential n Czech i ABSTRACT Republic Czech Republic s Slovakia The reservoir rocks of gas in the Zavod structure in the a Slovak part of the Vienna Basin are formed by fractured Austria B dolomites at a depth of approx. 4200 m underlying sandy- STRUCTURE clayey sediments of Neogene. Under specific conditions of a ZÁVOD fractured reservoir rock relatively large amount of gas is n n i blocked by active water. Depressurization of an aquifer n s creates conditions for release of this gas, enhancement of gas a e field recoverability and simultaneously gives possibilities of Vienna B i geothermal energy utilization. Bratislava V e Gas fields are in their final stage of production and without a i y b S r r l t a o s v g a u use of this possibility their abandonment in short time can be n u k A ia u H H u n n expected. The presented paper is treating these problems. g a ry a 1. INTRODUCTION D 0 25 50km 1 2 In the past no adequate attention was given to possibilities of geothermal energy utilization in the Slovak part of the Vienna Basin (Fig. 1). The lack of interest in geothermal Fig. 1 Position of Structure Závod in the Vienna Basin energy utilization had several reasons. 1 - Neogene Basins, 2 - Undivided pre-Neogene Alpine-Carpathian units. The first reason was the fact, that it is a case of typical oil and gas bearing basin and from the beginning of twentieth At present gas reservoirs in this structure are in their final century the main attention has been paid traditionally to the stage of production and it is necessary to evaluate the further reconnaissance, exploration and production of hydrocarbon procedure of operations. Newly found knowledge and the fields. In the last quarter of the twentieth century besides gradual decrease of gas production revived an interest in the that the attention was focused on the conversion of possibility of geothermal energy potential utilization of this hydrocarbon reservoirs into underground gas storage. structure. The second reason was the fact that not only from a In this paper there is evaluated the current state and given a worldwide but also Slovak viewpoint, geothermal activity of proposal of realization of work necessary for the recovery of the basin is relatively low. For this reason the research of gas reserves and gradual transition to geothermal energy possibilities of geothermal energy utilization was focused on utilization of the structure. The recovery of remaining gas other more active areas, as e.g. the Danube Basin. reserves and simultaneous utilization of geothermal potential can bring in an interesting economical effect in a relatively Several tens of wells drilled in the of twentieth century short time. within the framework of reconnaissance and exploration of hydrocarbon fields in Zavod structure (Fig. 1) and its close 2. GEOLOGICAL CHARACTERISTIC OF THE vicinity brought apart from oil-geological information also AREA valuable data on temperature, chemistry and partially also on amount of geothermal waters as in pre- Neogene underlier in The underlier of Neogene sediments in the southern part of deeper Neogene sedimentary fill of the basin. Data shows, the Vienna Basin in Slovakia is formed by units of the that at depths of 4000 to 5000 m it is possible to obtain Northern Limestone Alps. Young-Paleozoic, Mesozoic and geothermal waters with temperatures 130 °C to 150 °C from Paleogene sequences form a complex system of nappes, fractured carbonates (largely dolomites) of Mesozoic nappes slices and post-tectonic elements, which have been formed of the Northern Limestone Alps occurring here in the within Upper Cretaceous and Paleogene period. underlier of Neogene sedimentary fill Nappe units of Northern Limestone Alps form up to a 6 km thick rock complex, which is subdivided into three basic groups. In the Slovak part of the Vienna Basin lower nappes (bajuvaricum) are represented by Frankenfeld - Lunz slice 1 Drozd, Vana, Pereszlenyi system, middle nappes (Tirolicum) by Göller Nappe and quartzites, sandstones and variegated claystones. In the higher nappes (Juvavicum) by Schneeberg Nappe. Middle Triassic prevailed sedimentation of various types of shallow water limestones. At the beginning of the Upper The Neogene underlier in the Zavod structure is formed by Triassic sedimentation of carbonates was interrupted and for the Göller Nappe (Fig. 2). The underlier relief of the a short period was replaced by deep water sedimentation, concerned area was within the late Paleogene period early mainly of pelites. Later on in Upper Triassic sedimentation Lower Miocene intensively abraded and peneplanationed. of carbonates was renewed. In the Uppermost Triassic sedimentation with limestones and anhydrites took place. Z88 Younger Jurassic-Cretaceous sequences of the Göller Nappe Z94 Z96 0 80 -3 were not found in the studied area. Z75 Z85 Fig. 4 Lithostratigraphic units of the Göller Nappe Z89 Z74 Z72 1 - Evaporites and shale Z78 Z87 Zlambach formation 2 - Quartzites, sandstones and variegated shale Z73 Z84 Z76 15 Z90 Rhetian 3 - Gray to black bedded limestone intercalated 0 Dachstein 00 14 -4 Z77 Z92 Limestone with dolomite and in places by breccias or Z79 Z93 cherty dolomite Hauptdolomite 0 20 -4 Formation 13 4 - Light-coloured to white organodetrital -4 40 0 0 1 60 -3 12 limestone 2 0 0 -4 2 0 Z81 00 Opponitz 6 4 0 0 4 Hallstatt 3 00 - 0 80 -3 11 -4 -3 Upper Triassic Formation 5 - Gray and light-gray cherty limestone in places 00 Li m esto n e -46 4 5 0 Lunz Member Z97 -320 10 intercalated with marlstone 6 Reingraben Shale Z93 7 9 Carnian Norian Wett erstein 6 - Light-coloured to white organodetrital 7 g e n n e i o Dolomit t limestone and in places reef limestone m s 8 a e e Fig. 2 Structure Závod - Map of pre-Neogene top. R g n m i 7 - Dark to light limestone n o L i t 1 - Lower Göller Nappe, l s f i e e Wett erstein 8 - Dark-gray bedded dolomite overlain by light- m 2 - Lower slice of the Upper Göller Nappe, R Lad inian i Limestone 3 - Upper slice of the Upper Göller Nappe, L gray to white locally massive dolomite 5 6 4 - Isobaths, 9-Darkshale 5 - Thrust boundary, Steinalm 10 - Black or greenish sandy shale variably 3 Li m esto n e 4 6 - Cross section, Middle Triassic Anisian alterating with beds of dark-gray to greenish 7 – Well. Gutenstein Limestone sandstone W E 11 - Yellowish clayey limestone Z89 Z85 Z74 Z78 Z87 12 - Variegated limestone -3 600 2 130 13 - Main Dolomite - thick masses of gray massive Werfen Formation Scythian or reef dolomite and bedded lagoonal 135 Lo wer Triassic dolomite -4 000 140 C 14 - Light-gray to white bedded organodetrical A B B 1 145 limestone with variegated shale Ha se lge birge Permian 15 - Gray thick-bedded clayey limestone, shale Thuring ian De pt h [ m] A 150 and anhydrite -4 500 1 5 155 2 6 Fig. 4 Lithostratigraphic units of the Göller Nappe. 3 7 4 Z85 8 0500m The Main Dolomite of Upper Triassic is reservoir rock, in -5 000 which the reservoirs of gas and petroleum were found. It is simultaneously also a potential reservoir of geothermal Fig. 3 Structure Závod - Cross section with steadysate waters. Stratigraphic span of Neogene sediments is from temperatures. Lower Karpatian to Dacian. These sediments are A - The Lower Göller, Nappe characteristized by alternating sandy and clay layers in a B - Lower slice of the Upper Göller, Nappe variable ratio. Smaller amounts of geothermal waters with a C - Upper slice of the Upper Göller, Nappe lower temperature can be gained from Neogene sandstones. 1 - Neogene 2 - Upper Triassic 3 - Upper Triassic (Main Dolomite) 3. PRESENT SITUATION 4 - Lower, Triassic - Permian After seismic surveys in 1975 there was a proposal to verify 5 - Thrust plane the uplift of pre-Neogene underlier in the Zavod structure by 6 - Un-conformity exploration drilling. The Zavod-72 well (Fig. 2) at a depth of 7 - Isotherm 4200 m drilled Upper Triassic carbonates saturated by 8 - Well hydrocarbons. This result was followed by waste drilling exploration. Successively in the 80’s 20 wells were drilled, Well results show, that the Göller Nappe forms several slices which encountered pre-Neogene rocks in different depths in with a complex internal structure and relatively steep dip of interval of 3450 to 6450 m. beds (Fig. 3). Reservoir rock from which until now economically Characteristic lithological profile of the Göller Nappe is utilizable inflows of natural gas and petroleum have been shown in Fig. 4. The sequence begins in Permian by gained is the Upper Triassic Main Dolomite. Porosity is of evaporites and claystones, continuing in Lower Triassic by two types: primary (matrix porosity) which originated 2 Drozd, Vana, Pereszlenyi during sedimentation and consequent lithification of rocks, with relatively high amount of water. The second is an and secondary – substantially more important, characteristic independent hydrodynamic unit of Z-78 well. Z-78 well by a system of all directional fractures, which originated due produced in 2003 about 2 MM m3 of natural gas monthly. to tectonic stress during the processes of folding and thrusting of nappes.