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Paper: The future of oil shale mining related to the mining and hydrogeological conditions in the Estonian deposit

txt: 104 Doctoral school of energy- and geo-technology January 15–20, 2007. Kuressaare, Estonia The Future of Oil Shale Mining related to the mining and hydrogeological conditions in the Estonian deposit Abstract Due to high oil price in 2005 number of oil shale mining claims were requested by mining companies in Estonia, which is indicator of rapid oil shale mining development. There will be new mines opened in near future which causes changes in environmental conditions, mainly decrease of water level. Estonia is leading Oil shale mining country in the world. Oil shale has been mined for 90 years, the peak was 31 Mt in 1981 and has stabilised in level of 13 Mt annually in recent years. 95% of Estonian electricity is generated in oil shale power plants. About 20% of mined oil shale is used for oil and chemical production Keywords Mining, oil shale, hydrogeology, technology, modeling, GIS. Introduction Mining is performed equally in underground and surface mines accordingly with room and pillar and open cast mining with draglines. In low bedding surface mines, mechanical extraction with shovel- truck operations is used. Traditional depth in low surface mines is up to 15 m, 30m in open cast mines and 80 m in underground mines. Oil shale seam thickness is stable – 2,8 m, the layers are intersecting with hard limestone layers making selective mining or enrichment obligatory for getting required quality. 1 Modelling Modelling is relatively new approach for planning new and analysing abandoned mines. Modelling itself is convenient way for choosing and selecting and visualising the results but deciding for optimal modelling method and software is complicated task. There are three main tasks for modelling to solve: • Mining technology • Mining development • Mining influence 2 Technological modelling For modelling environmental influence mining locations and advancing speed are required. Depending on mining conditions, possible technology, availability of equipment and their productivity, mining areas were chosen. The main criteria for redistricting the deposit are possible mining technologies in certain mining conditions. The main criterion is thickness of overburden. Since the advancing speed of mining front depends on mining technology and its geometric parameters, the technology has to be modelled for expecting geometric parameters. Geometric models with GIS model allow easily explaining suitable mining technologies in every certain location. Strip and room-and-pillar mining were modelled with Excel software Visual Basic. In addition Surpac, Encom Discover and Modflow software were used for local cases. 3 Spatial modelling Because of big amount of available drill hole and survey data, GIS and mining modelling systems were used to solve spatial task. Spatial distribution and geographical data were retrieved with Vertical Mapper package. (Fig.1.) For further visualisation of geological data and mined areas Surpac Vision and Encom Discover were used. Ahtme2 Tammiku3 Ahtme4 Estonia5 Estonia 4 Surface mining areaAhtme3 Sirgala1 Ahtme1 Narva1 2Sirgala2 Estonia2 Sirgala3 8 Puhatu1 Underground mining area 9 1 7 Narva2 4 5 3 Puhatu2 Puhatu4 6 Permiskula1 Puhatu3 Puhatu5 Fig. 1. Possible technologies in mining fields depending on the given criteria More expensive or not practiced mining technologies in Estonian oil shale deposit give great increase in surface mining area and different mining influence according to water regime and landscape. Kohtla-Järve Kohtla-Järve Kohtla-Järve Kohtla-Järve Kohtla-Järve Kohtla-Järve Kohtla-Järve Kohtla-Järve Kohtla-Järve UljasteUljasteUljasteUljasteUljasteUljasteUljasteUljaste UljastePõhja-Kiviõli Põhja-Kiviõli Põhja-Kiviõli Põhja-Kiviõli Põhja-Kiviõli Põhja-Kiviõli Põhja-Kiviõli Põhja-Kiviõli Põhja-Kiviõli Vana Vana Vana Vana Vana Vana Vana Vana Vana Kohtla Kohtla Kohtla Kohtla Kohtla Kohtla Kohtla Kohtla Kohtla AiduAiduAiduAiduAiduAiduAiduAidu AiduSondaSondaSondaSondaSondaSondaSondaSonda SondaNarvaNarvaNarvaNarvaNarvaNarvaNarvaNarvaNarva SeliSeliSeliSeliSeliSeliSeliSeli SeliPeipsiPeipsiPeipsiPeipsiPeipsiPeipsiPeipsiPeipsiPeipsi Fig. 2. Development plan of Estonian oil shale mining areas, grid 5×5km 4 Hydrogeological modelling Due to the low mineral deposits and highly permeable overburden the groundwater has strong influence to oil shale mining, inhabitants and nature. Taking into consideration similar geological conditions, thickness of limestone overburden and bottom layer of the oil shale, the water level and drainage radius were interpolated with MapInfo Vertical Mapper software between measured observation well values. The model visualises mining advancing and changes in decreased water level until the year 2025 when four new mines will be developed. Generated models gave the possibility to give prediction about the wetlands or nature reserve areas what could be affected by mining activity and which mining technologies should be used for decreasing mining influence. Retrieved data gives boundary conditions for dynamic modelling with Visual Modflow software. Water level models show the relation of abandoned mines and water flow in mined area (Fig. 4.). In addition to modelling, surface miner was tested as landscape designing tool – creating new lake and river areas. Creating infiltration dams during stripping operations were tested to decrease drainage radius of the mine. Both tests showed good results for sustainable mining operations and gave data for further modelling. Ten oil shale mines in the middle of the deposit have been closed. Water level restoring in these mines gives good practical experience for expecting water level in the neighbourhood of future mines. In addition to good analysing possibilities the 3D models helps to explain water situation to the concerned people. (Fig. 3.) Open cast mining with draglines and conveyor bridges and combined stripping methods with excavators and bulldozers allow increasing mineable overburden thickness and moving mines in southern direction. (Fig. 1.) The development plan was chosen after analysing all potential technologies, risks and expenses. The plan allows evaluating environmental and social impacts of mining until year 2025. (Fig. 2.) Mined out area Surface mines Kukruse Kukruse Kukruse Kukruse Kukruse Kukruse Kukruse Kukruse Kukruse Käva 2Käva 2Käva 2Käva 2Käva 2Käva 2Käva 2Käva 2Käva 2 Küttejõu Küttejõu Küttejõu Küttejõu Küttejõu Küttejõu Küttejõu Küttejõu Küttejõu KiviõliKiviõliKiviõliKiviõliKiviõliKiviõliKiviõliKiviõliKiviõli Mining development until 2025 Kaevandus Kaevandus Kaevandus Kaevandus Kaevandus Kaevandus Kaevandus Kaevandus Kaevandus 2 2 2 2 2 2 2 2 2 Kaevandus Kaevandus Kaevandus Kaevandus Kaevandus Kaevandus Kaevandus Kaevandus Kaevandus 4 4 4 4 4 4 4 4 4 KohtlaKohtlaKohtlaKohtlaKohtlaKohtlaKohtlaKohtla KohtlaTammiku Tammiku Tammiku Tammiku Tammiku Tammiku Tammiku Tammiku Tammiku Surface mines Uus Uus Uus Uus Uus Uus Uus Uus Uus Kiviõli Kiviõli Kiviõli Kiviõli Kiviõli Kiviõli Kiviõli Kiviõli Kiviõli SompaSompaSompaSompaSompaSompaSompaSompa SompaViruViruViruViruViruViruViruViruViru AhtmeAhtmeAhtmeAhtmeAhtmeAhtmeAhtmeAhtmeAhtme OjamaaOjamaaOjamaaOjamaaOjamaaOjamaaOjamaaOjamaa Ojamaa Mining in 2006 OanduOanduOanduOanduOanduOanduOanduOanduOanduEstonia Estonia Estonia Estonia Estonia Estonia Estonia Estonia Estonia TuduTuduTuduTuduTuduTuduTuduTudu TuduPuhatuPuhatuPuhatuPuhatuPuhatuPuhatuPuhatuPuhatuPuhatu Underground mines Permisküla Permisküla Permisküla Permisküla Permisküla Permisküla Permisküla Permisküla Permisküla Haljala2 Haljala2 Haljala2 Haljala2 Haljala2 Haljala2Haljala2 Haljala2 Haljala2 Haljala4 Haljala4 Haljala4Haljala4 Haljala4 Haljala4Haljala4 Haljala4 Haljala4 UbjaUbjaUbjaUbjaUbjaUbjaUbjaUbjaUbja Tammiku Tammiku Tammiku Tammiku Tammiku Tammiku Tammiku Tammiku Tammiku Kohtla Kohtla Kohtla Kohtla Kohtla Kohtla Kohtla Kohtla Kohtla mine mine mine mine mine mine mine mine mine Tammiku3 Tammiku3 Tammiku3 Tammiku3 Tammiku3 Tammiku3 Tammiku3 Tammiku3 Tammiku3 Estonia5 Estonia5 Estonia5 Estonia5 Estonia5 Estonia5 Estonia5 Estonia5 Estonia5 Fig. 3. Water level model in mining area in year 2005 Käva 1Käva 1Käva Käva 1Käva Käva Käva 1Käva 1Käva Käva 2Käva Käva Käva Käva Käva Käva 2Käva Käva 111 1Kukruse Kukruse Kukruse Kukruse Kukruse Kukruse Kukruse Kukruse Kukruse 2222 222Põhja-Kiviõli Põhja-Kiviõli Põhja-Kiviõli Põhja-Kiviõli Põhja-Kiviõli Põhja-Kiviõli Põhja-Kiviõli Põhja-Kiviõli Põhja-Kiviõli surface surface surface surface surface surface surface surface surface mine mine mine mine mine mine mine mine mine Vanaküla Vanaküla Vanaküla Vanaküla Vanaküla Vanaküla Vanaküla Vanaküla Vanaküla Kohtla Kohtla Kohtla Kohtla Kohtla Kohtla Kohtla Kohtla Kohtla surface surface surface surface surface surface surface surface surface Mine Mine Mine Mine Mine Mine Mine Mine Mine 2 2 2 2 2 2 2 2 2 mine mine mine mine mine mine mine mine mine nr nr nr nr nr nr nr nr nr 4 4 4 4 4 4 4 4 4 KiviõliKiviõliKiviõliKiviõliKiviõliKiviõliKiviõliKiviõli KiviõliAidu Aidu Aidu Aidu Aidu Aidu Aidu Aidu Aidu surface surface surface surface surface surface surface surface surface mine mine mine mine mine mine mine mine mine SompaSompaSompaSompaSompa SompaSompaSompaSompaMine Mine Mine Mine Mine Mine Mine Mine Mine nr nr nr nr nr nr nr nr nr 2 2 2 2 2 2 2 2 2 Tammiku Tammiku Tammiku Tammiku Tammiku Tammiku Tammiku Tammiku Tammiku Aidu1BAidu1BAidu1BAidu1BAidu1BAidu1BAidu1BAidu1B Aidu1BViru Viru Viru Viru Viru Viru Viru Viru Viru mine mine mine mine mine mine mine mine mine AhtmeAhtmeAhtmeAhtmeAhtmeAhtmeAhtmeAhtme AhtmeEstonia Estonia Estonia Estonia Estonia Estonia Estonia Estonia Estonia mine mine mine mine mine mine mine mine mine Kohtla Kohtla Kohtla Kohtla Kohtla Kohtla Kohtla Kohtla Kohtla 1 1 1 1 1 1 1 1 1 Ahtme1 Ahtme1 Ahtme1Ahtme1 Ahtme1 Ahtme1Ahtme1Ahtme1Ahtme1 Ojamaa3 Ojamaa3 Ojamaa3 Ojamaa3 Ojamaa3 Ojamaa3 Ojamaa3 Ojamaa3 Ojamaa3 Viru2Viru2Viru2Viru2Viru2Viru2Viru2 Viru2Viru2Viru1AViru1AViru1AViru1AViru1AViru1AViru1AViru1A Viru1A Fig. 4. Water level model in oil shale mining area 5 Hydrogeochemical modelling Geochemical processes which determine seasonal variations were examined in 1979-1981 [8]. The mine water in closed mine were affected by sulphide oxidation. During the mining processes pyrite (FeS2) had been extensively mixed with air oxygen. Oxygen is a master variable in pyrite oxidation [9]. It acts directly in oxidizing the sulphide and the iron (II) as shown by the reaction [10, 11] FeS2 + 7/2O2 + H2O –> Fe2+ 2 SO42- + 2 H+ (1) or indirectly by generating Fe(III) which then oxidizes pyrite. The reaction formulas are as follows FeS2+14Fe3+ +8H2O –> 15Fe2+ + 2SO42- +16H+ (2) Fe2+ + 1/4O2 + H+ –> Fe3+ + 1⁄2 H2O (3) The dissolution of pyrite leads to high concentrations of sulphates. The water displayed neutral pH and positive Eh in the spring-summer than in other times [8]. These results reflect the increasing of the sulphide oxidation rate during the warm months, other time the sulphide oxidation rate was low, but depend on precipitation. During mining the water level drowning and increasing aeration zone cause intensive pyrite oxidation, which is the biggest groundwater pollution problem associated with underground mining. After mine closure the water level rising and pyrite oxidation decrease. The most noticeable change will take place in the sulphate content. 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The water washes water has been caused by oxidation of pyrite in well- the already oxidising pyrite products out of the aerated water, which percolates down through the limestone and the sulphate content in groundwater overburden. In the water, which fills underground will increase. The sulphate may distribute in a lateral mines, the content of this element is high (Fig. 5. – direction many times higher than in transversal Ahtme mine), but lowering and still stays 10 times direction. This may be explained with the higher than its natural background. permeability of groundwater aquifer or aquifer This is naturally accompanied by intensive removal of the sulphates recharging Ordovician carbonate system. Sulphate distribution in underground mine water in 2003 is shown in Fig. 7A. rocks. Significant enrichment of water with the In 2003, in the earliest closed underground mines sulphates takes place in the carbonate rocks in the (Kukruse, Mine no 2) the sulphate content was high aeration zone. There is increasing evidence that in the Lasnamäe–Kunda aquifer. In the western part portions of the water infiltrating through the soil of Tammiku mine the Lasnamäe–Kunda aquifer was surface may move rapidly through the aeration zone very high in sulphate (Fig. 10B). This is promoted along preferred flow paths such as macrospores and by karst and technogenic faults. The Ahtme mine fractures. In many cases, the water has low pH and water pool exerted a weak influence on the contains elevated levels of sulphate ions. Lasnamäe–Kunda aquifer. In the southern part of In recent years, in the area of oil shale mines, the chemical composition of groundwater has been stable. The content of SO4 in groundwater was 2 times higher in spring (Fig. 6.) than during the remaining seasons of the year. It can be caused by dissolution of pyrites in oxygen-abundant water in spring. Kohtla mine and in the northern part of Sompa mine the sulphate content in the Lasnamäe–Kunda aquifer was between 200-320 mg/l. Mine no 4 and also Käva mine pools water amount in the Lasnamäe– Kunda was lower, than in the other mines. In this region a relatively impermeable aquitard may be located between mine pool area and the Lasnamäe– Kunda aquifer. The distribution of sulphate in the Mine No 4 closed in 1975 and in 1990 it was water Lasnamäe–Kunda aquifer may be due to the filled. Mainly precipitation, groundwater flow from circumstance that the permeability of carbonated each side and rising water level caused fluctuations rock in a lateral direction can be up to 100 times in the sulphate content in Mine No 4. The sulphate higher than in a transversal direction. The same content in the water filling up mine is high; in the effect is observed in the Keila–Kukruse aquifer. 1600 1400 1200 Tammiku Sompa Kohtla Ahtme Sulphate content, mg/l 1000 800 600 400 200 0 2003 2002 2004 Fig. 5. The sulphate content of groundwater in underground mines: Tammiku observation well no 0714; Sompa – 486; Kohtla – 0705; Ahtme – 16122 600 S02.2001 42 500 41l /gm, tnetnoce tahplu400 . l.s.a300 40200 m , levelr etawdnuorG39100 0 03.2001 13804.2001 12.2001 05.2001 01.2001 6.2001 Sulphate Groundwater level Fig. 6. The sulphate content in the water of the underground oil shale Mine no 4 in 2001 106 A B Fig. 7. The sulphate content in the Ordovician Keila–Kukruse aquifer (A) and Lasnamäe–Kunda aquifer (B) of underground oil shale mine area. 5. Conclusions There is no single software package for modelling complicated mining development plan as a country’s main mineral mining in a rapid increase period. All available packages have to be tested from both simplicity and information exchange side and from advanced results and analysing side. The results depend form local conditions like people, geology, mining traditions and software availability. For Estonian Oil Shale mining modelling – in addition to traditional office software, MapInfo, Vertical Mapper and Modflow have shown good results. In addition to analysing capabilities the visualisation aspect has shown strong importance for working with development plans. In closed mine workings form underground water basins with higher sulphate content, which may be exacerbated due to the mining methods and underground mining operations. The main results may be summarized as follows: 1. the hydrogeological regime in oil shale mines is controlled by the thickness of the aeration zone, tectonical faults and fractures in the geological section, alteration of hydraulic gradients causing changes in flow direction and rate; 2. closing and flooding of underground mines has changed the groundwater forming conditions in the Lasnamäe–Kunda aquifer and sulphate content within it; 3. due to technogenic impact the water of closed mines is connected with the Lasnamäe–Kunda aquifer. This study is related to EstSF grant G5913 “Usage of mined out area”. References: 1. Reinsalu Enno, Changes in Mine Dewatering After the Closure of Exhausted Oil Shale Mines, Oil Shale, Estonian Academy Publishers, Tallinn, 2005, 261 – 273 2. Reinsalu Enno, Lind Helena, Valgma Ingo, Technogenic water body in closed oil shale mines, Oil Shale, Estonian Academy Publishers, Tallinn, 2006, - 3. Reinsalu Enno, Valgma Ingo, Geotechnical Processes in Closed Oil Shale Mines, Oil Shale, Estonian Academy Publishers, Tallinn, 2003, 398 – 403 4. Tammeoja Tauno, Oil shale in Estonian power industry, Oil Shale, Estonian Academy Publishers, Tallinn, 2003, 135 – 142 5. Valgma Ingo, An evaluation of technological overburden thickness limit of oil shale open casts ny using draglines, Oil Shale, Estonian Academy Publishers, Tallinn, 1998 6. Valgma Ingo, Estonian oil shale resources calculated by GIS-method, Oil Shale, Estonian Academy Publishers, Tallinn, 2003, 404 – 411 7. Taiex Workshop on EU Legislation as it Affects Mining. Department of Mining of Tallinn University of Technology in co-operation with Society of Mining Professors and TU Bergakademie Freiberg 8. Karise, V., Pill, A., Johannes, E., Erg, K. 1987. Water chemical content forming in Estonian oil shale mining area. Manuscript in Institute of Geology. Tallinn, 286. 9. Perens, R., Andresmaa, E., Antonov, V., Roll, G., Sults, Ü. 2001. Groundwater management in the northern Peipsi-Narva river basin. Background report, seminar ”Support to Estonian-Russian Joint Peipsi-Narva Transboundary Water Commission through Capacity Building and Development of Recommendations” (CEE 008, supported by the Swedish Environmental Protection Agency), Tartu, Estonia April 18-19. 10. Singer P. C., Stumm, W. 1970. Acidic mine drainage: the rate – determining step. – Science, 167, 1121–1123. 11. Erg, K. 2005. Groundwater sulphate content changes in Estonian oil shale mining area. – Oil Shale, 22, 3, 275-289. 107

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