Numerical Modelling and Prediction of Abandoned Mine Methane Recovery: Field Application from the Saar Coalfield, Germany Sevket Durucan, Ji-Quan Shi and Anna Korre Minerals, Energy and Environmental Engineering Research Group Department of Earth Science and Engineering Royal School of Mines Imperial College London Page 1
Outline Background Abandoned mine methane extraction in the Saar Coalfield Abandoned mine model development History matching of field production data (Hangard shaft) Production forecasts for two new boreholes Summary and conclusions Page 2
Background The coal bearing strata surrounding an abandoned coal mine constitutes a naturally fractured, naturally stimulated reservoir with up to 80% of the original gas resource being available for subsequent extraction and utilisation. This very favourable characteristic presents abandoned coal mine reservoirs as a very attractive prospect for unconventional natural gas production. Saar Coalfield represents one of the best examples of abandoned mines methane utilisation in Europe Page 3
Location: Saar Coalfield and Study Area in Germany Saarland Page 4
Geology and Mining History in the Saar Basin Upper Carboniferous age (formed 350-285 Million years ago) Section 1-1 Study Area Saarbrucken Settelachse (vermutet) Muldenachse (vermutet) Umbiegungsachse (vermutet) Auf- bzw. Uberschiabung (vermutet) Abschabunbg (vermutet) Melaphyr Lagerang Holzer Konglomerat Abbaugrenze Shnittlinie Stefan B+C Schacht Stefan A Bohrung Westfal D Staatsgrenze Westfal C Basislinie Section 2-2 Page 5
Mining History 1816 Mining activities began 1879/82 Construction of shafts Frankenholz 1 and 2 and later Frankenholz 3, 4 and 5 (5=Hangard) 1903 Start of production 1908 Known CH 4 gas explosions in Saarland workings 1930 2.822 workers produced 484.228 tons coal 1941 Large gas explosion with 41 fatalities and subsequent mine closure 1946 Reopening of Frankenholz colliery 1954 Opening of St. Barbara colliery connection to Frankenholz 1959 Both mines closed Scale 1: 50,000 1960 Connection of Allenfeld and Hangard shafts and upcast ventilation from Hangard 1983 Filling of Hangard shaft (= Frankenholz 5) 1992 Filling of Anna shafts 1 and 2, later known as Kohlwald Frankenholz Colliery is known as the most gassy mine in Europe Page 6
Saar Coalfield: Coal Mines, Power Stations and Gas Network Mine Volume flow rate per day Methane Concentration Hangard 40,000 m 3 73,0 % Kohlwald 75,000 m 3 52,8 % Sinnerthal 24,000 m 3 35,4 % Reden 50,000 m 3 34,5 % Itzenplitz 102,000 m 3 42,0 % Erkershöne 71,000 m 3 30,8 % Camphausen 125,000 m 3 37,3 % Göttelborn 75,000 m 3 29,1 % Alsbach 127,000 m 3 34,6 % Delbrück 174,000 m 3 50,0 % Velsen 130,000 m 3 43,6 % Warndt 195,000 m 3 49,2 % Nordschacht 18,000 m 3 32,9 % Until 2002 DSK produced mine gas from 13 shafts, with methane concentrations in the produced gas varying from 30 to 90%. In 2003 the gas production activities have been transferred to a regional energy producer, STEAG Saar Energie AG, and later in 2011 STEAG Power Saar GmbH.
Seam Depth and Thicknesses: Frankenholz and St Barbara Mines Mittelfeld Ostfeld 1 Ostfeld 2 Seam depth (m) thickness (m) depth (m) thickness (m) depth (m) thickness (m) Scale 1: 50,000 Up to 32 seams of varying thickness between 0.3 3 m in the Frankenholz - St. Barbara mining complex, dipping in Northwest direction. Tonstein 1 56 0.40 8 0.54 - - Fl Kallnbrg 81 1.24 36 0.50 - - Floz Serlo 100 2.00 63 0.94 - - Floz Kliver 103 1.00 81 0.45 - - Floz A 159 0.85 136 0.85 - - Floz HGd 2 189 0.6 170 0.45 - - Floz HGd 1 199 0.95 176 0.95 159 0.59 Floz B 205 0.98 182 0.75 164 0.30 floz 1 211 0.88 186 0.50 170 1.50 floz nn 216 0.30 193 0.30 180 0.20 floz nn 244 0.45 203 0.30 200 0.90 floz 2 267 1.33 237 0.75 212 1.25 floz nn 272 0.38 252 0.38 227 0.25 floz nn 280 0.35 260 0.20 - - floz 3 311 1.05 276 0.80 241 1.55 floz 4 330 0.40 284 0.40 246 0.95 floz 5 338 0.60 312 0.60 265 1.60 floz nn 375 0.30 320 0.35 275 0.25 floz 6 385 0.66 337 0.97 291 0.80 floz 7 407 2.12 352 1.65 303 1.20 floz 8 422 1.70 362 0.80 310 2.70 floz 9/10 439 2.20 383 1.60 336 1.45 floz 11/12 446 1.30 395 1.50 347 1.50 floz 13 457 0.51 408 0.50 364 0.65 floz 14 461 0.37 414 0.70 373 1.10 floz 15 475 0.55 427 0.70 389 1.95 floz 16 484 1.80 431 1.70 393 1.35 floz 17 502 0.50 449 0.65 399 1.20 floz18 505 0.74 457 0.70 404 2.45 floz 19 511 1.10 460 0.71 408 0.40 floz 20 535 0.20 469 1.10 414 0.40 floz 21 541 0.80 475 0.30 425 0.55 floz 22 554 0.40 486 0.40 432 0.15 floz 23 560 1.00 492 0.45 437 0.24 floz 24 576 1.60 455 1.35 445 1.30 floz 25 581 1.86 462 1.30 465 1.70
Frankenholz St. Barbara Mining Complex Mining sequence, mined out areas and seam footprint, methane extraction shafts Hangard Shaft Frankenholz I and II Shafts Mined out Area Allenfeld Shaft St Barbara Shaft Between levels 1 and 11 (-470 m), the total thickness of coal is 40 metres in 430 metres of coal measures strata. From 1833 to 1959, Frankenholz and St. Barbara Collieries jointly mined a total coal surface area of 4.5 km 2
Historical gas production from the Hangard shaft Annual production (10 3 m 3 ) 30000 25000 20000 15000 10000 5000 0 methane total gas Hangard Shaft backfilled Kohlwald Shaft backfiled 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year 1 0.8 0.6 0.4 0.2 It was measured that Hangard has vented an average of 6 million m 3 of methane per annum between 1981 and 1984. After the filling of Hangard shaft in 1984 the free methane gas in the mine air was also recovered. Assuming that an average volume of 6 million m 3 methane was lost through ventilation in the period from 1960 to 1984, the total methane flow rates at the Hangard shaft were plotted. 0 Methane concentration Gas extraction from the Hangard shaft reached over 26 million m 3 per annum with a methane concentration of over 57% in the first few years of production. The back-filling of Hangard Shaft in 1984 resulted in an immediate recovery in both the gas rates and methane concentration, reaching approximately 20 million m 3 per annum and 55% respectively. The produced gas quality was further boosted to a high of 90% methane following the filling of the Kohlwald Shaft in 1992. Annual methane rates (million m 3 ) 25 20 15 10 5 0 drainage only drainage + ventilation 1960 1970 1980 1990 2000 Year
Abandoned Mine Reservoir Model Only gas phase (above mine water level) is accounted for While the free gas phase in old mine workings and other voids is a mixture of methane, nitrogen, carbon dioxide and other minor gas components, the adsorbed phase consists of mainly methane, which is at equilibrium with the methane partial pressure in the mine. By working with methane (partial) pressure and an immobile water phase, our in-house CBM simulator (METSIM) was used to describe methane flow in an abandoned mine reservoir. Page 11
Abandoned Mine Reservoir Model An areal model with a uniform thickness of 40 m in 430 metres of coal measures strata (the net thickness of all the seams down to -470 m) was built. I A uniform grid of 710 active grid blocks (100m x 100m) used. Hangard shaft A total coal area of 5.1 million m 2 in the mined out Northeast region (I) and 2 million m 2 in the unmined Southwest region (II) yielding a net coal volume of 7.1 million m 2 x 40 m = 284 million m 3. II Initial gas content: Zone I = 11.5 m 3 /t Zone II = 9.7 m 3 /t Total initial methane in-place @ average gas content of 11 m 3 /t = 4,060 million m 3 Total gas emission until abandonment in 1960 = 2,200 million m 3 Residual methane after abandonment = 1.800 million m 3 Page 12 Methane content (m 3 /t) 20 15 10 5 0 (40, 11.5) (30, 9.7) 0 10 20 30 40 50 60 70 80 Sorption pressure (bar)
Abandoned Mine Reservoir Simulation The model domain covers the entire mined surface areas and natural geological features such as faults are used to form the boundaries of model domain. An enhanced permeability zone is introduced in order to represent the impact of early mining activity on the permeability of the strata surrounding the mine workings. The magnitude of the enhanced permeability to be determined through history matching the field gas production data. The initial gas content represents the average in situ gas content of both the coal seams and other methane-bearing strata in the region. In order to account for historic methane emissions, a pre-extraction production period is introduced. This approach has the advantage that a spatial gradient in the residual gas content is established in the domain to reflect the fact that the remaining seams have been subjected to different degrees of degassing prior to abandonment in 1961. Page 13
Historical Gas Emission and Residual Gas Content Distribution II I Hangard shaft By 1960, when the mines were abandoned, approximately 2,200 million m 3 methane had been emitted since the start of mining back in 1880 s. In order to account for this gas depletion in the model, a pre-extraction production period is included in the simulations. Using a base permeability of 1 md, methane is drained from an imaginary shaft situated at the centre of the mined out region and, when the predefined amount of methane has been drained, the production switches to Hangard shaft. Base permeability Permeability 1 md Enhanced permeability 1960 Hypothetical shaft Page 14
Field Permeability Characterisation History matching of field production data at Hangard Shaft using Imperial College in-house Coalbed Methane simulator METSIM Suction pressure (mbar) 300 250 200 150 100 50 0 field model 1960 1970 1980 1990 2000 Year Annual methane rates (million m 3 ) 30 25 50 md 20 40 md 15 10 30 md 5 field 0 1960 1970 1980 1990 2000 Base Permeability 1 md Enhanced permeability Year Permeability of mining affected zone (I) = 40md Permeability of unaffected zone (II) = 1 md Page 15
Production Forecast for the Frankenholz St. Barbara Complex 1960 Methane pressure distribution with production from Hangard shaft only Hypothetical shaft 2000 Hangard Shaft Allenfeld Shaft Annual methane production (million m 3 ) 9 8.5 8 7.5 7 with Allenfled & Frankenholz Hangard shaft only with Allenfeld Page 16 6.5 2001 2003 2005 2007 2009 2011 2013 2015 Year
Production Forecast for the Frankenholz St. Barbara Complex 2000 Residual methane contents with production from Hangard shaft only Hangard Shaft 2000 Residual Gas Content (m 3 /t) 7.5-8 7-7.5 Allenfeld Shaft Hangard shaft Frankenholz shafts 1 & 2 6.5-7 6-6.5 5.5-6 5-5.5 4.5-5 4-4.5 3.5-4 3-3.5 2.5-3 Allenfeld shaft 2-2.5 1.5-2 1-1.5 Hangard shaft 2015 Residual Gas Content (m 3 /t) 7-7.5 6.5-7 6-6.5 5.5-6 5-5.5 4.5-5 4-4.5 3.5-4 3-3.5 2.5-3 2-2.5 1.5-2 1-1.5 Page 17
Methane Production Forecast from the Allenfeld Shaft Allenfeld shaft was seen to be emitting 5m 3 /min (2.6 million m 3 per annum) CH 4 Annual methane prodcution (million m 3 ) 4 3.5 3 2.5 2 1.5 Allenfeld 2001 2003 2005 2007 2009 2011 2013 2015 Year Residual methane contents with production from both Hangard and Allenfeld shafts Predicted annual methane production at Allenfeld Allenfeld shaft Hangard shaft 2015 Residual Gas Content (m 3 /t) 6.5-7 6-6.5 5.5-6 5-5.5 4.5-5 4-4.5 3.5-4 3-3.5 2.5-3 2-2.5 1.5-2 1-1.5 Page 18
Methane Production Forecast from the Frankenholz Shaft Annual methane prodcution (million m 3 ) 13 12 11 10 9 8 7 6 Frankenholz 2006 2008 2010 2012 2014 Year Residual methane contents with production from Hangard, Allenfeld and Frankenholz shafts Predicted annual methane production at Frankenholz Allenfeld shaft Hangard shaft Frankenholz shaft 1 & 2 2015 Residual Gas Content (m3/t) 6.5-7 6-6.5 5.5-6 5-5.5 4.5-5 4-4.5 3.5-4 3-3.5 2.5-3 2-2.5 1.5-2 1-1.5 Page 19
Methane Production Forecast from the Allenfeld Shaft Annual methane production (million m 3 ) 25 23 21 19 17 15 13 11 9 7 5 2001 2003 2005 2007 2009 2011 2013 2015 Year Hangard + Allenfeld + Frankenholtz Hangard + Allenfeld Hangard Allenfeld shaft Hangard shaft 2000 Frankenholz shafts 1 & 2 Residual Gas Content (m 3 /t) 7.5-8 7-7.5 6.5-7 6-6.5 5.5-6 5-5.5 4.5-5 4-4.5 3.5-4 3-3.5 2.5-3 2-2.5 1.5-2 1-1.5 Predicted annual methane production for all three shafts draining Allenfeld shaft Hangard shaft Frankenholz shaft 1 & 2 2015 Residual Gas Content (m3/t) 6.5-7 6-6.5 5.5-6 5-5.5 4.5-5 4-4.5 3.5-4 3-3.5 2.5-3 2-2.5 1.5-2 1-1.5 Allenfeld shaft Hangard shaft 2015 Residual Gas Content (m 3 /t) 6.5-7 6-6.5 5.5-6 5-5.5 4.5-5 4-4.5 3.5-4 3-3.5 2.5-3 2-2.5 1.5-2 1-1.5 Page 20
Summary and Conclusions A general gas-water two-phase CBM simulator METSIM has been modified to simulate methane extraction from abandoned coal mines Reservoir characterisation was carried out and abandoned mine models were developed for an abandoned coal mine complex in the Saar coalfield of Germany A methodology for reservoir characterisation of abandoned mines has been formulated An areal model to represent the lumped effect of all coal seams that contribute to methane emission Predictions carried out at Imperial College led to two new boreholes, Allenfeld and Frankenholz to be planned and implemented in 2001 and 2006 Page 21