Effects of caprock sealing capacities on coalbed methane preservation: Experimental investigation and case study

Abstract

Caprocks play an important role in the trapping of coalbed methane (CBM) reservoirs. To study the sealing capacities of caprocks, five samples with different lithologies of Neogene clayrock, Paleogene redbeds, Permian sandstone, Permian mudstone and Permian siltstone were collected and tested using experimental methods of microstructure observation, pore structure measurement and diffusion properties determination. Results indicate that with denser structures, lower porosities, much more developed micropores/transition pores and higher pore/throat ratios, mudstone and siltstone have the more ideal sealing capacities for CBM preservation when comparing to other kinds of caprocks; the methane diffusion coefficients of mudstone/siltstone are about 6 times higher than sandstone and almost 90 times higher than clayrock/redbeds. To further estimate the CBM escape through caprocks, a one-dimensional CBM diffusion model is derived. Modeling calculation result demonstrates that under the same thickness, the CBM sealing abilities of mudstone/siltstone are almost 100 times higher than those of clayrock/redbeds, and nearly 17 times higher than sandstone, which indicates that the coal seam below caprocks like clayrock, redbeds or sandstone may suffer stronger CBM diffusion effect than that below mudstone or siltstone. Such conclusion is verified by the case study from III3 District, Xutuan Colliery, where the coal seam capped by Paleogene redbeds has a much lower CBM content than that capped by the Permian strata like mudstone, siltstone and sandstone.

Keywords

caprock / sealing capacity / coalbed methane / preservation

References

[1] TaoM-x, ShiB-g, LiJ-y, WangW-c, LiX-b, GaoBo. Secondary biological coalbed gas in the Xinji area, Anhui province, China: Evidence from the geochemical features and secondary changes [J]. International Journal of Coal Geology, 2007, 71(2): 358-370

[2] JinK, ChengY-p, WangW, LiuH-b, LiuZ-d, ZhangHao. Evaluation of the remote lower protective seam mining for coal mine gas control: A typical case study from the Zhuxianzhuang Coal Mine, Huaibei Coalfield, China [J]. Journal of Natural Gas Science and Engineering, 2016, 33: 44-55

[3] SiddiquiF I, PathanA G, ÜnverB, ErtunçG. Sustainable lignite resource planning at Thar coalfield, Pakistan [J]. Journal of Central South University, 2018, 25(5): 1165-1172

[4] WangS-f, LiX-b, WangD-ming. Mining-induced void distribution and application in the hydrothermal investigation and control of an underground coal fire: A case study [J]. Process Safety and Environmental Protection, 2016, 102: 734-756

[5] ZhangG-c, HeF-l, LaiY-h, JiaH-guo. Ground stability of underground gateroad with 1 km burial depth: A case study from Xingdong coal mine, China [J]. Journal of Central South University, 2018, 25(6): 1386-1398

[6] ZhangH, CuiY-j, TaoM-x, PengG-l, JinX-l, LiG-hong. Evolution of the CBM reservoir-forming dynamic system with mixed secondary biogenic and thermogenic gases in the Huainan Coalfield, China [J]. Chinese Science Bulletin, 2005, 50(1): 30-39

[7] JinK, ChengY-p, WangL, DongJ, GuoP-k, AnF-h, JiangL-min. The effect of sedimentary redbeds on coalbed methane occurrence in the Xutuan and Zhaoji Coal Mines, Huaibei Coalfield, China [J]. International Journal of Coal Geology, 2015, 137: 111-123

[8] LeungD Y C, CaramannaG, Maroto-ValerM M. An overview of current status of carbon dioxide capture and storage technologies [J]. Renewable and Sustainable Energy Reviews, 2014, 39: 426-443

[9] WuY-d, JuY-w, HouQ-l, HuS-b, PanJ-n, FanJ-jia. Comparison of coalbed gas generation between Huaibei-Huainan coalfields and Qinshui coal basin based on the tectono-thermal modeling [J]. Science China Earth Sciences, 2011, 54(7): 1069-1077

[10] BaoY, WeiC-t, NeupaneB. Generation and accumulation characteristics of mixed coalbed methane controlled by tectonic evolution in Liulin CBM field, eastern Ordos Basin, China [J]. Journal of Natural Gas Science and Engineering, 2016, 28: 262-270

[11] McintoshJ C, WarwickP D, MartiniA M, OsbornS G. Coupled hydrology and biogeochemistry of Paleocene-Eocene coal beds, northern Gulf of Mexico [J]. Geological Society of America Bulletin, 2010, 122(7): 1248-1264 8

[12] QinY, TangX-y, YeJ-p, JiaoS-hong. Characteristics and origins of stable carbon isotope in coalbed methane of China [J]. Journal of China University of Mining and Technology, 2000, 29(2): 113-119

[13] KędziorS, KotarbaM J, PękałaZ. Geology, spatial distribution of methane content and origin of coalbed gases in Upper Carboniferous (Upper Mississippian and Pennsylvanian) strata in the south-eastern part of the Upper Silesian Coal Basin, Poland [J]. International Journal of Coal Geology, 2013, 105: 24-35

[14] SaghafiA, PinetownKThe role of interseam strata in the retention of CO₂ and CH₄ in a coal seam gas system [C]//10th International Conference on Greenhouse Gas Control Technologies, 2011311731244

[15] SuX-b, LinX-y, ZhaoM-j, SongY, LiuS-bo. The upper Paleozoic coalbed methane system in the Qinshui basin, China [J]. AAPG Bulletin, 2005, 89(1): 81-100

[16] MengY-j, TangD-z, XuH, LiC, LiL, MengS-zhi. Geological controls and coalbed methane production potential evaluation: A case study in Liulin area, eastern Ordos Basin, China [J]. Journal of Natural Gas Science and Engineering, 2014, 21: 95-111

[17] PalmerI D, MetcalfeR S, YeeD, RuriRCoalbed methane reservoir valuation and exploitation [M], 1996, Xuzhou, China University of Mining and Technology Press(in Chinese)

[18] DuS, ShanX-l, YiJ, LiJ-yan. Controlling factors of high-quality volcanic reservoirs of Yingcheng Formation in the Songnan gas field [J]. Journal of Central South University, 2018, 25(4): 892-902

[19] ZhaoZ-l, ZhaoJ-z, RenH-j, LiJ, WuW-tao. Characterization and formation mechanisms of fractures and their significance to hydrocarbon accumulation: A case study of Lower Ordovician mid-assemblage Formations in central Ordos Basin, China [J]. Journal of Central South University, 2018, 25(11): 2766-2784

[20] ChengY-p, WangH-f, WangL, ZhouH-x, LiuH-y, LiuH-b, WuD-m, LiWeiTheories and engineering applications on coal mine gas control [M], 2010, Xuzhou, China University of Mining and Technology Press(in Chinese)

[21] SaghafiA, JavanmardH, RobertsDParameters affecting coal seam gas escape through floor and roof strata [C]//Proceeding of the 10th Underground Coal Operators Conference, 2010210216

[22] ZhangP-h, JinX-l, LiuY-h, WangZ-x, LiuN-na. Synthetical analysis on geological factors ccontrolling coalbed methane [J]. Procedia Earth and Planetary Science, 2011, 3: 144-153

[23] BaoY, WeiC-t, WangC-y, WangG-c, LiQ-guang. Geochemical characteristics and generation process of mixed biogenic and thermogenic coalbed methane in Luling coalfield, China [J]. Energy & Fuels, 2014, 28(7): 4392-4401

[24] HongF, SongY, ChenZ-h, ZhaoM-j, LiuS-b, QinS-f, FuG-you. Study on process and model of CBM dissipating [J]. Chinese Science Bulletin, 2005, 50(1): 134-139

[25] LiuG-d, ZhaoZ-y, SunM-l, LiJ, HuG-y, WangX-bo. New insights into natural gas diffusion coefficient in rocks [J]. Petroleum Exploration and Development, 2012, 39(5): 597-604

[26] SaghafiA. Potential for ECBM and CO₂ storage in mixed gas Australian coals [J]. International Journal of Coal Geology, 2010, 82(3): 240-251 4

[27] NelsonJ S, SimmonsE C. Diffusion of methane and ethane through the reservoir cap rock: Implications for the timing and duration of catagenesis [J]. AAPG Bulletin, 1995, 79(7): 1064-1073

[28] HuangZ-l, HaoS-sheng. Study on sealing of gas concentration and diffusion in overlying gas reservoirs [J]. Acta Petrolei Sinica, 1996, 17(4): 36-41(in Chinese)