| The air quality, especially the PM2.5(Particulate Matter, the diameter is less than 2.5 μm), troubles the healthy life of residents in China. In 2012, the Greenpeace reported that the coal burning and coal dust are the main sources of PM2.5 in our country, especially in the northern region dominated by coal-fired heating. Traditional coal mining method has restricted the sustainability development of environment in China. China should focus its efforts on the management of air quality pollution and take the important strategies to find the main course of the air pollution and to effectively solve it. Underground Coal Gasification(UCG) is the chemical technology which makes the coal convert into the gas, and it is one of the new energy technology strategy of the country. It is a kind of advanced technology to promote the clean and efficient use of coal resources in China. The normalization and industrialization of underground coal gasification technology has important strategic significance for the sustainable utilization of coal resources in China. In China,the research and production of underground coal gasification was launched after 1980 s. China University of Mining Technology(Beijing) has taken its successful underground coal gasification trial in Xuzhou, Tangshan and other places. They have developed a new underground gasification technology of ?long tunnel, large section, two stages‘ to obtain the hydrogen content of up to 60%-65% of water gas. Over the past 30 years, after the efforts of many scholars and engineers, underground coal gasification technology has made significant progress in China. The successful operation of underground coal gasification were always established based on the study on various control factors in underground coal gasification, especially on studies of the influence of groundwater on the sustained, stable, efficient and safe gas production in UCG. Therefore, study on the mechanism of groundwater in the gasification furnace roof aquifer entering the gasifier has gradually drawn the research agenda, and it also has made preliminary progress so far. The traditional methods for cacultaing the induced the water fracture zone mainly focused on the empirical formula method, simulation method and downhole measurement method. The most recognized method, whch is the most basic, simple and practical one, is also the empirical formula method to calculate the maximum height of the induced water fracture zone, especially for the coal seam which is still slowly inclined and nearly horizontal in China in UCG. Thus, the empirical formula method is widely used for the calculation of the induced water fractured zone in China. However, the practices have proved that the deviation of results which is calculated by adopting the empirical formula is too large, especially for the coal seam roof which is mudstone or shale. The high gasification temperature(1000℃ or higher) makes the physical character of the roof mudstone change, resulting that the actual hight of induced water fracture zone will be unable to use the empirical formula to calculate. With the development of computer technology, large number of numerical simulations was used by the research institutions to study the development of the induced water fractured zone. But there is rare study relating to the conceptual model and numerical model which accurately draws the characterization of temperature distribution of mining area based on the law of time-varying temperature. Additionally, there are also numerous studies performed by the domestic scholars to explore the groundwater seepage field by building the numerical models in China, however, researches on establishing the three-dimensional groundwater model to explore the dynamic changes of groundwater flow based on the development mechanism of the induced water flowing fractured zone are still lacking.On the basis of the above practical needs and current research, this paper takes the development law of the induced water fractured zone and the spatial-temporal characteristics of groundwater seepage field in Meiguiying mine of ENN in Wulanchabu as the research object and presents a comprehensive analysis of the natural geography, social economy, engineering geology, hydrological and geological data and the results of previous studies, and mainly does the main work as follows: Firstly, based on the review of previous studies and the results of the field experiment, it presents a comprehensive analysis of the changing law with temperature of the physical parameters(Young’s modulus, Poisson’s ratio, thermal conductivity, tensile strength, etc.) for the mudstone and mudstone combination in the gasification zone; secondly, this study also draws an engineering geological concept model with the temperature distribution of the roof and then to establish the numerical model for the calculation of induced water fractured zone under different temperature using FLAC3 D based on the composite analysis of the influences on the damage of overlying rock strata in the different gasification stage(the three temperature field), dissecting the development mechanism of the induced water fractured zone under different temperature, gasification stage, gasification depth, thickness of coal seam, length of gasification furnace; moreover, the study also established a 3D groundwater to investigate the mechanism that the water of the roof aquifer enters to the gasification furnace using Visual MODFLOW in the different gasification stages according to the hydrological parameters that were got from the results of the spatial distribution of the induced water fractured zone and the hydrological data. The characteristic of the groundwater seepage field and the water inflow were also obtained according to the hydrologic model; finally, the paper presents the optimized water pumping and injection scheme based on the flow flied model to ensure safety in production and meet the sustainable groundwater environment. The following main conclusions are reached:1. The study area is located in the mid latitude region and its climate belongs to the mid temperature and semi-arid monsoon climate. The gasification coal seam is located below erosion base, and the surrounding rocks, mainly consisted of the mudstone with knot better and argillaceous sandstone, have weak water abundance. The sources of the deposits are mainly Quaternary and Neogene pore fissure water. The groundwater is mainly static reserves and the water abundance of the aquifer is medium. Hydrogeological types are the No.1 and No.2 class, which are the medium mineral deposit mainly filled with the pore and fissure water. What‘s more, the geologic structure of the study area is simple. The exposed strata is mainly made of the sandstone and mudstone with weak cementation of Quaternary and Neogene, and the strength of the rock is relatively low which is loose and soft rocks with poor stability.2. In the mining process, the part of the combustion heat will be lost to the surrounding rock, and the rock thermal physical parameter determines the loss of heat conduction, thereby affecting the temperature distribution of surrounding rock in mining field. Meanwhile, deformation and failure characteristics, strength and physics parameters of the rock(elastic modulus, Poisson ratio, tensile strength and compressive strength) will change with the temperature changes under the influence of surrounding rock temperature. In addition, the mining depth, the width of mining face, the characteristics of rock assemblages and other factors will also affect the height of induced water fractured zone in the gasification process. The resuls of the numerical simulation also show that the induced water fractured zone will be developed persistently, the hight of the induced water fractured zoon will be 35 m in 150-200 days after mining, and then the cracks would be conducting upper coal seam aquifer, threatening the safe and efficient production of gasification. Moreover, the major fractured zoon mainly happens around the initial phase of the gasification, and the highest induced water fractured zone is around 65 m.3. The multi-scenario hydrological models using Visual MODFLOW effectively investigate the dynamic change of the groundwater flow field which is affected by the failure of overlying strata. The study also presents a method of setting the hydrological parameters for the hydrological models based on the development of the induced water fractured zone(these zones are cousidered that the vertical permeability are bigger than the horizontal one), breaking the traditional research pattern. The characteristic of the groundwater seepage field in the upper aquifer system remained largely unchanged in the process of gasification which indicated the hydrological environment of the upper aquifer system was not affected. However, the drawdown funnels were obviously formed in the roof aquifer due to the water pumping before breakover of the coal seam and the aquifer, and the water level would gradually resume a normal level after stopping water pumping. The roof aquifer would be breakover with the continued gasification. At the beginning, the ?inverse drawdown funnels‘ were formed around the fractured zone in the roof aquiclude, and the characteristic of the flow field would be developed to a similar situation to the roof aquifer with the passage of time. That‘s because that the groundwater enters to the roof aquiclude, and produces the similar spatial characteristics of the groundwater level.4. The Pumping wells should be assigned around the induced water fractured zone to lower the water level of the region, ensuring the safe mining. The results of the numerical model indicate that the assigned pumping well has an impact on the groundwater flow field; the drawdown funnels have been obviously formed around the two wells, and the center of the cones decreases with the increase of the pumping rate; the groundwater level researches the minimum value(1165m) on the pumping rate of 253m3/d, and there will been drainage region formed in the induced water fractured zone when the pumping rate is over 290m3/d which indicates the aquifer has been dry and it is meaningless to the continuous water pumping. Thus, the optimal pumping rate is 253m3/d combining with the economic reasons. For the efficient use of the pumped water and the groundwater ecological environmental, we also assign an injection well(the rate is decided according to the need of the production) outside the influenced radius of the water pumping. The simulation results indicate that the added injection well is helpful for the water level recovery of the groundwater, and the ?inverse drawdown funnel‘ will be obviously formed after the injection; the injection scheme is beneficial to the balance of groundwater ecological environment. In addition, the flow field of the gasification area has not obviously been affected by the added injection well, and the influenced radius has not researched the gasification area. Therefore, the injection scheme with the rate of 160m3/d is reasonable and achievable.This paper typically established an engineering geological concept model with the temperature distribution of the roof on the basis of the modification law of the mudstone and mudstone combination under the high temperature and the composite analysis of the influences of the multiple factors from the mining depth, the width of mining face and the characteristics of rock assemblages, and then built a FLAC3 D numerical model considering the time-varying characteristics of the temperature to investigate the development of the induced water fractured zone in the gasification; what‘s more, it also creatively presents a 3D hydrogeology numerical model based on the development of the induced water fractured zone to explore the space-time variation characteristics of the groundwater seepage field, and provides a valuable research mechanism and theoretical basis for the studied of dynamic change of the groundwater flow field under the influence of the induced water fractured zone in UCG. |
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