By Gu Dazhao
General Manager, Department of Science and Technology Development, Shenhua Group
Water Shortage: A Constraint to Coal Mining in Western China
The western provinces and autonomous regions such as Shanxi, Shaanxi, Inner Mongolia, Gansu, and Ningxia, are bestowed with abundant resources. Such resources are characterized by large, integrated coalfields with optimal geological conditions, the most concentrated deposits, and the highest quality coal in China. After years of development, this area has become a coal production base featuring fully- mechanized modern mining techniques. As a major coal supply region, western China sets the standard for China’s coal industry and plays an important role in the country’s coal supply network. In 2012, production from the five provinces and autonomous regions reached 2.59 billion tonnes, accounting for 71% of the overall coal production in China.
China is constantly challenged by severe droughts and water shortages. With the world’s fourth largest fresh water reserve, the country has 2800 billion m3 of freshwater, accounting for 6% of the global total. The water resource per capita, however, is merely 2200 m3, only one quarter of the global average. Western China is exceptionally dry with scarce and unevenly distributed water, by location and season. For instance, the northern part of Shaanxi Province is an inland area with high evaporation and little rainfall. The local per capita water resource is merely 927 m3.
It is unavoidable that large-scale and intensive coal mining operations in this area will have negative impact on local water resources. The movement and state of surface and underground water can be affected by coal mine tunnels and depleted gobs (waste rock from coal mining). Examples of potential effects on water include changing the underground water cycle and that can lead to dried-up streams, groundwater recession, and drastic decrease. Together with the enormous amount of mine water discharge, all these problems pose a severe threat to the local ecosystem. Water conservation, therefore, is recognized as the key to sustainable development in this region. However, conventional water conservation technologies are not easily employed in this region due to unique climatic characteristics and the geological formation of coal deposits. It is therefore highly necessary to develop and study new technologies tailored for mining operations in this region.1–3
Water Conservation Development by the Shenhua Group
Shenhua Group is the world’s largest coal production company; in 2012, the raw coal production was 460 million tonnes. The Shenhua Group’s coal resources are mainly located in western China. Since 2003, the Shenhua Group has launched a number of technical projects on the coordinated exploitation of resources and environment. These research projects have been focused on water conservation and utilization technologies. Based on Coordinated Development Technology of the Resources and Environment in Super-large Mine Groups, Conservation and Utilization Technology of Water Resources in Mining of Ten Million Tonnes Mine Group, Underground Water and Surface Ecological Conservation Technology of Shendong Mine Area and demonstration projects, etc., underground water migration directions have been revealed, some technological problems of distributed underground reservoirs of coal mines, such as water migration pathways in the reservoir, construction of coal pillar dam and artificial dam, water scheduling among reservoirs, water treatment technology, and safety monitoring, have been solved. A technology system of distributed underground reservoirs of coal mines has been established.4,5
Distributed Underground Reservoirs of Coal Mines
Large quantities of mine water must be drained to ensure safety during coal mining. However, there are many disadvantages to draining mine water: It results in extreme waste of the water; in addition, mine water drained to the ground surface results in pollution. Moreover, because it is arid in western China, evaporation occurs rapidly and most drained water will evaporate before it can be effectively utilized.
To conserve and utilize water resources effectively, water resource protection and utilization technology must be developed. However, the location of the coal seam is shallow in western China, and coal mining will most certainly impact aquifers and result in the loss of groundwater. Therefore, continued innovation is essential to improve beyond the traditional approaches to water preservation during coal mining. If the purification and storage of the mine water under the colliery without drainage can be accomplished, it will lead to the realization of water-preserving mining in this area.
As discussed in previous sections, the key challenge related to water preservation while mining in western China is finding means not to drain the mine water. One concept is to use the goafs (i.e., the part of the mine from which the coal is extracted) formed by underground mining to store the mine water. If this storage is supplemented by engineering measures, the stored water can be utilized by connecting the water to the surface by drilling holes. The connections among multiple goafs of water storage through artificial pipelines result in an interconnected underground water system, which is distributed throughout coal mine underground reservoirs.6,7
Construction of Distributed Underground Reservoirs of Coal Mines
The coal mine underground reservoirs technology system is composed of design technology, construction technology, and operation technology.8
The design stage primarily includes assessment of three areas: migration patterns of groundwater, site selection of ground- water reservoirs, and capacity design of groundwater reservoirs. Through geophysical exploration, geological drilling, and by assessing similar material in the lab experimentally and with 3D numerical simulations, the migration patterns of groundwater can be assessed to provide the basis for the construction of groundwater reservoirs. The key characteristics to be explored include the hydrogeology, aquifer, aquiclude (barrier to the movement of water), water-filling conditions of mines, ponding of neighboring goafs, and water gushing in mines. By researching the properties of the stratum in the goafs and the permeability of the roof and floor stratums, the construction sites for the groundwater reservoirs can be determined by combining the production technologies and mining parameters as well as the configuration of the working face. With the studies on the spatial dimensions of goafs, stratum conditions, roof caving, and density of the loose body, the optimal reservoir capacity can be determined.
The engineering and construction of the groundwater reservoirs consists of damming and seepage prevention. The damming forms the water storage goafs by preserving the coal pillars and purposefully sequencing mining arrangements at the working face. Seepage prevention includes conducting geological exploration on the palisades and coal walls in the goafs, assessing their permeability, and carrying out seepage-proofing engineering at important locations.
The operation of the groundwater reservoirs includes the reservoir monitoring, coordination, and control of reservoirs. Therein, groundwater reservoir monitoring mainly includes water quality monitoring, security, and protection technology. The water quality sensors can measure the water quality in real time. The security and protection of the groundwater reservoirs is designed to prevent a dam break by monitoring the water level and dam seepage.
The distributed underground reservoirs of coal mines are connected by artificial tunnels or pipelines, which can regulate the water volume in different reservoirs and maximize the utilization of the reservoir capacity.
The Daliuta coal mine in the Shendong mining area is located in Shenmu county of Shaanxi Province; this coalfield is approximately 120 km2. Located on the southeast edge of the Mu Us Desert, it is a drought-stricken area with an annual rainfall of 441.2 mm and total evaporation potential of 2111.2 mm. Given the shallow depth and thick coal seam, there is a severe impact to the groundwater due to artificial drainage caused by mining and water loss due to mining- related fractures in aquifers. The conventional approach to mitigate impacts to groundwater is to pump all underground mine water to the surface for treatment, keeping a small fraction to meet the demands of the mining operations while discharging the rest. This approach not only wastes the precious water resources, but also is a source of pollution to the surface environment. Before introduction of distributed underground reservoirs, the normal water influx at Daliuta was approximately 450 m3/h, with only 180 m3/h recycled, meaning a large amount of water was not being utilized effectively.
A distributed underground reservoir demonstration project was developed at the Daliuta mine, which resulted in zero water discharge, as illustrated in Figure 1. The 2-2 coal seam is divided into 6 panels. Except for panel 1 which is still being used for coal production, all the rest of the panels have been fully recovered, providing sufficient space for underground water reservoirs. Based on analysis of factors such as the engineering geology of the gobs, strength and permeability of the coal wall, and water demand on the ground, appropriate gobs were selected to be water storage areas. No. 1, No. 2, and No. 3 underground reservoirs have been built; these reservoirs are connected by tunnels or pipes to form the distributed underground reservoirs. The No. 4 underground reservoir is under construction in panel 3 of the 5-2 coal seam. Mine water from the 5-2 coal seam, treated by small underground water treatment equipment, can be pumped to the 2-2 seam via drainage pipes. The water will then flow through injection holes into storage areas and then will be purified by gangues (i.e., non-valuable material left behind) in the gobs. The pumping stations then pump the purified mine water to grade level for further treatment before being used for industrial, irrigation, and/or domestic purposes. The remaining water is returned to the gobs for storage through inverted wells.
Built in 2010, the distributed underground reservoirs in Daliuta now have a storage capacity of 2.35 million m3. While ensuring near-zero discharge, the project has also resulted in efficient and integrated utilization of mine water.
The technology of underground storage, treatment, and utilization of mine water has been applied to other mines in the Shendong mining area such as Bulianta, Shangwan, and Wulanmulun, which can be considered an important step taken towards “water preservation” during mining.
Distributed Underground Reservoirs of Coal Mines as a Strategic Measure to Protect Water Resources
Western China is China’s major coal producing area. The aridity and the shortage of water are characteristics caused by natural climatic conditions in this area. Large-scale, intensive coal mining has damaged groundwater and resulted in drainage of large amounts of mine water, which wastes water and has polluted the surface ecology. in addition, a large amount of water has evaporated without being sufficiently utilized because of the high rate of surface evaporation. The process of coal mining can inevitably destroy the overlying strata and forms water-flowing fissures. The traditional water preservation strategy emphasizes “blocking” and “cutting”, that is, trying to block up or cut off the migration of the groundwater during mining by means of water source protection, mining with filling, impermeable section protection, or repair. However, it is exceedingly difficult from both a technical and engineering perspective, and often satisfactory results are not achieved. Therefore, a change from the traditional water-preservation mining policy and adoption of the “draining” strategy is urgently needed. This policy should be based on research and an understanding of the influence of mining on groundwater, transfer of the mine water produced to a safe underground storage space (so as to realize the underground storage and allocation of the mine water), utilization of the water according to the local needs, and achieving the target of no drainage of mine water.
To achieve these goals, this article has proposed construction of distributed coal mine underground reservoirs to reduce and prevent the drainage of mine water, realize the underground storage and purification and filtration of water, optimize the storage of water resources through recharging water pipelines, and realize the effective utilization of water resources by taking advantage of the pumping pipelines. The demonstration at the Daliuta coal mine in the Shendong mining area has demonstrated that the technologies and processes of distributed underground reservoirs in coal mines are feasible with extensive applicable scope and favorable water preservation effects and can become a strategic measure for water resource protection with mining operations in the ecologically vulnerable western region.
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- Gu Dazhao, et al., Water Resource Protection and Utilization for Coal Mining in “Energy Golden Triangle”, 2012: Beijing, Science Press.