Hydrological effects of the underground hydraulic curtain with different design parameters based on numerical modeling for a co-exploitation of coal and uranium

https://doi.org/10.1016/j.coal.2022.104011Get rights and content

Highlights

  • Separating mining the coalfield have a great influence on the water level of overlying uranium deposit.

  • Compared with separate mining coal, the water level in uranium deposit under hydraulic curtain has no longer decreased.

  • A divide is formed between the two ores under the optimal hydraulic curtain.

  • The optimal water injection efficiency realizes the co-exploitation of coal and uranium.

Abstract

Coal is both the most widely utilized traditional energy resource and a raw material for chemical industries. Uranium is an important nuclear fuel used to produce clear electric energy. The coal mine is widely distributed, while the uranium deposit is scarce. However, a large storage of both the two ore deposits, with the uranium deposit lying above the coal seam, was found in the Tarangaole coalfield in Ordos Basin, China, which caused a co-exploitation of coal and uranium with a high risk of radioactive pollution. A hydraulic curtain approach was proposed to realize the safe co-exploitation; the hydrological effects of separately mining the two ores and different-design hydraulic curtain, including different shapes (polyline, straight and arc), different locations (100 m, 300 m and 500 m from uranium deposit), number of injection wells (167, 207 and 247), spacing between injection wells (50 m, 100 m and 150 m), single well injection rates (50, 75, 100 m3/d), and injection positions (Zhiuo-Yanan Formation and Zhiluo Formation), were quantified by numerical simulation. The water level of groundwater dropped from 72 to 300 m with separately mining Plan A-D at the end of the 20th year. The drawdown in the uranium deposit of the polyline curtain shape was only 99.49 m, which was half that of separating coal mining and also smaller than that of the straight and arc-shape curtains. 167 injection wells, 50 m injection well spacing, 100 m3/d single well injection rate, and injection location in Zhiluo-Yanan Formation were the best combination to reach a high injection efficiency of water and protect from the minimum rise of water level (8.69 m). The hydraulic curtain of the polyline, 8300 m length, and 100 m distance to the uranium deposit had a high feasibility for the two resources recovery together. Compared with separate mining plans, the advantages of the hydraulic curtain method could make the two ore deposits not influenced by each other in the co-exploitation of the same time. This study offers a new way to make combined uranium and coal mining practicable, and the findings and methods could be applied to other places with similar problems.

Introduction

Uranium deposits are economically important in many places of the world because they provide a critical raw material for clear nuclear energy to achieve the peak carbon dioxide emissions and carbon neutrality (Forsberg, 2009; Dai et al., 2015). Coal is frequently co-located with uranium deposits but uranium and coal are not in the same strata in some sedimentary basins, and the uranium deposit is usually lying above the coal seam (Fuchs et al., 2015; Atkins et al., 2016). Consequently, Coal and uranium mining influences each other (Akhtar et al., 2017; Li, 2016; Xu et al., 2016). Uranium is also a toxic and radioactive heavy metal and mining uranium often cause groundwater contamination (Cui et al., 2019). Uranium-bearing water may enter the coal mining roadway, causing pollution of coal resources and economic losses. In recent years, researchers in India, Korea and the United States have conducted the surveys of uranium-containing groundwater to evaluate health risks to humans, and the concentrations of uranium in the groundwater are generally higher than the limit values in the World Health Organization's provisional guideline (Coyte et al., 2018).Long-term human exposure to such conditions of uranium contamination can lead to kidney problems and potential toxicity in bone (Shin et al., 2016; Burow et al., 2017). Some researchers have even suggested to raise groundwater quality standard (Coyte et al., 2018). To prevent from these risks of radioactive pollution, in-situ leaching is currently the most widely used method for extracting uranium to reduce uranium-bearing water leakage (Mudd, 2001a, Mudd, 2001b; Haque and Norgate, 2014). Besides, due to different reasons such as faults, unconformities and hydraulically induced fractures, there may be stress changes between coal seam and overlying strata during coal mining (Salmachi and Karacan, 2017). The stratum above the goaf was at risk of collapsing, posing a threat to uranium mining. Therefore, the backfill mining and pressure relief methods are used to solve the collapse above the goaf on account of which could reduce the pressure of the coal wall on the stratum (Zhang et al., 2016; Yu et al., 2021). Since the hydraulic curtain method has the advantages of continuous water resistance, reduction of seepage flow and seepage pressure, installing an underground hydraulic curtain between the uranium and coal deposits is considered to keep the water level in the coal mine stable and prevent uranium-contaminated water entering into the tunnels during the coal mining.

The major purpose of a hydraulic curtain is to raise the groundwater level by regulating the water injection rate and injection well spacing to form a hydraulic wall that has a blocking effect (Mohajit, 2015; Clark et al., 2005; Wang et al., 2018; Li et al., 2016). There are numerous numerical simulations to reasonably design and install underground hydraulic curtains (Wang et al., 2019; Yamamoto and Pruess, 2004). For example, the hydrological effect of a hydraulic curtain under various design parameters (the water injection intensity, duration and location) was evaluated by Mahesha (1996) and Lathashri and Mahesha (2015), and it was recommended that the hydraulic curtain approach might be utilized to reduce seawater incursion. Liange et al., (2009) used an abrupt interface model to ascertain the relationship between the single well injection, spacing of injection wells, and effectiveness in preventing seawater intrusion. Allow (2012) employed a three-dimensional finite-difference model and the variable density solute transport model to simulate the effect of seawater intrusion, and the hydraulic curtain is found to be more effective than the solid barrier in preventing seawater infiltration. Jemcov (2019) monitored the long-term and the short-term water levels in the reservoir, indicating that the grout curtain could detect and remedy the water leakage. Bryson et al. (2014) designed a simulated grout curtain in a seepage model, and they found that a linear relationship exists between hydraulic conductivity and the grout curtain installation sequence. Hu et al. (2019) pointed out that the injection efficiency of water curtains should be adjusted according to the geological conditions. Although the hydraulic curtain has been used successfully to prevent seawater intrusion and to reduce leakage of a dam, it has not been used in a co-exploitation of coal and uranium worldwide including the Tarangaole coalfield.

The Tarangaole coalfield in the Ordos Basin, a basin of the coal-oil-ore enrichment of China, contains considerable coal and uranium resources, with buried at different depths (Li and Wang, 2007; Li et al., 2014; Bolin et al., 2009). Several studies have been published to realize the safe mining of the two mines problem, for example, Yang et al. (2010) put forward an integrated method for multi-energy mineral co-exploration but did not include a plan for synchronous extracting the two minerals. A research paper published in 2014 proposed an orderly method based on the uranium deposit hosting features of coal deposits in the Ordos area, but the method gave up the mining of coal to prevent the collapse of the goaf (Li et al., 2014). By examining the permeability of the formations, Cui et al. (2019) estimated the scope and manner of coal mining, which would not affect the mining of uranium resources but would limit coal mining. Zhang et al. (2021) simulated the groundwater seepage field during coal and uranium mining, but they did not design the relevant coordinated mining plan. In recent years, a hydraulic curtain was proposed as a new method to be installed between uranium and coal mines for a co-exploitation of the two minerals. However, no related study has been conducted to simulate the hydrological effects of the hydraulic curtain with different design parameters (shapes, locations and injection efficiencies) for a co-exploitation and further suggest the optimal scheme of co-exploitation to maximize the recovery of resources.

Therefore, a three-dimensional numerical model was established by MODFLOW, which was based on the hydrogeological survey in the study area to select the optimal parameters of the hydraulic curtain, and the model was calibrated and verified by using the data from four observation wells in different periods. The influences of coal mining with different hydraulic curtains on uranium deposit were determined as the simulated drawdown under the condition of the separate mining and the co-exploitation with hydraulic curtains in the next 20 years. The study aims to answer the following questions: (1) How much influence do different coal mining plans have on the water level in the uranium deposit above a coal seam? (2) What are the influences of the different hydraulic curtains on the seepage pressures of formation? (3) Which is the best design of the hydraulic curtains that can guarantee the safe co-exploitation of coal and uranium mines?

Section snippets

Study area

The Tarangaole coalfield is located in the north Ordos Basin, China. The area is characterized by the arid-semi-arid temperate plateau continental climate, with an average elevation ranging from 1410 to 1530 m (Fig. 1). The surface water system in the study area is part of the Yellow River system. The geologic structure of the area is an erosive hilly plateau, and the main strata in the area were formed during the Cenozoic Era and Mesozoic Era. Quaternary Period (Q) in the area is widely

Conceptual model

The southern and eastern boundaries of the study area were the natural groundwater divide, and those boundaries were generalized into the no flow boundary. The western boundary of the Tarangaole was generalized as the drainage boundary. As the streamline of the lower part of the river was perpendicular to the simulated strata, and the river was regarded as a no flow boundary. At the northern boundary of the study area, groundwater flows from south to north, which could be seen as a general head

Hydrological effect of separately mining coal

The water level in the uranium deposit of Plan A dropped by nearly 200 m after the first mining year. This result was possibly since the first working face of the scheme was closest to the uranium deposit. The hydrological effects in the uranium deposit of Plan B and C were to roughly the same extent throughout the mining period, with the water levels beginning to fall in the 3rd year after mining. The maximum and minimum differences of water level between the two schemes were 31 m and 3 m,

Extent of coal exploitation influencing on uranium deposit

Leaching mining uranium has the advantages of less investment, short working time, no destruction of farmland and forests, but it has the disadvantages of polluting the groundwater environment (Yang et al., 2015; Van Lien et al., 2020). The disadvantage is more pronounced in the Tarangaole coalfield because contaminated water from the uranium deposit may seep into the lower coal seams (Zhang et al., 2017). Uranium recovery would not only pollute the groundwater environment but also be a

Conclusions

Four coal mining schemes have had a significant impact on uranium deposit. The water level drops most during coal mining, even more than 300 m at the end of 20 years. Plan D has the least effect on the uranium deposit, and the water level in the uranium deposit has dropped by only 72 m during the whole mining period. The effects of Plan B and Plan C on uranium mining are similar, with water levels falling by 199 m and 218 m, respectively. Plan C was chosen for coal mining because of the less

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors are thankful for the help and support of the Xi'an Research Institute Co., Ltd of China Coal Technology Engineering Group Corp. This study is supported financially by the National basic Research Program of China (973 program, No.2014CB744702).

References (53)

  • N. Haque et al.

    The greenhouse gas footprint of in-situ leaching of uranium, gold and copper in Australia

    J. Clean. Prod.

    (2014)
  • R. Kumar et al.

    Underground mining of thick coal seams

    Int. J. Min. Sci. Technol.

    (2015)
  • U. Lathashri et al.

    Simulation of saltwater intrusion in a coastal aquifer in Karnataka, India

    Aquat. Procedia

    (2015)
  • G. Li

    Coal reservoir characteristics and their controlling factors in the eastern Ordos basin in China

    Int. J. Min. Sci. Technol.

    (2016)
  • Z. Li et al.

    Design and operation problems related to water curtain system for underground water-sealed oil storage caverns

    J. Rock Mech. Geotech. Eng.

    (2016)
  • Y. Li et al.

    A numerical procedure for modeling the seepage field of water-sealed underground oil and gas storage caverns

    Tunn. Undergr. Space Technol.

    (2017)
  • M.G. Li et al.

    Statistical and hydro-mechanical coupling analyses on groundwater drawdown and soil deformation caused by dewatering in a multi-aquifer-aquitard system

    J. Hydrol.

    (2020)
  • L. Lien

    Advances in coal mining technology

  • W. Shin et al.

    Distribution and potential health risk of groundwater uranium in Korea

    Chemosphere

    (2016)
  • T. Van Lien et al.

    Study on leaching systems and recovery for PALUA–PARONG low grade uranium sandstone ores

    Hydrometallurgy

    (2020)
  • Z. Wang et al.

    Hydro-geochemical analysis of the interplay between the groundwater, host rock and water curtain system for an underground oil storage facility

    Tunn. Undergr. Space Technol.

    (2018)
  • X.W. Wang et al.

    Evaluation of optimized depth of waterproof curtain to mitigate negative impacts during dewatering

    J. Hydrol.

    (2019)
  • H. Xu et al.

    Evaluation of coal macrolithotypes distribution by geophysical logging data in the Hancheng Block, Eastern margin, Ordos Basin, China

    Int. J. Coal Geol.

    (2016)
  • H. Yavuz

    An estimation method for cover pressure re-establishment distance and pressure distribution in the goaf of longwall coal mines

    Int. J. Rock Mech. Min. Sci.

    (2004)
  • J. Zhang et al.

    Research and application of roadway backfill coal mining technology in western coal mining area

    Arab. J. Geosci.

    (2016)
  • L. Zhang et al.

    Hydrothermal mineralization in the sandstone–hosted Hangjinqi uranium deposit, North Ordos Basin, China

    Ore Geol. Rev.

    (2017)
  • Cited by (1)

    View full text