A substantial reduction in groundwater level, exacerbated by coal mining activities, is intensifying water scarcity in western China’s ecologically fragile coal mining areas. China’s national strategic goal of achieving a carbon peak and carbon neutrality has made eco-friendly mining that prioritizes the protection and efficient use of water resources essential. Based on the resource characteristics of mine water and heat hazards, an intensive coal-water-thermal collaborative co-mining paradigm for the duration of the mining process is proposed. An integrated system for the production, supply, and storage of mining companion resources is achieved through technologies such as roof water inrush prevention and control, hydrothermal quality improvement, and deep-injection geological storage. An active preventive and control system achieved by adjusting the mining technology and a passive system centered on multi-objective drainage and grouting treatment are suggested, in accordance with the original geological characteristics and dynamic process of water inrush. By implementing advanced multi-objective drainage, specifically designed to address the “skylight-type” water inrush mode in the Yulin mining area of Shaanxi Province, a substantial reduction of 50% in water drillings and inflow was achieved, leading to stabilized water conditions that effectively ensure subsequent safe coal mining. An integrated-energy complementary model that incorporates the clean production concept of heat utilization is also proposed. The findings indicate a potential saving of 8419 t of standard coal by using water and air heat as an alternative heating source for the Xiaojihan coalmine, resulting in an impressive energy conservation of 50.2% and a notable 24.2% reduction in carbon emissions. The ultra-deep sustained water injection of 100 m3·h−1 in a single well would not rupture the formation or cause water leakage, and 7.87 × 105 t of mine water could be effectively stored in the Liujiagou Formation, presenting a viable method for mine-water management in the Ordos Basin and providing insights for green and low-carbon mining.
At present, over 70% of China’s coal production is centralized in ecologically fragile regions in western China—a consequence of the depletion of shallow coal resources in the middle east (Fig. 1) [1]. The structure of underground aquifers is directly damaged by the intensive mining, with the ratio of groundwater damage to coal mining being estimated at approximately 1:2 [2]. A survey has indicated that the average national coalmine water richness coefficient was 1.87 in 2018, with a corresponding minewater production of 6.88 billion cubic meter. However, only 35% of the mine water is utilized for the coal industry, domestic water, and ecological system, due to the high degree of mineralization that raises the treatment cost [3]. The large-scale treatment and utilization of mine water pose significant challenges in China’s “Energy Golden Triangle” (i.e., Shanxi, Shaanxi, Inner Mongolia, and Ningxia Hui), which has a water abundance of 5.52.
Water scarcity and complex water inrush hazards present formidable obstacles to the advancement and application of coal resources in the Ordos Basin of the Yellow River Basin, where extensive and widely distributed Jurassic coal strata are found [4]. The per capita water availability in the region is only 1318 m3, with water resources per unit area accounting for just 1/10 of China’s average. Restricted by the high cost of extensive treatment, policies prohibiting the storage of water in goafs, and the local economy, the utilization rate of water resources in most coalmines is lower than 40%, except for the Shendong mining area. Over one billion cubic meter of mine water is discharged annually, indicating that a considerable portion of the mine water cannot be efficiently utilized and stored, or no longer participates in the groundwater cycle [5], [6]. By leveraging the spatial advantage of the underground goaf in the Shendong mining area, underground reservoirs have been established to store excess mine water, supplying more than 95% of the water needed in the mining area. The stability and storage capacity of the underground reservoir during the mining process are crucial factors that will determine the feasibility and potential widespread adoption of this technology [7].
Groundwater is the primary—and often the sole—water source in arid regions, serving as a vital source for maintaining a favorable ecological environment. Advanced drainage of the groundwater—the most common method used to reduce the water yield and inrush risk in many water-rich mines in China [8]—is extremely unstainable, as the high-volume extraction of groundwater over time has the potential to induce ecological disasters. Desertification, the decline of groundwater level, and land subsidence in mining areas further diminish the regional ecological carbon-sequestration capacity. This trend contradicts the development goals of coal mining, which aim for green, low-carbon, and scientific production capacity [9]. The contemporary chemical processing of coal, which is characterized by high water consumption and discharge, exacerbates water resource depletion and intensifies the conflict between coal mining and groundwater protection. The cumulative area exhibiting a water level decline exceeding 15.0 m is as large as 306.8 km2 in the Yushenfu mining area of the Ordos Basin, with 79.3% of the area concentrated within zones characterized by high and extreme mining intensities [9]. The alterations in water level are greater than the usual natural variations in groundwater levels and can be directly attributed to coal-extraction activities. These issues, along with the regulatory mandate by the environmental protection authorities for zero mine water discharge, has motivated researchers to explore effective solutions for enhancing the disposal and utilization of mine water in ecologically fragile mining areas in western China.
In the long-term practice of water inrush control, a water-conserving coal mining method based on filling mining has been gradually established in recent years [10], [11]. This method, which involves controlling the development of the water-conducting fracture zone, can be summarized as overburden grouting and goaf filling in accordance with the water-loss mode of the roof aquifer and the specific filling location [12]. In overburden grouting, a relatively recent method to reduce water inflow and surface subsidence [13], filling material is injected into the separation to reconstruct the aquifer and seal the water-seepage channel, creating an aquitard that weakens the trend of groundwater seepage toward the working face. Goaf filling has a positive effect on supporting and reducing the deformation of a coal seam roof, effectively controlling secondary-damage water inrush induced by the collapse of overburden rock. Commonly used mine backfill materials include gangue filling, high-water filling, and cemented paste filling [14].
Coal development and utilization activities contribute about 80% of China’s total carbon emissions. As part of the national strategy to achieve a carbon emission peak and carbon neutrality, the coordinated development of safe mining, eco-environmental protection, and cleaner production is emphasized [15], [16]. In the process of solving water inrush and high temperature damage, beneficial energy resources are transformed and liberated, increasing the contribution of self-generated water and heat-resource recycling to the development of low-carbon energy-saving mining. This promotes the complementary synergy of multiple resources while reducing energy consumption and carbon emissions [17], [18], [19]. In this way, coal mining can evolve from a sole focus on coal production to an optimized combination of coal and clean energy.
Wang et al. [20] and Wang et al. [21] evaluated the geothermal reserves in the main coal-endowed areas in China and established a development framework of coal and energy. Wan et al. [22] simulated geothermal water extraction from the coal seam floor and evaluated the sustainable heat-recovery capacity of thermal storage. Guo et al. [23] effectively utilized the exhaust heat from mine water for cooling and heating in deep mines by means of a heat pump. The focus of research on mine water has transitioned from water hazard control to protection and hydrothermal utilization. Taking mine water as an alternative water resource, a mode focusing on control, treatment, utilization, recharging, and ecological protection has gained prominence as a mine water management approach. This mode works to achieve a balance between mine water damage, regional water resources, and eco-environments within mining areas by integrating the water cycle with the natural and socioeconomic systems of the surrounding environment [24], [25]. Energy projects using the heat energy contained in associated mine water for district heating have reached maturity and can effectively reduce carbon dioxide emissions from energy systems [26], [27], [28]. The coal- and energy-endowed areas in the Ordos Basin spatially coincide, exhibiting an average geothermal gradient of 27 °C·km−1 and a geothermal flux value of 64.1 mW·m−2. These values indicate a background of high heat flow, which implies a significantly enhanced carbon reduction potential through the utilization of high-temperature mine water, air exhaust, and geothermal resources from deep mines [29].
Considering the existing imbalance between green mining and the utilization of associated mine water and geothermal energy in ecologically fragile areas in west China, a full-time coal-water-thermal synergistic co-mining mode based on the concept of water-retaining mining and cleaner mine production is proposed. This mode aims to achieve a positive transformation of water hazards and heat damage in western mining areas through source reduction, process resource utilization, and the harmless disposal of mine water. It also takes into consideration the utilization of self-consumed clean energy to promote the low-carbon transformation of an industrial mining park’s energy system. This work provides a preliminary discussion of the primary framework and crucial techniques, and reports on demonstration tests and an effect evaluation that were carried out in the Yushen mining area in Shaanxi Province, Ordos Basin.
2. Construction of a collaborative mining model
2.1. Conceptual model
The essence of the full-time coal-water-thermal co-mining (CWTC) mode lies in fully leveraging the respective strengths of coal and its associated water and heat. The system is grounded in the coordination of mine water control, treatment, and utilization at each temporal stage throughout the entire coal-mining life cycle. It incorporates mine water hazard prevention and energy-conservation technologies, with the associated thermal energy serving as the primary heat source for the energy supply within the mining area. The co-mining strategy considers multiple objectives, including constraints related to groundwater level and carbon emissions, ensuring that coalmines have a sustained energy supply capacity during the transition period.
As shown in Fig. 2, the CWTC model gives rise to a multidimensional system composed of three main modules: water-source control, process utilization, and terminal storage. In time series, the process of mine water exploration, drainage, prevention, utilization, and discharge has experienced key technologies such as mine water inrush prevention, hydrothermal extraction, and tailwater deep well injection. During the water-control stage, the primary focus is on water-conserving mining by controlling and reducing water inflow for mining safety. Although carbon reduction is a secondary priority, the indirect reduction of carbon emissions is achieved by reducing the drilling material and electricity consumption associated with drainage. Direct carbon emissions are also reduced by replacing boiler heating with cleaner geothermal heating. Similarly, reinjection and storage technology at the end of the mine water treatment will reduce indirect carbon emissions by minimizing the electricity consumed by the deep processing of mine water. The spatial field of the model has two manifestations: ① The establishment of a 3D water-control system is achieved through the construction of pumping infrastructure and grouting drilling on the ground, as well as the drilling and drainage of water-rich underground zones; and ② deep single-well injection and storage is extended to directional branch injection within the same stratum, multi-aquifer injection within the same well, and multi-well joint injection, creating an integrated underground injection structure. This ultimately leads to the establishment of a green development model that integrates the practices of water hazard prevention, efficient heat energy utilization, and water-resource consumption and protection within mining areas.
2.2. Module I: Reducing source water and preventing dynamic inrush
The mining areas in ecologically fragile areas in west China are characterized by thick and shallow coal seams, and high-intensity large-scale mining inevitably destroys the aquifer structure. The dynamic recharge of an indirectly recharged aquifer, vertically activated by the water-conducting fracture zone, heightens the risk of water inrush and threatens the safety of mining and the safeguarding of the ecological water level [30], [31]. Through an analysis of geological structure integrity and the dynamic recharge processes of water, two types of derived dynamic water-inrush modes have been identified: leakage recharge and skylight recharge (Fig. 3).
The direct driving force of leakage-recharge water filling is the water head difference between direct and indirect water-recharge aquifers. Mining disturbance enhances aquitard permeability, which in turn increases the hydraulic conductivity of the seepage channel and exacerbates the crisis of mine water inrush to the working face. Skylight dynamic water inrush is caused by the absence of an aquitard to form a seepage-dominant area. The primary skylight can be attributed to discontinuous distribution of the original primary effective aquitard, caused by depositional extinction. A derived skylight gradually forms during mining, as the bearing capacity of the overlying roof is weakened due to mining damage. A separate space that becomes a water-filled water source is created by the upper stratum’s bends and sinks. Alternatively, the integrity of the aquiclude is compromised, leading to a gradual increase in the hydraulic head difference between the upper and lower aquifers in the weak area of the aquiclude. Groundwater outside the development range of the water-conducting fracture zone gathers and participates in the water inrush process. This situation is more susceptible to sudden and hazardous large-scale water inrush and the loss of shallow water resources because the direct aquifer at the skylight is closely hydraulically connected to the indirect aquifer.
Fundamental prevention and control of roof water inrush involves weakening the permeability of the seepage channel; this can be done by adjusting the coal mining method to minimize the active development and spatial distribution of water-conducting fracture zones. In addition, rebuilding the roof aquifer by combining advanced drainage with grouting reinforcement can weaken the hydraulic connection between aquifers, significantly reducing and controlling the water flow within a safe operation range. In this way, it is possible to reduce the mine water emissions from the mine water source. Based on this method, an innovative quality-control process to address the issue of dynamic mine water inrush via a combination of active mining and passive measures is proposed (Fig. 4).
A water-inrush active prevention and control (WAPC) system controls the water inflow to the working face and protects the aquifer structure by optimizing the mining methods used. The essence of WAPC lies in regulating the extent of rock disturbance induced by mining in order to effectively manage the height of the water-conducting fracture. Vertical expansion of the fracture is limited to the bottom boundary of the safeguarded aquifers—a factor that is contingent upon mining methods and working face parameters such as the width, coal pillar dimensions, and coal seam thickness. The current water-conservation mining modes include height limit/stratification mining, strip mining, and filling mining. A procedure that can be referred to as “three studies, one determination, two verifications, tracking twice” is formed: First, a thorough exploration and evaluation is conducted of the hydrogeology and overburden failure conditions of the mining area, and the type of coal mining (e.g., short-wall mining or long-wall mining) and corresponding parameters are determined accordingly. Subsequently, physical and numerical simulations are combined to verify whether the requirements for water control and mining are being met, and the effectiveness of the water-control measures is monitored by observing surface damage and water level variations [32].
A water-inrush passive prevention and control (WPPC) system improves the control of water inflow by decreasing the water pressure of the working face. This is achieved by artificially interrupting the hydraulic connections between the direct and indirect aquifers, thereby reducing the overflow replenishment of the Quaternary phreatic layer in the western mining area. ① For leakage recharge, multi-objective drainage technology [33], [34], [35] is adopted, with a drainage execution scheme being constructed by solving a groundwater management model constrained by the water inflow, resource amount, and economic constraints, using a linear target programming algorithm. The optimal solution set of multiple objective functions for production safety, water-resource protection, and efficient drainage is used as a reference to optimize the drainage borehole layout, drainage water volume, and drainage timing. ② For skylight recharge, multi-objective drainage technology and grouting transformation technology are utilized (Fig. 3), ensuring zonal and time-sharing mining. Aquitard grouting reconstruction is conducted at the direct contact interface of the two aquifers (i.e., the skylight area before mining), the mine-deterioration area is repaired by grouting through the mining overburden fractures after mining, and multi-objective drainage is adopted in the leakage area during production.
The technical process for multi-layer directional grouting treatment is shown in Fig. 4. Based on an exploration of hazardous water-rich areas and flow-field distribution characteristics, the target grouting area is determined according to the monitoring overburden damage [36]. Grouting material is injected into the caving zone, separation stratum, or bottom of the indirect recharge aquifer, and a vertical stratified grouting reinforcement system is established, resulting in a comprehensive combined treatment approach. To ensure effectiveness, downhole drilling is simultaneously implemented in order to verify the treatment effect, and the grouting is replenished in abnormal areas as needed. Current water prevention and control technologies and their advantages and disadvantages, according to the application status, are summarized and detailed information is provided in Table 1 [10], [33], [34], [37], [38], [39], [40], [41].
2.3. Module II: Carbon reduction and hydrothermal process improvement and utilization
Under the premise of safe mining, it is essential to develop green and low-carbon energy both underground and at the surface, in order to gradually reduce energy consumption and improve the sustainable energy-supply capacity of mining areas [42]. Firstly, by extracting the low-grade thermal energy contained in mine ventilation, mine water, and the rock surrounding the tunnel, the threat of underground heat hazards can be mitigated, ensuring safe mining. Secondly, the extracted thermal energy can be utilized for heating, preheating the intake air shaft, and other industrial applications, thereby increasing the proportion of clean energy consumption. In the proposed model, heat is extracted and transported to the surface through coal exploration channels, and a multi-energy complementary and regulatory system is built that combines solar and wind energy to optimize and upgrade the energy supply system (Fig. 5). The system is composed of four main modules: energy extraction, energy conversion, energy allocation, and energy storage.
The abundant water, heat, and space resources underground provide advantages for the utilization of the hydrothermal energy in the mine. In the proposed model, the thermal energy of the mine water is increased and effectively harnessed as a low-carbon heating and cooling heat source by means of an open- or closed-loop system involving a groundwater source heat pump (GWHP). High-temperature ventilation air from shallow mining shafts insulates and reduces energy dissipation during the collection and transportation of hot fluids. As the mining depth increases, the benefits of the thermal energy in the surrounding rock become increasingly evident. To take advantage of these benefits, ground source heat pump (GSHP) technology is gradually being developed that can more efficiently extract the deep-mine geothermal energy stored in soil or rocks. The developed technologies include a high-temperature exchange machinery system (HEMS), enhanced geothermal technology, backfilled buried pipe technology, and coaxial casing buried heat-exchange technology [43], [44], [45], [46]. High-quality mine water is used as the circulating medium to exchange heat with the surrounding rock. Solar collectors and air-source heat pumps (AHPs) are used as auxiliary energy storage to meet the peak heat demand of mining areas.
The energy allocation module makes it possible to achieve a balance between heat supply and efficient energy utilization by considering the uncertainty in the spatiotemporal load distribution, energy endowment and transmission characteristics, equipment selection, and capacity configuration over an annual time scale. During the design of the management mode and operation scenarios, energy equilibrium theory is used to match the terminal thermal energy demand. The specific cascade utilization process is as follows: The temperature of the associated water and air is increased to over 80 °C by means of the heat pump; the heat energy is then predominantly utilized for industrial and district heating applications, including membrane distillation and the multi-stage evaporation distillation of mine water. Heat energy in the range of 30-80 °C is also utilized for residential heating and laundry water. Heat energy below 30 °C is suitable to prevent air shaft freezing, for agricultural greenhouses, and to meet the demand for summer cooling. An energy-monitoring network monitors the temperature of each user and feeds it back to the control platform, facilitating a gradual extraction that optimizes the energy flow from high grade to low grade.
2.4. Module III: Reducing terminal carbon and storing mine tailwater
2.4.1. Deep multi-well ex situ storage
Traditional mine water treatment strategies include surface discharge, reverse osmosis desalination, and evaporation ponds, among others. However, these solutions present economic challenges, and it is difficult to obtain permits for discharging waste brine into the environment [47]. Deep well injection is recognized as an environmentally safe and economically viable method for the disposal of large volumes of brine waste [48], and this method has been well-established and successfully applied in industry and oil fields in the United States and Canada. Nevertheless, deep well injection has not been widely adopted in most Asian countries, especially for the management of mine water in coal mining. The first attempt to transfer mine water to an Ordovician limestone aquifer was carried out at the Wutongzhuang coalmine in China, using a single well with a depth of 1200 m and an injection rate of 258.4 m3·h−1 [49].
As shown in Fig. 6, the high salinity mine water that exceeds the saturation capacity of the ground secondary concentration and evaporative crystallization treatment is the object of deep injection. First, thermal energy is extracted from the mine water via a heat exchanger and a heat pump unit. Next, the mine water that meets recharge standards is concentrated or dispersed for recharging into an ultra-deep formation away from the coal seam floor. By sealing off of the geological formation, the mine water can be protected ex situ and stored as a resource for an extended period of time. The water-injection monitoring system consists of monitoring the variation in groundwater level in a monitoring well, microseismic monitoring, and logging inversion techniques. The directional water intake, interlayer inrush, and water absorption profile characteristics of the injection well are identified. Three layout schemes for injection well groups are considered: (I) orbital layout, (II) checkerboard layout, and (III) diagonal matrix centralized layout (Fig. 6). The spacing between the wells and the number of injection wells are determined and regulated according to the need for mine water disposal and the desired injection effect.
The safe storage of injection fluid within the strata is an important prerequisite for the continuous operation of deep well injection. The impact of the water injection dynamic on the stability of the reservoir structure is primarily determined by how the water injection affects the constraints of the reinjection pressure and the water absorption capacity of the reservoir. The injection pressure must be carefully managed to ensure that it remains greater than the pressure of the target formation, as well as the waterproof pressure from the target strata to the coal seam bottom. In cases where the reservoir’s water absorption capacity is insufficient, perforation and fracturing are potential options that may be implemented, based on the lithological characteristics of the formation. From a mesoscopic perspective, the attenuation of the water absorption capacity in low-permeability sandstone formations induced by physical clogging and water-rock action cannot be ignored, including formation damage caused by water sensitivity, speed sensitivity, and salt sensitivity. It is proposed that the water quality accuracy be used to determine the water-quality decision-making boundary matching the reservoir [50].
2.4.2. Optimization of the injection parameters based on proxy mode
The deep well injection process is described in Fig. 7. First, the selection of the target storage reservoir involves a comprehensive assessment of the water storage performance, which must satisfy the principles of stratum sealing, permeability, storage availability, and geochemistry. The selection can be performed quantitatively or qualitatively using the hydrogeological conditions, historical drilling records, and reservoir characteristic parameters (rock matrix porosity, permeability, fissure development, etc.). After determining the preferred parameters, multi-stage directional hydraulic fracturing penetration enhancement is implemented according to the physical properties and microscopic seepage characteristics of the reservoir unit in different strata. Numerical simulation of the injection is employed to visualize and analyze the dynamic flow field under different well layout schemes (I/II/III), various injection rate schemes (i.e., constant speed, intermittent constant speed, and intermittent variable speed injection rate), and different water-quality and temperature conditions. An optimization design of the water injection parameters is performed based on the well bottom pressure, groundwater level, and injecting flow, which reflect the alignment of the reservoir with the injection parameters.
The optimization based on the proxy model incorporates the experimental design and response surface methods into the simulation. It specifically focuses on the optimization of the injection parameters and the well distance, as the uncertainty of the reservoir’s implicit parameters exceeds the control scope. The process is shown in Fig. 7; following the completion of the 3D geological model within the injection area, the parameters influencing storage sustainability are identified. The value range for these parameters is determined according to the geological and production conditions, including factors such as flow, well distance, water quality, temperature, and more. Numerical simulations are performed with a designed test matrix, and a proxy model linking the control parameters to the reservoir response is constructed based on the additional criteria following the regression statistical analysis of the reservoir responsiveness data. The optimal parameter-setting combination is determined by means of the multi-response optimization method, with the aim of enhancing the storage efficiency of high-salinity mine water while avoiding potential formation damage such as reservoir roof rupture and scaling clogging.
3. Application cases and analysis
3.1. WPPCs in a typical Yushen mining area
3.1.1. Hydrogeological conditions
The Caojiatan coalmine, whose main coal seam was formed in the Jurassic, is located in the Yushen mining area of the Ordos Basin, China. It covers a field area of 5298 km2. The #2-2 coal seam is the current mining coal seam; it has a thickness ranging from 11.55 to 12.03 m, with an average thickness of 11.8 m. The mine’s hydrogeological data indicate that the primary aquifers above the #2-2 coal seam are a Quaternary pore submersible aquifer, a weathered bedrock pore fracture aquifer, and a bedrock fracture aquifer. In addition, there is a laterite aquifer erosion loss zone in the western part of the mining area, which forms a typical skylight recharge water inrush. As shown in Fig. 8, the inflow of the 122108 and 122109 working faces—primarily derived from the overflow recharge of the Quaternary pore submersible aquifer and the lateral recharge of the weathered bedrock aquifer—was stabilized after mining at 350 and 650 m3·h−1. The water inflow into the 122107 working face has consistently increased since mining, with current inflow rates exceeding 700 m3·h−1. Sharp increases in the water inflow were observed at distances of 1600 and 1900 m from the cut hole. The water hazard risk assessment indicated potential inrush risks within 2500 m of the cut eye, necessitating exploration and drainage efforts.
3.1.2. Multi-objective drainage
Optimization of the drainage water volume, drilling position, and evacuation time of the 122107 working face was configured as advanced multi-target management, limiting the inrush flow to ensure production safety and controlling the groundwater loss of the Quaternary submersible aquifer to protect the ecological environment. Based on the water abundance zone, water inrush characteristics, and laterite thickness distribution characteristics of the Caojiatan coalmine, a drainage drilling layout was designed via 0-1 integer planning, and ten control nodes were set at equal intervals within the missing laterite aquifer zone. These nodes were strategically set to cover various locations, such as the center, edge, and thicknesses at 1.5, 2.6, and 3.7 m of the laterite loss zone (Fig. 9). The management period was set to 12 months, extending from the remining coal seam to a distance of 2000 m from the cut hole, and was divided into four management stages. A unit impulse response matrix of the groundwater was obtained under a single-drilling drainage flow of 10, 20, 30, 40, and 50 m3·h−1, and a multi-objective management model of advanced drainage was constructed with the inrush water flow, leakage flow, and economic constraints [33], [34].
According to the drainage experience from the 122109 working face, a multi-drilling synchronous drainage scheme with 20 equally spaced boreholes and a single drilling drainage flow of 30 m3·h−1 was designed for the 122107 working face. This was regarded as a traditional scheme, compared with the optimization scheme of the multi-objective management model. As shown in Table 2, the drainage holes in the latter were reduced by 50.0%, and there was a 42.6% reduction in the average total drainage flow per unit of time. In addition, the average water inrush flow and the increase in leakage of the Quaternary aquifer decreased by 10.35% and 52.09%, respectively. In the optimization scheme, wherein both the boreholes and the released water are halved, the average reduction in water inflow remains relatively stable, and there is a substantial reduction in the increase of overflow volume.
Grouting treatment was carried out at the 122109 working face. The target layer was 15 m below the weathered bedrock at the top of the coal seam roof, and the grouting holes were combined with small branched vertical holes and horizontal holes. The inflow rate served as a direct indicator to evaluate the effectiveness of the grouting. The water inflow of the 122109 working face exhibited a significant and continuous downward trend. After two months of grouting work, the water inflow decreased from 456 to 273 m3·h−1, for a reduction of 40%.
3.2. The ex situ deep storage of concentrated mine water
3.2.1. Hydrogeological characteristics
The Xiaojihan coalmine is situated in the northeast of the Yuheng mining area in the Jurassic coal field in northern Shaanxi Province, China. Over the past three years, the site has experienced water inflow ranging from 1280 to 1650 m3·h−1. Original or high-concentration mine water was initially stored in the goaf. However, an increase in the mineralization and volume have been observed due to leakage at a cut hole, which exceeded the disposal capacity of the evaporation crystallization system and increased the economic burden of mine water treatment. Drawing inspiration from an injection project dealing with oil and gas field wastewater in the Ordos Basin, deep well injection was applied to solve this problem. The Triassic Liujiagou Formation (LJGF), a sandstone formation with frequent regional well losses, was preliminarily selected as the target storage strata.
A comprehensive histogram of the LJGF in Xiaojihan coalmine (Fig. 10) shows that the formation was developed from interbedded medium-coarse sand, fine sand, siltstone, and mudstone. Scanning electron microscope (SEM) testing shows that quartz and feldspar are the dominant minerals; it also reveals the presence of microfractures. The horizontal fractures at the lithological interface have good connectivity, which is conducive to the dominant seepage of injected water. The shallow resistivity logging curve shows significantly vertical heterogeneity, with vertical and longitudinal fracture zones developed in local rock formations. In particular, the existence of natural and artificial water-conducting channels enhances the water storage capacity of the LJGF as the target injection strata. The groundwater flow was found to be limited, with an original permeability coefficient of 5.31 × 10−6 m·d−1 [51]. The potential for mine water storage in the LJGF was evaluated, and it was found that the formation has the capacity to safely store 7.87 × 105 t of high-mineralization water annually.
Piper water-quality diagrams for the Xiaojihan coalmine water and the original water in the LJGF are shown in Fig. 11(a). The hydrochemical type of the orignial mine water is mainly SO4-Na·Ca, with an average total dissolved solids (TDSs) concentration of 3487 mg·L−1. The hydrochemical type of the formation water in the LJGF is Cl-Ca·Na with a TDS concentration of 64 000 mg·L−1, which is higher than the mineralization concentration of the injection mine water (RO2) with 53 000 mg·L−1. The chemical compositions of the fluid are given in Table 3. The scaling trends of the reinjection water and formation water (i.e., calcium carbonate) are predicted at a ground temperature of 50 °C using the saturation index (SI) method and the stability index (SAI) method; the injection water is stable without a scaling trend, due to the low concentration of hydrogen carbonate ions.
An analysis of the mixed compatibility results (without any precipitation/dissolution reactions) using Phreeqc software is shown in Fig. 11(b). The amount, quality, and temperature of the injection water collectively determine the solubility and SI of the formation minerals. The mineral phase can be considered as being in thermodynamic equilibrium if −0.3 ≤ SI ≤ 0.3 [52]. With the injection, the dissolution trends of aragonite, calcite, and dolomite in the LJGF decrease. When the mixing ratio is 5:5, calcite changes from a dissolved state to a precipitated state, and the precipitation trend continues to rise. The temperature near the injection well gradually decreases with an increasing volume of injected water, which makes the carbonate precipitation unstable. The minerals tend to dissolve under the saturated state of injection water; thus, it is easy for clogging and damage to the deep formation to be caused by endogenous suspended particles.
3.2.2. Injection numerical modeling
A conceptual brine mine water deep well injection model of the study area was developed based on the hydrogeological information and a field investigation, in order to assess the injection and storage performance in the LJGF. The research region was built with a size of 1000 m × 1000 m × 400 m, the target strata was located within a vertical depth range from −1700 to −2000 m, and the effective water injection section corresponded to the thickness of the LJGF. The boundary condition was set to be a Dirichlet condition, and the top and bottom of the storage strata were aquifer boundaries. The initial condition assumed a hydrostatic pressure gradient distribution, and the injection water followed Darcy’s law of seepage. The general parameters used for the simulation model were set using data from the literature and from project engineering [53]. The hydrogeological properties of the target strata are set as follows: the permeability coefficient (k) is 6.31 × 10−14 m2; the porosity (n) is 0.05; the specific storage (S) is 5; the initial water level is −200 m, and the radius of the injection well is 0.1 m. The confined aquifer was assumed to be a homogeneous layered medium, and a homogeneous porosity in the geological formation of 5% was assumed.
The pressure head and seepage radius were selected as the safety criteria for the deep well injection. Fig. 12 shows the evolution of the aquifer water pressure during continuous water injection with a fixed flow rate of 100 m3·h−1. The water head is continuously raised to form a mound centered on the injection well, due to the dynamic energy dissipation caused by porous medium resistance. The recharge pressure gradually decreases with the radius of hydraulic diffusion, and the stable water pressure at the injection well is 23.9 MPa after ten years. The water inrush coefficient of the upper coal seam floor was calculated to be 0.015 MPa, which is lower than the upper safety limit of 0.06 MPa, indicating that the water injection would not pose a threat to the upper resources. The pressure head after the injection is still lower than the maximum mechanical strength of saturated rock, 32 MPa; thus, the injection water will not break the rock formation and leak into the upper aquifer.
4. Effective carbon mitigation via mine water utilization and disposal
4.1. A low-carbon mode for mine water management
Mine water has the dual attributes of environmental pollution and resource utilization, along with characteristics that can contribute to carbon emission reduction or carbon sequestration. Zero mine water discharge based on energy substitution is a management strategy designed to treat mine wastewater effectively and optimize the utilization of mine water; it offers the potential to minimize environmental pollution and improve the sustainability of mining operations [54]. Section 2 elaborates on the crucial links in coal-water-thermal mining and discusses the direct and indirect carbon emissions produced by the consumption of electricity, water, fuel, and other resources at each stage. In the CWTC mode, mine water is regarded as an unconventional water resource; in the water-management process, carbon emissions are reduced through energy recovery instead of relying on raw materials, while the carbon sequestration capacity of terrestrial ecosystems is concurrently enhanced by protecting the groundwater cycle. Fig. 13 depicts the reduction of carbon emissions throughout the entire CWTC process.
The energy consumed in the processes of water drainage, transmission, utilization, and injection directly impacts the equivalent indirect carbon dioxide emissions in the coalmine, and the coupling of the thermal cycle with the carbon cycle is ignored in current mine water management. Considering the coupling of water, heat, and carbon, the construction of a mine water allocation model based on energy substitution requires a clarification of the balance relationship between each unit of water and heat consumption. Moreover, in order to allocate and calculate the corresponding water resources, heat consumption, and carbon emissions of each node in the system, it is possible to optimize the utilization of water resources while minimizing waste and emissions. The preliminary model construction steps are as follows:
(1)Identification of the supply and demand units of mine resources. The mine water source side (i) mainly includes the advanced drainage water, working face inrush water, and goaf drainage. The water distribution department (j) is primarily responsible for allocating industrial, domestic, ecological, and injection water. When solar energy and air source heat pumps are not designed to assist in heat storage regulation, the mine return air and mine water heat are the main heat sources (m), and the heat consumer (n) mainly includes building heating, domestic hot water, and the preheating and freeze prevention of the air shaft.
(2)Construction of an objective function. The objective function is constructed by considering the net benefit obtained from the substitution of mine water and heat resources and the carbon emissions of the production unit as the target factors.
where f1(x) is the economic objective function and f2(x) is the carbon emission function; aij is the energy efficiency coefficient of the water supply provided to the jth user from the ith water source; I and J represents the total water sources and water users, respectively; amn represents the energy efficiency coefficient of the heat supply provided to the nth user from the mth heat source; M and N denote the number of heat sources and heat users, respectively; cij and cmn denote the unit cost factor of water and heat supply, respectively; Qij and Qmn represent the supply of water and heat resources, respectively; Crelease and Ccapture refer to the quantity of carbon emissions and carbon capture per cubic meter of water; Qpro denotes the quantity of mine water generated; and Qinj is the amount of water utilized for ecological protection purposes, including replenishment of the ecological base flow and injections for restoration; and T represents the total management period.
(3)Constraints. ① Water supply and demand constraints: The production capacity of the mine water ensures a rigid demand for water in the mining area. ② Ecological constraints: The drawdown in advanced drainage cannot exceed the maximum allowable drawdown, and the maximum leakage flow of the Quaternary aquifer due to drainage must be as close as possible to the recharge flow. ③ Heat-supply constraints: The basic heat load of the mine must be met. ④ Carbon balance constraints: The carbon emission intensity must be lower than that required by local policies.
where Qw′ denotes the water demand required to manage the mine; Qw″ represents the maximum allowable amount of water that can be utilized for mining operations; and s(k,t) and s′(k,t) represent the minimum and maximum allowable drawdown of constraint point k in the t period, respectively. The function β(k,x,t) represents the unit impulse response of the drawdown at constraint point k during period t, resulting from pumping and drainage activities at mining point x. Qh′ is the total heat load of the mine. C represents the net carbon emissions of the system, encompassing both energy-consuming and non-energy-consuming emissions, which are determined by converting the tons of coal water and tons of coal energy used.
4.2. Heating potential of the clean utilization of mine water
Given the actual production situation of the mine, the current normal water flow of the Xiaojihan coalmine is 1445 m3·h−1, with water and exhaust air temperatures recorded at 20 and 25 °C, respectively. The clean utilization of heat from the mine return air and mine water provides a viable alternative to conventional coal-fired boilers used for heating. The corresponding equipment configurations include coal-fired boilers, waste heat boilers, mine water source heat pumps, and air source heat pumps. To assess the effectiveness of these approaches, three scenarios are considered:
Scenario 1: Obtain the heat supply solely from the mine return air.
Scenario 2: Obtain the heat supply solely from the mine water.
Scenario 3: Comprehensively utilize heat from both the mine return air and mine water.
With an annual average temperature of 13 °C and a total mine heating load of 32.5 MW, the entire year is divided into the winter heating season (144 days), summer cooling season (120 days), and transition season. Assuming a coefficient of performance (COP) value of five for the mine water source heat pump, the available heat energy contained in the Xiaojihan mine water can be calculated using Eq. (4) [55]. After the mine water passes through the compressor unit of the heat pump, the heating side produces an output heat energy of 11.7 MW, and the heat energy output of the air compressor side is 18.2 MW. The total heat is capable of heating a local building with an area of 429 000 m2, based on a heat index of 70 W·m−2 for a residential building. In the winter heating season, there would be a heating deficit if the return air for heating was solely relied on. However, the comprehensive utilization of both water and air heat proves to be sufficient to meet the heat load supply, considering the energy conversion efficiency of the equipment and the heat transmission losses.
where EER is energy efficiency ratio; H is heating load; Qh is heating capacity in winter; and Eh is energy consumption of the air conditioning system during winter.
The heating energy consumption of the heat pump system was calculated with a system EER of five. The system was found to use 8419 t less of standard coal each year in comparison with coal-fired boilers, resulting in an energy-conservation of 50.2%. In addition, it can directly decrease the heating carbon emissions of 48965.4 t (24.2%) compared with the carbon emissions from the total production energy. Moreover, it can release carbon emission rights worth about 2.2034 million CNY, while effectively reducing other polluting gas emissions. Therefore, the energy-conservation and emission-reduction benefits the CWTC mode are significant.
5. Conclusions
In the ecologically fragile mining areas in west China, the interplay between coal mining and the ecological environment is becoming increasingly evident. Establishing a low-carbon mining paradigm that prioritizes the protection and sustainable utilization of associated resources is essential for adjusting and optimizing mining in these areas.
(1)In this work, a coal-water-thermal collaborative mining mode was proposed. Based on the multi-objective constraints of safe mining, ecological water protection, and low carbonization, the technological system contains active and passive water inrush prevention, a multi-energy complementary heating supply, and ultra-deep ex situ mine water storage, successfully reducing water inrush, utilizing heat, and providing deep terminal mine water storage.
(2)Based on a case study of a coalmine with skylight recharge water inrush in the Yushen mining area of western China, advanced mine water drainage based on multi-objective constraints was implemented. Half of the original number of holes were no longer required, leading to a significant reduction of 42.60% in the average total drainage precipitation per unit time; the average water inflow was also decreased by 10.35%. The water inflow was stabilized and maintained, indicating that the approach is effective for ensuring safe mining operations.
(3)A low-carbon mine water management model was preliminarily established, and the effective carbon mitigation from mine water utilization and disposal was analyzed. In comparison with existing coal-fired boilers, the utilization of the abundant water heat and wind heat of the Xiaojihan coalmine to supply building heating reduced the standard coal consumption by 8419 t, the corresponding annual carbon emissions by 48965.4 t, and the carbon emissions from heating by 24.2%. The potential of mine-water storage in the LJGF was evaluated, and the formation was found to have the capacity to safely store 7.87 × 105 t of Xiaojihan high-mineralization water per year.
In future research, there will be further development of the concept of the collaborative allocation and utilization of mine water between mining areas, as well as research on the utilization of multi-mode water and heat cascades supplemented by surface wind and solar thermal energy. Other future research directions include establishing a theoretical system to determine carbon emissions under the CWTC and investigating the evolution and potential of long-term carbon reduction based on scenario analysis.
Acknowledgments
This research was financially supported by the National Key Research and Development Program of China (2021YFC2902004) and the National Natural Science Foundation of China (42072284, 42027801, and 41877186). The authors would like to thank the editor and reviewers for their constructive suggestions.
Compliance with ethics guidelines
Xiaoxiu Liu, Yifan Zeng, Qiang Wu, Shihao Meng, Jiyue Liang, and Zhuping Hou declare that they have no conflict of interest or financial conflicts to disclose.
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