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Abstract:The broken pattern of the overburden strata induced by mining has a non-ignorable effect on overlying strata movement, failure, and safety in mining production. To study the caving pattern of overlying strata and determine the calculation method of fracture pathway parameters due to roof caving induced by coal mining, the trapezoidal broken models were developed to explain and prevent water leakage, and even water inrush, during the mining process. By incorporating the variation of the volume expansion coefficient, a connection among the parameters of the fracture pathways and fracture angles, face width, and mining height could be established, which shows that the larger the degree of the fracture angle is, the smaller the value of the volume expansion coefficient and face width is with a relatively larger mining height. This relationship was also used to determine the eventual evolution configuration of the trapezoidal broken model. The presented approaches may help us to better understand the movement of overburden strata and provide an idea to help settle conflicts related to fracture space calculations induced by coal mining.

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Large scale roof strata caving that occurs during coal extraction can irreversibly damage floor strata and result in riskier mining operations. Four research models incorporating floor water pressure were assessed for floor strata failure, using eight methods and two classification systems. A connection between floor strata failure and the coefficient of impact risk was developed. The derived equations represent a potentially effective method for providing a preliminary assessment of the risks associated with floor strata failure due to caving. A classification system of floor failure potential can be constructed to minimize risks during mining.

Site Ordovician limestone karst water plays a key role in the release of heavy metals from abandoned coal gangue or filled-in goafs in closed coal mines. In this study, a series of immersion experiments was performed to evaluate the leaching behavior of heavy metals in coal gangue from the Baizhuang coal mine in Shandong Province. The experiment was conducted with different immersion solutions, including site Ordovician limestone karst water (pH 7.3) that was compared to three solution pH levels (pH 5.3, 7.3, and 9.3), over a maximum duration of 648 h. The surface structure, chemical composition of the original coal gangue and post-leached residue, and multiple water quality parameters [pH, oxidation reduction potential (ORP), electrical conductivity (EC), total dissolved solids (TDS), and dissolved oxygen (DO)] of the immersion solutions were analyzed using scanning electron microscopy–energy-dispersive spectroscopy (SEM–EDS), X-ray fluorescence spectrometry (XRF), and a Hach Hydrolab multiparameter water quality analyzer, respectively. The results indicated that the site karst water presented quite different implications for heavy metal dissolution compared to laboratory solutions having the same and different pH levels as the karst water, which often showed the lowest values of heavy metal concentrations for the four solutions. Although the heavy metal releases did not change in order of increasing solution pH levels, the dissolved quantities of heavy metals in the four solutions showed a generally similar development trend with processing using increasing immersion times and increased almost to the maximum time used, except that Pb exhibited a strong increase followed by a decline. Additionally, the pH, ORP, EC, TDS, and DO values for the coal gangue at different solutions over time showed marked differences. Additionally, the SEM–EDS and XRF results showed that stable heavy metals, including Mn and Pb, released more readily in site karst water or neutral solutions than in acidic and alkaline solutions but unstable heavy metals, including Fe, Zn, and Cu, more easily released under acidic and alkaline solutions. In abandoned coal mines, coal gangues under the effects of site Ordovician limestone karst waters may pose a lesser environmental problem than indicated by the laboratory results.

In order to explore the potential effects of abandoned coal mines on the water quality of Ordovician limestone aquifers, water-rock interaction simulations were conducted. After the closure of the coal mine, the karst water in the goaf area and the waste gangue had a geochemical reaction, and the above-mentioned water-rock process was simulated by an indoor static immersion experiment to explore the differences in the effect of different types of karst water on the dissolution of gangue. The basic water quality parameters pH, EC (electrical conductance), and ORP (oxidation-reduction potential) showed different trends in karst hydro-immersion solution and ultra-pure hydro-immersion solution; pH and EC had greater fluctuations in two sets of ultrapure hydro-immersion solutions, while ORP fluctuated more widely in three groups of karst hydro-immersion solutions. In addition, gangue minerals dissolved more significantly in bodies of water where limestone was added. The results of chemical component clustering showed that TDS (total dissolved solids) and EC were homopolymerized in each immersion solution, and subsequent correlation analysis showed that TDS and EC clusters were more significantly affected by mineral properties in ultrapure water-immersion solutions, and more affected by dominant ions in karst water-immersion solutions.

Hydraulic fracturing is a widely used reservoir stimulation technique to exploit CBM in China. Compared with the original permeability of coal reservoir, the postfracturing permeability is more critical for the productivity evaluation of CBM wells. Based on hydraulic fracturing, microseismics monitoring, and well test data of the CBM wells in Zhengzhuang block of Qinshui Basin, this paper proposes a simple method to obtain the fitting pressure by the classic fracturing pressure decline analysis theory under the condition of corrected dynamic overall leakoff coefficient. In addition, the postfracturing permeability of coal reservoir is predicted by the injection well testing theory. The morphology of hydraulic fractures in the study area is mainly determined by in‐situ stress, and most fractures are vertical ones propagating along the direction of the maximum principal stress. The fracturing curves of the study area can be classified into stable curves, stably fluctuating curves, descending curves, ascending curves, and fluctuating curves, where the stable and descending fracturing curves represent better fracturing effect. The length of hydraulic fractures is much greater than the height. The PKN model has been selected as the hydraulic fracture propagation model to analyze the fracturing pressure decline. The slope value of liner segment of G(δ,0) curve is proposed as the criterion for the fitting pressure calculation. The postfracturing permeabilities of coal reservoir range from 1.55 to 8.83 × 10−3 μm2, all of which are remarkably higher than that of the original reservoir.