Shallow Coal Research Articles

Fracture Characteristics of Overlying Bedrock and Clay Aquiclude Subjected to Shallow Coal Seam Mining

Some of the coal deposits in the northwest region of China are at relatively shallow depths, covered by a thin layer of bedrock and a thick layer of wind-blown sand. We studied the mechanics and permeability of a clay aquiclude by X-ray diffraction and triaxial loading tests and the fracturing of the overlying bedrock and aquiclude in physical simulation tests. The results indicate that if the bedrock thickness is 90 m or more, the height, distribution pattern, and damage to the water-conducting fractured and caving zones (“belts”) are normal. If the bedrock thickness is 30 m or less, the “three belts make one” phenomenon occurs, and caving extends into the soil layer; the overlying rock is completely perforated by vertical fractures and the collapse spreads to the surface. When the bedrock thickness is between 30 and 90 m, the height of the water-conducting fractured zone is inhibited by the weathered rock and cohesive soil layers, and the height of the “two belts” is significantly decreased. This can be used to prevent and control coal mine water hazard.

Evolution Characteristics of Pressure-Arch and Elastic Energy during Shallow Horizontal Coal Mining

Evolution Characteristics of Pressure-Arch and Elastic Energy during Shallow Horizontal Coal Mining

Coalbed methane accumulation and dissipation patterns: A Case study of the Junggar Basin, NW China

The Junggar Basin is a potential replacement area of coalbed methane (CBM) development in China. To improve the efficiency of CBM exploration, we investigated CBM accumulation and dissipation patterns of coal profiles located in the northwestern, southern, eastern, and central Junggar Basin based on the following criteria: burial depth, hydrogeological zone, CBM origin, CBM phase, and CBM migration type. We identified four types of CBM accumulation patterns: (1) a self-sourcing CBM pattern containing adsorbed gas of biogenic origin from shallow-depth coal within a weak runoff zone; (2) an endogenic migration pattern containing adsorbed gas of thermogenic origin from the medium and deep coals within a stagnant zone; (3) an exogenic migration pattern containing adsorbed gas of thermogenic origin from deep coal within a stagnant zone; and (4) an exogenic migration pattern containing adsorbed and free gas of thermogenic origin from ultra-deep coal within a stagnant zone. We also identified two types of CBM dissipation patterns: (1) shallow-depth coal within a runoff zone with mixed origin CBM; and (2) shallow and medium-deep coal seams with mixed origin CBM. CBM migration in low-rank coals was more substantial than that adsorbed in high-rank coal. CBM in shallow coal could easily escape, in the absence of closed structures or hydrogeological seals. CBM reservoirs occurred in deep coal where oversaturated gas may accumulate. Future exploration should focus on gas-water sealing structures in shallow coalbeds. CBM that occurred in adsorbed and free phases and other unconventional natural gas dominated by free gas in the coal stratum should be co-explored and co-developed.

A New Closed‐Form Solution of the Side Abutment Pressure Distribution of Roadway

Instability of coal wall is one of the hot‐button and difficult issues in the study of coal mine ground control. The shallow side coal of roadway in the coal measures is usually weak and consequently easy to bring about failure. Hence, the side abutment pressure redistributes and dramatically influences the roadway stability. Since the previous closed‐form solutions of the side abutment pressure do not take into account all the necessary parameters which include the properties of the coal and the interface between coal and roof/floor, the roadway height, and the support strength, a mechanical model is established based on the equilibrium of the plastic zone, and a new closed‐form solution is derived in this paper. Moreover, a numerical investigation is conducted to validate the accuracy of the closed‐form solution. The numerical results of the side abutment pressure distribution are in good agreement with the closed‐form solution. Afterwards, a parametric analysis of the width of the plastic zone is carried out, and the results show that the width of the plastic zone is nearly negatively linearly correlated with the friction angle and the cohesion of the coal, the interfacial cohesion, and the support strength. By contrast, it is positively linearly correlated with the roadway height and negatively exponentially correlated with the interfacial friction angle. The results obtained in the present study could be useful for the evaluation process of roadway stability.

Numerical Modeling of Pot-Hole Subsidence Due to Shallow Underground Coal Mining in Structurally Disturbed Ground

Stability analysis of underground mining is, generally, complex in nature and is difficult to carry out through analytical solutions more so in case of pot-hole subsidence prediction. Thus, application of numerical modeling technique for simulating and finding a solution is preferred. This paper reports the development of a methodology for simulating the pot-hole subsidence using FLAC3D. This study is restricted to geologically disturbed areas where presence of fault was dominating factor for occurrence of pot-hole subsidence. The results demonstrate that the variation in the excavation geometry and properties of immediate roof rocks play a vital role in the occurrence of pot-hole subsidence.

Structure and deformation measurements of shallow overburden during top coal caving longwall mining

Mining induced pressures are strong and overburden failure areas are large in top coal caving longwall mining, which constrains high production and safety mining. By employing the combination of the full view borehole photography technique and the seismic CT scanner technique, the deformation and failure of overlying strata of fully mechanized caving face in shallow coal seam were studied and the failure development of overburden was determined. Results show that the full view borehole photography can reveal the characteristics of strata, and the seismic CT scanner can reflect the characteristics of strata between the boreholes. The combined measurement technique can effectively determine the height of fractured and caved zones. The top end of the caved zone in Yangwangou coal mine employing the top coal caving longwall mining was at the depth of 171m and fractured zone was at the depth of 106–110m. The results provide a theoretic foundation for controlling the overburden strata in the shallow buried top coal caving panel.

Experiment on Seepage Property and Sand Inrush Criterion for Granular Rock Mass

Water and sand inrush is one of the most serious threats in some shallow coal mines in China. In order to understand the process of sand inrush, experiments were performed to obtain the criterion for sand inrush. First, seepage tests were carried out to study the hydraulic properties of granular sandstone. The results indicate that seepage velocity has a linear relation with the porosity and particle-size distribution parameter. Then, sand inrush tests were conducted to investigate the critical conditions for sand inrush occurrence. It is determined that the sand inrush zone can be clearly distinguished based on the values of porosity and particle-size distribution parameter. Additionally, sand inrush tended to happen in the conditions of high porosity, high seepage velocity, and large particle-size distribution parameter. Further, general principles for preventing the water and sand inrush were proposed, such as reducing the porosity, improving the pore structure, and decreasing the seepage velocity. The proposed principles have been successfully used in situ to control the water and sand inrush.

HyLogging unconventional petroleum core from the Cooper Basin, South Australia

ABSTRACTThe Cooper Basin is currently Australia's premier onshore hydrocarbon-producing province and hosts a range of unconventional gas play types, including the very extensive basin-centred and tight gas accumulations in the Gidgealpa Group, both shallow and deep dry coal seam gas associated with the Patchawarra and Toolachee formations, as well as the shale gas plays in the Murteree and Roseneath shales. Characterisation of mineralogical properties of shales is critical to hydraulic fracture design and development of unconventional hydrocarbon resources. Properties are sensitive to variations in mineralogy, especially clays, and historically, acquisition of a suite of rock mechanics analyses has been on a point-source basis rather than on a continuous sampling approach, which is uncommon. In this paper, a near-continuous 382 m cored section, comprising the lower Permian Daralingie Formation, Roseneath and Murteree shales, the Epsilon Formation and the top portion of the Patchawarra Formation from Holdfast-1 well (DH261958) in the southern Cooper Basin, was measured using robotic core scanning spectroscopy with automated mineral identification. HyLogger 3 incorporates visible, near infrared, shortwave infrared and thermal infrared hyperspectral measurements recorded every 0.8 cm along the core at the rate of approximately 1 m per minute. These data are co-registered with a high-resolution image and a laser profile of the core. X-ray diffraction and thin-section petrology from the Holdfast-1 well-completion report (Beach Energy, 2011) provided relative proportions of minerals including quartz, kaolinite, dickite, muscovite, illite, chlorite and siderite that enabled the modelled vs actual mineralogies to be constrained in the thermal infrared with a correlation coefficient of 0.85 for quartz, 0.79 for muscovite and 0.68 for kaolin; illite and siderite were more poorly correlated. HyLogger logging, when integrated with other petrophysical and analytical core data, provides a useful tool to better understand unconventional reservoirs and may assist in identification of zones that are conducive to fracture stimulation.

Learnings from a failed nitrogen enhanced coalbed methane pilot: Piceance Basin, Colorado

A nitrogen flood, ECBM micro pilot was carried out in a deep, low permeability Cameo coal zone of the Mesavede Group in the Piceance Basin. The pilot project entailed the injection of about 28mmcf (7.9E5m3) of nitrogen over a period of 15days at pressures about 650psig (4482kPa) above the initial reservoir pressure, followed by a soak period of 15days, and subsequently 11days of flow back. The nitrogen flood was a failure inasmuch as there was no pressure change nor nitrogen or methane breakthrough in three closely adjacent monitoring wells, which were considered necessary to meet the metrics required for a field scale economic ECBM project.The results of the pilot test, in conjunction with subsequent laboratory field simulation tests and numerical modeling, are consistent in that they show injection of nitrogen, of necessity above reservoir pressure, neither displaced methane nor promoted methane desorption from the low permeability Cameo coal. The results suggest the injection of nitrogen above reservoir pressure increases the total adsorbed gas resulting in swelling of the coal, which increases the effective stress and hence decreases permeability, the reverse of what is intended. The field, laboratory, and numerical results indicate that for a nitrogen ECBM flood to initiate, in coal such as studied here, the in situ permeability must be high enough that nitrogen can displace, and thus reduce the partial pressure for methane. The results thus indicate that ECBM production by nitrogen flooding will only work in coals where the pre-existing permeability is high enough that conventional coalbed methane production is possible or has already been carried out.Results of our studies provide learnings relevant to carbon dioxide sequestration and ECBM for coals of similar properties to those of this study. An initial high permeability is critical, particularly for carbon dioxide injection, due to its higher volumetric strain coefficient and greater adsorption affinity than methane or nitrogen. Such high in situ permeability exists mainly in shallow coals, which also have the potential of being exploited in the future as a resource and hence may not be viable sites for long term carbon dioxide sequestration.

Using a Fluid–Solid Coupled Numerical Simulation to Determine a Suitable Size for Barrier Pillars When Mining Shallow Coal Seams Beneath an Unconsolidated, Confined Aquifer

Barrier pillars are an effective and fundamental measure to prevent water inrush when mining shallow coal seams under an unconsolidated, confined aquifer. Based on the complex geological and hydrogeological conditions in the southern area of the Qidong coal mine, the no. 61 coal seam there was selected for a research demonstration. A fluid–solid coupled numerical simulation was carried out using the universal distinct element code. The hydraulic pressures and seepage rates in overlying strata were analyzed for two mining cases, near the aquifer and near the fault. The results showed that the degree of interconnection between the bed-separated and vertical fractures, and increases in hydraulic pressures and seepage rates in overlying strata were key factors in predicting potential water inrush when mining shallow coal seams under an unconsolidated, confined aquifer. Combining the numerical simulation results with China’s coal mining requirements, the no. 61 coal seam can be mined up to 90 m beneath the unconsolidated, confined aquifer, which limits mining to an altitude of −509.36 m. The width of the barrier pillar should be 30.7 m near the fault.

Large-depth hydrogeological detection in the North China-type coalfield through short-offset grounded-wire TEM

The North China-type coalfield is the most important coalfield in China. With the development of Chinese economy, shallow coal resources (<600 m) have been exhausted and large-depth detection is being conducted. Water-inrush is disastrous for coal mining. However, large-depth (600–1200 m) detection of water-filled zones in the North China-type coalfield is a difficult geophysical task due to its low-resistivity coverage. The traditional loop-source transient electromagnetic method (TEM) cannot realize large-depth detection. On the other hand, high-resolution detection of LOTEM is limited by the problem of recording points and large volume effect. To explore the large-depth targets with high-resolution, Short-offset grounded TEM (SOTEM) was applied in the North China-type coalfield. SOTEM is one branch of grounded-wire TEM. The response of SOTEM is calculated using superposing-dipole method rather than the direct-dipole approximation, which is used in the calculation of the response of LOTEM. This paper analyzes the comparison of the resolution and detection depth between SOTEM and loop-source TEM at first. Compared to the loop-source TEM with frequently used parameters, the detection depth of SOTEM with the usual setup parameters is about twice of magnitude, and the resolution capability of SOTEM at large depth is higher than that of loop-source TEM. Then, the resolution capability and detection depth between LOTEM and SOTEM are analyzed. For the grounded-wire TEM, offsets are strictly related to the detection depth. So, the detection depth of LOTEM with larger receiver–transmitter offset will be greater. However, at the depth of 600–1200 m, the resolution capability to conductive targets of SOTEM is higher than that of LOTEM. At last, SOTEM with a 1000-m long source and 1000–2000 m offset is applied to the 1200 m-depth exploration of water-filled zones in the North China-type coalfield. The induced voltage is observed and transformed into magnetic field. Then, the data is inverted using the least mean fourth algorithm. The distribution of water on the top and bottom surface of the large-depth No. 4 coal seam is given, and is verified by the drilling result.

The Height of Water-Conducting Fractured Zones in Longwall Mining of Shallow Coal Seams

The height of the water-conducting fractured zone is of significant importance for both the prevention of water inrush induced disasters and the design of longwall mines. To ensure the mining safety of shallow coal seams under water-rich aquifers, an approach for forecasting the height of the water-conducting fractured zone is proposed by analyzing the damage zone distribution in FLAC3D model. First, 30 numerical models with different face lengths and mining thicknesses of no. 1101 coal face at Luxin Mine, China, a typical shallow coal face, were built using FLAC3D. Then, the heights of water-conducting fractured zones and the thicknesses of protective layers with different face lengths and mining thicknesses were obtained from numerical simulations and empirical formulae. Results show that the height of the water-conducting fractured zone increases with both the face length and mining thickness. And there is a critical value of face length, which is about 300 m in no. 11 coal seam at Luxin Mine. To mine no. 11 coal seam, a slicing mining method is suggested and the face length and mining thickness of the first slice are recommended as 200 and 3.0 m, respectively.

A Case Study of Damage Energy Analysis and an Early Warning by Microseismic Monitoring for Large Area Roof Caving in Shallow Depth Seams

Shallow depth coal seams are widely spread in Shendong mining area, which is located in the Northwestern region of China. When working face is advanced out of concentrated coal pillar in upper room and pillar goaf, strong strata behaviors often cause support crushing accidents, and potentially induce large area residual pillars instability and even wind blast disaster. In order to predict the precise time when the accident happens, guaranteeing life-safety of miner, microseismic monitoring system was for the first time applied in shallow coal seam. Based on damage mechanics correlation theory, the damage energy model is established to describe relationship between damage level and cumulative energy of microseismic events. According to microseismic monitoring data of two support crushing accidents, the damage energy model is verified and an effective early warning method of these accidents is proposed. The field application showed that the early warning method had avoided miners suffering from all support crushing accidents in Shigetai coal mine.

门克庆矿综采工作面区段煤柱合理尺寸研究

随着浅部煤炭资源的日益枯竭，开采活动逐渐转入深部，鉴于安全高效生产和资源回收的原则，区段煤柱留设的合理宽度变得尤为重要。本文以门克庆矿31201工作面区段煤柱留设为工程背景，利用载荷估算法和塑性理论进行了煤柱宽度的初步计算，分别得出煤柱宽度为36.03 m和18.59 m。并利用数值模拟计算的方法分别对15 m、25 m、35 m、40 m煤柱宽度条件下，回采巷道在一次采动影响下的塑性区和支承压力分布情况进行了研究，结果显示，煤柱宽度为35 m、40 m时，巷道的塑性区破坏最小，综合分析认为，区段煤柱的合理尺寸应为35~40 m。 With the shallow coal resources exploitation exhausts, coal mining gradually changes into the deep mining. In view of the principle of safe and efficient production and resource recovery, the reasonable width of section coal pillar has become more important. On the engineering background of coal pillars of 31201 working face of Menkeqing Mine, by using load estimation method and plastic theory, the paper calculates the pillar width and obtains that the coal pillar widths are 36.03 m and 18.59. By using the method of theoretical calculation and numerical simulation, the plastic zones of roadway affected by the one-time mining and its abutment pressure distribution in the condition of 15 m, 25 m, 35 m and 40 m coal pillar widths are studied. The results show that the roadway has minimal damage when coal pillar widths are 35 m and 40 m. The reasonable size of the section coal pillar should be 35 - 40 m.

3D topographic stress perturbations and implications for ground control in underground coal mines

It is well known that the perturbed stress field beneath valleys can result in roof instabilities in shallow underground coal and stone mines. Quantitatively predicting the magnitude of these stress perturbations, particularly beneath complicated three-dimensional (3D) topography, has not become commonplace in mine planning, perhaps due to the complexity and time-consuming nature of the problem. Here we utilize 3D digital elevation models and the 3D boundary element method (BEM) approach to efficiently calculate the pre-mining topographically perturbed stress field in the vicinity of the Carroll Hollow coal mine in eastern Ohio. We find that regions of elevated compressive stress in the mine correspond to areas in which cutter roof failure is a common source of roof instability. Furthermore, both the magnitude and inclination of the principal stresses calculated from the 3D topographic BEM model are found to be consistent with observed failure distributions within the mine. We propose that the approach outlined in this study can be efficiently applied to the mine planning process in order to mitigate or avoid potentially hazardous mining conditions.

Expansive behavior of coal mine spoils treated with calcium-based stabilizers

Sulfate-induced swelling, resulting from the chemical treatment of sulfate rich soils, has been known to cause significant damage to pavements and other structures particularly in the south-western United States. This paper addresses the problem of sulfate-induced swelling in coal mine spoils, formed as a result of shallow strip coal mining, after treatment with calcium-based stabilizers. Samples of coal mine spoils were obtained from the Hawthorn coal mine area in Sullivan County, Indiana, and treated with the following calcium-based stabilizers: hydrated lime, quick lime and Portland cement, at dosages ranging from 1.5% to 9% by dry mass of the spoils. The swelling tests, conducted over a period of one month, were performed using the sand bath method in a custom setup housed in an environmental chamber. All specimens of the spoils treated with the stabilizers were found to exhibit swelling greater than that measured on the untreated spoils, with the greatest swelling observed following treatment with 3% hydrated lime. Swelling of the treated spoils was initially delayed and approached a constant value after approximately one month.X-ray diffraction and thermo-gravimetric analyses provide evidence of the presence of ettringite in all spoil–stabilizer specimens. While the formation of this mineral appears responsible for the observed expansion, the measured swelling strain could not be correlated to the amount of ettringite formed, and appears, instead, governed by the restraint provided by the stabilized matrix.Tests were also conducted to examine the effectiveness of the following two mitigation techniques: pre-compaction mellowing and double lime treatment. A pre-compaction mellowing period of 2–3days led to swelling strains comparable to those of the untreated spoils. No advantages could be identified in using double lime treatment.

Deep-hole pre-split blasting mechanism and its application for controlled roof caving in shallow depth seams

The shallow depth coal seams mined in the Shendong Mining Area has led to large area roof weighting frequently due to large thickness, high tensile strength and small overlying load of the main roof. As a result, many accidents have happened including hydraulic support being iron-bound, water inrush through the penetrating cracks along coal wall, and potentially induce large area residual pillars instability and even wind blast damage particularly in the conditions beneath a room mining goaf. To prevent the large area roof weighting accidents, deep-hole pre-split blasting technology for controlled roof caving is a suitable method and has been widely applied in the mines, and many in situ trials have already achieved a sound performance. According to the field conditions of the shallow depth coal seams, this paper employs cylindrical cavity expansion theory to calculate the three blasting-induced divisions including break zone, fracture zone and elastic vibration zone. The software of LS-DYNA3D was applied to establish a deep-hole pre-split blasting model, which presented the rock stress field and the break scope affected by the high energy explosion stress wave. The simulation results revealed the blasting for controlled roof caving mechanism and also optimized the blasting parameters. The field observations showed that, with the application of deep-hole pre-split blasting technology in Shigetai Mine, Shendong Mining Area, the first main roof weighting length was about 17.4m, and neither a hydraulic support being iron-bound nor a serious roof subsidence in the form of step convergence in the mining face happened during the first weighting period. The field application of deep-hole pre-split blasting for controlled roof caving has achieved the expected effects.

Interpretation of Voids or Buried Pipes using Ground Penetrating Radar Modeling

Abstract: In coal mines abandoned works or voids at shallow depth, might be a potential hazard and therefore, such areas are considered for redevelopment. As voids and fractures accumulate groundwater which may be hazardous for the mines work. Subsidence, fire, flooding and some other kinds of environmental hazards related to shallow coal works, necessitate for better understanding of voids and their interpretation for Ground Penetrating Radar (GPR) data. Whereas, in civil engineering, GPR is used for detection of the buried pipe, however, it becomes difficult to differentiate betweem the pipes used for water supply and for electrical cable. In order to better interpretation of voids and buried pipes we present a finite-difference time-domain (FDTD) solution based on Maxwell’s equations that allows accounting for the frequency dependence of the dielectric permittivity and electrical conductivity of many near-surface materials. The algorithm and the results presented here, however, offer the perspective to improve some of these inherent problems and thus help to make 2-D structure a more reliable and effective tool for probing the shallow subsurface. Also it is found that the different dispersion mechanisms cause significant amplitude and phase differences, which may be relevant to amplitude variation with offset (AVO) and stratigraphic investigations using Ground Penetrating Radar.

Mining Design Optimization Research on the Northern Section of 8 Coal Seam in West No.1 Mining Area in Pansan Mine

Water in Lower Aquifer of Quaternary is one of the mine water inrush sources during working faces being mining in the Northern Section of 8 Coal Seam in West No.1 Mining Area. Water flowing fractured zone, F1 fault, F22-a fault and F48 fault are main water-conducting channels in mining. According to the analysis of geology and hydrographical geology of the mine area, and combined with “Water Prevention Regulation of Coal Mines” and “Regulations of pillar leaving and coal mining under building, water, railway and mail shaft and tunnel”, stope layout was optimized, i.e. the minimum waterproof coal(rock) pillar between working face and Lower Aquifer Quaternary is 70 meter, the waterproof coal(rock) pillar between working face and F1 fault, F22-a fault, F48 fault is 70~95 meter, 70~75 meter, 80~90 meter respectively. Each shallow waterproof coal (rock) pillar of fault is narrow at the top and tends to increasingly widen at the bottom.

A Study on the Roof Fracture Mechanism of Large Cutting Height Workface in Shallow Thick Coal Seam

Based on the high mining height of large cutting height workface in shallow thick coal seam and the few falling waste rock in goaf, the key roof can not be supported effectively, the facture mechanical model of key roof was established. The theoretical calculation formula of key stratum fracture step and working resistance of support were obtained by using fracture mechanics. The results show that the fracture step of key roof relate to not only the mechanical character of key roof and the load of overlaying rock seam, but also the working resistance of support and horizontal pressure in key roof. The reasonable working resistance of support and the step of roof fracture were analyzed in 1-2coalmine 51104 face of a mine in Dongsheng area. The theoretical results are well agreeable with the field measured results.