Characteristics of Permeability Changes in Bituminous Coal Under Conditions of Stress Variation Due to Repeated Mining Activities

The factors affecting permeability change under repeated mining of coal seams are important study aspects that need to be explored. This study combined various stress variation characteristics of protective seam mining and simplified the stress path of repeated mining in protective seam mines. Based on the results from the bespoke gas flow and displacement testing apparatus, seepage tests for simulated repetitive mining were carried out. The results simulated the actual behavior very well. With any drastic increase in the mining influence, the axial deviation stress in the stress path increased, and the greater the difference in coal permeability during the unloading and stress recovery stage, the more substantial the increase in permeability. The change in coal permeability was significantly influenced by the severity of simulated repeated mining cycles. When the mining stress exceeded a critical value, the permeability of the coal sample increased with the increase in the number of loading and unloading cycles, but the reverse was true when the mining stress was lower than the critical value. The effective sensitivity of seepage to the applied stress decreased with an increase in the number of stress cycles. With a decrease in the deviation stress, that is, with lower severity of mining influence, the effective sensitivity of coal seepage to stress gradually decreased.

     

Dual-Porosity Coupled Borehole Gas Flow Model: A New Method for Inversion of Coal Seam Permeability

The permeability of a coal seam is an important index for coal mine gas control and coalbed methane development, and its magnitude determines the degree of difficulty of gas drainage. To obtain the permeability value, a dimensionless mathematical model for dual-porosity borehole gas-coupled flow in a coal seam was established and adopted using a simulator developed by our group. A new method of inversion was developed to determine the fracture permeability coefficient λ_f and the matrix micro-channel diffusion coefficient K_m by fitting the simulated results with onsite measured data. A range of simulations quantified the effects of different dimensionless parameters on gas migration. The results verified the feasibility of the inversion method based on the high matching degree of the fitted results, and the dimensionless mathematical model was accurate. The desorption and release of adsorbed gas from the center to the surface in coal matrices were heterogeneous, and unsteady states and gas migration times in coal matrices cannot be neglected. The new method can be introduced to analyze the problem of gas migration in different coal reservoirs, simplify the corresponding calculation and computational processes, and provide guidance in determining the permeability of coal seams.

     

A Trapezoidal Three-Dimensional Model for Gas Extraction Based on Shapes of Caved Overlying Strata and Numerical Calculation

Gas emission from the working face in a gas-bearing coal seam impairs safe production. Especially in the coal face, influenced by mining, pressure-relief gas in adjacent seams flows into the working face through the goaf. Moreover, caved overlying strata in different regions show differences in pore and seepage characteristics. Thus, the caving of overlying strata in the goaf of a working face was explored by carrying out physical similarity simulation. Then, based on the characteristics of the geometric shapes of caved overlying strata after mining, a trapezoidal three-dimensional model for gas extraction was established. According to the calculation result, the parameters of field high-level boreholes were optimized. Then, the controllability of gas concentration at the working face after gas extraction was assessed by applying statistical process control. The result showed that after the observed surface of the physical similarity model was lightened, it was more favorable for conducting the test. Moreover, the maximum gas concentration in the goaf was negatively correlated with the diameter of high-level boreholes and the negative pressure for gas extraction. A statistical process control chart revealed that the gas concentrations at the working face were safe after gas extraction based on high-level boreholes, which also validated the feasibility and effectiveness of the model.

     

Study of Effects of Hard Thick Roof on Gas Migration and Field Experiment of Roof Artificially Guided Pre-splitting for Efficient Gas Control

Due to high gas content, high geo-stress and complex geological conditions, gas disasters occur frequently in deep coal mining. The hard thick roof (HTR) greatly increases the difficulty of coalbed gas control besides causing dynamic disasters. In this paper, the effects of HTR on gas migration were numerically analyzed based on a multi-field coupling model. Results indicated that the hanging arch leads to remarkable stress concentration and induces a “cap-shaped” low-permeable zone above the gob, which greatly prevents gas from migrating upwards. Meanwhile, HTR hinders the subsidence movements of the upper rock strata, contributing to very few roof fractures and bed-separated fractures. Without the formation of roof-fractured zone, coalbed gas completely loses the possibility of upward concentration and will accumulate in the gob, forming a major safety hazard. To overcome these problems, borehole artificially guided pre-splitting (BAGP) technology was proposed. Three different pre-splitting boreholes were constructed as a group to generate artificial fractures in advance in HTR via deep-hole blasting, promoting the evolution of roof fractures. With the effects of mining stress, a fracture network is eventually formed in HTR, which provides a preferential passage for the upward flow of coalbed gas. Moreover, the controllable breaking of HTR was achieved and the roof strata could deform and subside regularly, forming an “O-shaped” roof-fractured zone above the gob which greatly improves the gas extraction efficiency of roof high-level boreholes. In addition, after BAGP, several extraction measures can be applied in the gob-side entry to drain the gas in different concentrated areas. In the field experiment, the roof periodic breaking length was reduced by half, and the average gas extraction rate was increased by 4 times to 67.7%. The synergetic controls of HTR and coalbed gas were effectively realized. This study provides valuable insight into gas control in other deep coal mines with similar geological conditions.

     

Numerical Simulation Analysis of the Permeability Enhancement and Pressure Relief of Auger Mining

Auger mining (AM) is an effective and safe way to excavate an extremely thin protective layer. This method can relieve pressure and enhance the permeability of an ultra-contiguous coal layer with high gas capacity. However, there have been few studies on AM. Based on the conditions of a coal mine in Shanxi Province, China, theoretical analyses, laboratory tests and numerical simulations were used to analyze the evolution law of the overburden permeability in an AM face. A stress–damage–permeability coupling model was proposed, and a numerical simulation algorithm for fluid–solid coupling with FLAC software was established. Through this method, the evolution law of stress and permeability and its influencing factors of the overburden of the AM face were found. The intermediate coal pillar (ICP) width and the AM height and length are the main factors influencing the permeability evolution of the AM face. The first factor determines the damage state of the ICP in the goaf, and the last two factors influence the zone size with permeability enhancement of the protected layer. Therefore, reasonable AM parameter design is the key to both safe mining operations in the AM face and pressure relief and permeability enhancement.

     

Distribution Characteristics of In Situ Stress Field and Vertical Development Unit Division of CBM in Western Guizhou,China

In situ stress is not only a vital indicator for selecting explorative regions of coalbed methane (CBM), but also a pivotal factor affecting CBM production. The present study explored whether in situ stress affected the development potential of CBM in western Guizhou, China. To this end, we collected injection/falloff well test data and gas content data from 70 coal seams in 28 wells. The study found that from top to bottom, strike slip fault stress fields (<?500 m), normal fault stress fields (500–1000 m) and strike slip fault stress fields (>?1000 m) were successively developed in western Guizhou. The distribution features of vertical permeability in western Guizhou are consistent with the stress fields' transformation location. The coal permeability in the western part in Guizhou presents a tendency of increase followed by decrease as a result of increased burial depth. The vertical development characteristics of coal seam gas content are controlled mainly by reservoir pressure, and the relationship between reservoir pressure and buried depth shows a linear increase. The CBM in western Guizhou is divided vertically into three development potential regions dependent on the characteristics of burial depth, permeability and gas content of coal seams. The most favorable vertical development potential region in western Guizhou is 500–1000 m. This region exhibits high gas content, high permeability and moderate burial depth, which are favorable for the production of CBM. These research results can provide basis for geological selection and engineering implementation of CBM in western Guizhou.

     

Analysis of Spatial–Temporal Evolution of Mining-Induced Fracture Field: A Case Study Using Image Processing in the Shaqu Coal Mine,China

Natural Resources Research - Mining-induced fracture plays a key role in gas drainage for gas burst-prone underground coal mines, especially for closely multilayered coal seams. The layout and...     

Optimization and Field Application of CO2 Gas Fracturing Technique for Enhancing CBM Extraction

Since most coalfields in China are commonly characterized by high gas content and low permeability, there is an urgent need to improve coal seam permeability and further enhance coal bed methane (CBM) extraction efficiency. As an emerging fracturing technology, the CO₂ gas fracturing (CGF) technology has been widely used because of its advantages of low cost, environmental protection and high fragmentation efficiency. In order to improve the fracturing ability of CGF technique and optimize the release orifices of discharge head, computational fluid dynamics model was used in this paper to simulate the flow fields of dynamic pressure of gas jet released from the orifices with different structures and other geometrical parameters. The results show that the orifice structure has a great influence on the flow field of gas jet, but little influence on the magnitude of the dynamic pressure. Besides, the maximum dynamic pressure of gas jet linearly decreases with the increase in the number of release orifices. Based on a series of simulation results, the discharge head which has single group of orifices with structure c, diameter of 24 mm can be identified as the best choice for fieldwork. Then, two field experiments were conducted in Pingdingshan and Changping coal mines to evaluate the enhanced CBM extraction efficiency by CGF. The results indicate that the CGF can effectively create a large number of cracks in a large range around the fracturing borehole in the coal seam and further significantly improve the permeability. And the CBM extraction efficiency can be improved to a higher level from a lower level and maintained for a long time. Besides, the effective influence radii caused by CGF in Pingdingshan and Changping coal mines are 15.19 m and 12.5 m, respectively. Compared with other fracturing techniques, the CGF technique has a promising application prospect.

     

Present‐day stress orientation in the Clarence‐Moreton Basin of New South Wales,Australia: a new high density dataset reveals local stress rotations

Early phases of the Australian Stress Map project revealed that plate boundary forces acting on the Indo‐Australian Plate control the long wavelength of the maximum horizontal present‐day stress orientation in the Australian continent. However, all numerical models of the stress field to date are unable to predict the observed orientation of maximum horizontal stress in the northeast of New South Wales, Australia. Recent coal seam gas exploration in the Clarence‐Moreton Basin, eastern Australia, provides an opportunity to better evaluate the state of crustal stress in this part of the continent where only limited information was available prior to this study. Herein, we conduct the first analysis of the present‐day tectonic stress in the Clarence‐Moreton Basin, from drilling‐induced tensile fractures and borehole breakouts interpreted using 11.3 km of acoustic image logs in 27 vertical wells. A total of 2822 drilling‐induced stress indicators suggest a mean orientation of N069°E (±23°) for the maximum horizontal present‐day stress in the basin which is different from that predicted by published geomechanical‐numerical models. In addition, we find significant localised perturbations of borehole breakouts, both spatially and with depth, that are consistent with stress variations near faults, fractures and lithological contrasts, indicating that local structures are an important source of stress in the basin. The observation that structures can have a major control on the stresses in the basin suggests that, while gravity and plate boundary forces have the major role in the long wavelength (first‐order) stress pattern of the continent, local perturbations are significant and can lead to substantial changes in the orientation of the maximum horizontal present‐day stress, particularly at the basin scale. These local perturbations of stress as a result of faults and fractures have important implications in borehole stability and permeability of coal seam gas reservoirs for safe and sustainable extraction of methane in this area.     

Sampling Methane-Bearing Coal Seams by Freezing Method: Coalbed Methane Desorption and Inhibition Characteristics Under Freezing Temperature

Methane content in coal seam is an essential parameter for the assessment of coalbed gas reserves and is a threat to underground coal mining activities. Compared with the adsorption-isotherm-based indirect method, the direct method by sampling methane-bearing coal seams is apparently more accurate for predicting coalbed methane content. However, the traditional sampling method by using an opened sample tube or collecting drill cuttings with air drilling operation would lead to serious loss of coalbed methane in the sampling process. The pressurized sampling method by employing mechanical-valve-based pressure corer is expected to reduce the loss of coalbed methane, whereas it usually results in failure due to the wear of the mechanical valve. Sampling of methane-bearing coal seams by freezing was proposed in this study, and the coalbed gas desorption characteristics under freezing temperature were studied to verify the feasibility of this method. Results show that low temperature does not only improve the adsorption velocity of the coalbed gas, but also extend the adsorption process and increase the total adsorbed gas. The total adsorbed methane gas increased linearly with decreasing temperature, which was considered to be attributed to the decreased Gibbs free energy and molecular average free path of the coalbed gas molecular caused by low temperature. In contrast, the desorption velocity and total desorbed gas are significantly deceased under lower temperatures. The process of desorption can be divided into three phases. Desorption velocity decreases linearly at the first phase, and then, it shows a slow decreases at the second phase. Finally, the velocity of desorption levels off to a constant value at the third phase. The desorbed coalbed gas shows a parabolic relation to temperature at each phase, and it increases with increasing temperature at the first phase, and then, it poses a declining trend with increasing temperature at the rest phases. The experimental results show that decreasing the system temperature can restrain desorption of coalbed methane effectively, and it is proven to be a feasible way of sampling methane-bearing coal seams.

     

采煤塌陷地水域淹没范围模拟分析

采煤塌陷改变了矿区周边的地形地貌特征,使大量的耕地、园地变成水域。选择山东省龙口采煤塌陷区为研究对象,运用ArcGIS软件采集五期1:1万等高线和离散高程点数据,生成1×1 m2正方形网格的数字地面模型(DEM),以地表没有发生塌陷前的1978年DEM为标准高程数据,与其他各期DEM数据进行差值运算处理,得到不同时期采煤塌陷区面积。然后根据水利部门48年来的观测数据,计算出采煤塌陷区不同频率年份丰水期水文特征值,采用"无源淹没"分析法,计算给定水位条件下的采煤塌陷区水域淹没范围。     

Non-uniform Distributions of Gas Pressure and Coal Permeability in Coalbed Methane Reservoirs Induced by the Loess Plateau Geomorphology: A Case Study in Ordos Basin,China

Natural Resources Research - Coal mine methane (CMM) accumulation and permeability distribution characterization are the bases for a successful CMM drainage design. Before extending degassing...     

Theory and Application of Pseudo-Reservoir Hydraulic Stimulation for Coalbed Methane Indirect Extraction in Horizontal Well: Part 1—Theory

Pseudo-reservoir stimulation in horizontal well is an effective technique for indirectly extracting coalbed methane (CBM) in soft coal from the surrounding rocks (pseudo-reservoir). However, systematic studies of the theory and on-site application of this technique are still lacking, which severely hinders its application. In this paper, the technical principles of pseudo-reservoir stimulation are analyzed firstly, and then, the technical advantages are demonstrated by experimental tests and theoretical analysis. The results show that the pseudo-reservoir generally possesses considerable gas adsorption capacity, with the gas content of 1.56–4.22 cm³/g (avg. 2.51 cm³/g) in Well XC-01, which can be extracted as supplementary resources. The fracability of the pseudo-reservoirs is 0.73–0.92, which is much higher than that of the coal seam, i.e., 0.03–0.43. Meanwhile, the compressive and tensile strength and cohesion of the pseudo-reservoir are higher than those of the coal seam, indicating pseudo-reservoir stimulation is more conducive to forming fracture network, and maintaining wellbore stability and fracture conductivity. The technical feasibility of pseudo-reservoir stimulation is determined by the regional geological conditions, showing simple tectonic conditions and well-developed surrounding rocks with high fracability and mechanical strength but low permeability, water sensitivity and water content are beneficial for the technique application. Note that the fracture conductivity in pseudo-reservoir is more stable and higher than that in coal seam, pseudo-reservoir stimulation is beneficial for the CBM extraction from both hard and soft coal seams. By minimizing the gas diffusion distance, this technique overcomes the technical obstacles to the CBM commercialized production in soft coal.

     

Coal Permeability Evolution Under Different Water-Bearing Conditions

Natural Resources Research - The seepage problem of coal-bed methane (CBM) has attracted increasing attention from the research and industry communities. Coal permeability, which is a key parameter...     

Permeability and Energy Evolution Characteristics of Heterogeneous Coal and Rock Mass

Natural Resources Research - The coupling characteristics of stress, fracture, and seepage fields are significant to gas extraction and water-preserved mining. In this paper, a theoretical model...     

Geomorphology of coal seam fires

Coal fires occur in underground natural coal seams, in exposed surface seams, and in coal storage or waste piles. The fires ignite through spontaneous combustion or natural or anthropogenic causes. They are reported from China, India, USA, South Africa, Australia, and Russia, as well as many other countries. Coal fires lead to loss of a valuable resource (coal), the emission of greenhouse-relevant and toxic gases, and vegetation deterioration. A dangerous aspect of the fires is the threat to local mines, industries, and settlements through the volume loss underground. Surface collapse in coal fire areas is common. Thus, coal fires are significantly affecting the evolution of the landscape. Based on more than a decade of experience with in situ mapping of coal fire areas worldwide, a general classification system for coal fires is presented. Furthermore, coal seam fire geomorphology is explained in detail. The major landforms associated with, and induced by, these fires are presented. The landforms include manifestations resulting from bedrock surface fracturing, such as fissures, cracks, funnels, vents, and sponges. Further manifestations resulting from surface bedrock subsidence include sinkholes, trenches, depressions, partial surface subsidence, large surface subsidence, and slides. Additional geomorphologic coal fire manifestations include exposed ash layers, pyrometamorphic rocks, and fumarolic minerals. The origin, evolution, and possible future development of these features are explained, and examples from in situ surveys, as well as from high-resolution satellite data analyses, are presented. The geomorphology of coal fires has not been presented in a systematic manner. Knowledge of coal fire geomorphology enables the detection of underground coal fires based on distinct surface manifestations. Furthermore, it allows judgments about the safety of coal fire-affected terrain. Additionally, geomorphologic features are indicators of the burning stage of fires. Finally, coal fire geomorphology helps to explain landscape features whose occurrence would otherwise not be understood. Although coal fire-induced thermal anomalies and gas release are also indications of coal fire activity, as addressed by many investigators, no assessment is complete without sound geomorphologic mapping of the fire-induced geomorphologic features.     

Experimental Study on the Effect of Moisture Content on Bituminous Coal Porosity Based on 3D Reconstruction of Computerized Tomography

Bituminous coal in the Xutuan Coal Mine of the Huaibei Mining Bureau (China) is the research object of this study. The influence of moisture content on the porosity of the bituminous coal was investigated from a microscopic perspective by using a high-solution 3D X-ray micro-analyzer. The threshold segmentation method was used to segment the scanning slices of the coal samples. The threshold values of the various media were in the following order (from large to small): minerals, water, matrices, and fractures. The scanning volume and actual volume proportions of the different media in the coal samples with different moisture contents were calculated. The accuracy of the computerized tomography (CT) scanning method in measuring the coal moisture content was verified by comparison with the results obtained using the weighing method. 3D reconstructed coal samples, with different moisture contents, were analyzed, as well as separately reconstructed fractures and water in the coal samples with different moisture contents. The heterogeneity and anisotropy of the coal mass were explained quantitatively by the CT scanning intensity. A commonly used fracture classification method indicated that the primary fracture in the coal sample was a type A fracture. The results of the analysis of water in the coal fracture indicated that the porosity of bituminous coal decreased with the increase in moisture content in conditions of atmospheric pressure and a short immersion period. However, a certain level of porosity remained evident, and the degree of fracture development of the coal samples remained unchanged. This is attributed to the minor volumetric change in the minerals in the coal samples, as the water does not completely occupy the fractures in the coal samples, and the dissolution of the minerals by water is therefore not significant. The reasons for the moisture content affecting gas adsorption, seepage, and strength of a coal body were analyzed from a microscopic perspective. In addition, the types of fractures and water in the coal samples were classified by employing statistics and analyses of volume, surface area, specific surface area, and aspect ratio of the fractures and the water in the coal samples with different moisture contents.

     

Permeability Characteristics of Broken Coal and Rock Under Cyclic Loading and Unloading

Natural Resources Research - During longwall mining, the permeability evaluation in a caving zone is one of the most challenging risks for mine operators because it controls the behavior of gas...     

瞬变电磁法解释煤矿地下含水构造研究—以内蒙古东明露天矿为例

内蒙古呼伦贝尔东明露天煤矿矿坑边帮地下水渗漏的问题日益严重,矿区原有的疏干降水井排水能力及效果已不能满足生产需求,同时对矿区生态环境也造成一定程度破坏。目前,对矿区周边的水文地质条件和水力联系缺乏精准全面的认识和数据。本文通过瞬变电磁法对矿区地下水渗漏较严重的西北部进行三条勘探线(77个物理点)勘查,结果表明该区存在三处视电阻率低阻异常带,判定其主要为含水断层(裂隙)所致;结合矿区地形、地层岩性与矿区地下水矿化度,对上述异常带进行了科学合理的综合解释,并结合新施工的疏干降水井抽水验证了上述异常的可靠性,为矿区后期布置疏干降水井圈定了靶区,对当地同类型矿区地下水渗漏灾害治理起到了应用示范作用。