Seams In China Research Articles

A numerical modelling study of longwall face stability in mining thick coal seams in China

Longwall face failures remain a major ground control and safety hazard in Chinese coal mines. This paper develops a better scientific understanding of stress and displacement distribution in the face area through 2-D finite element modelling of a typical longwall face in China. The results show: 1) most of the yielding occurs in the upper part of the face; 2) most vertical displacement occurs near the upper part of the coal face, while most horizontal displacement occurs around the centre part; 3) most face falls should, therefore, occur in the upper part of the coal face; 4) face stability is most affected by the load carrying characteristics of caved gob; 5) increased loading capacity of shields has limited effect on the stress distribution ahead of the face; 6) currently used face plate pressure distribution does not have a major effect on the redistribution of stress in the face area.

A numerical modelling study of longwall face stability in mining thick coal seams in China

Longwall face failures remain a major ground control and safety hazard in Chinese coal mines. This paper develops a better scientific understanding of stress and displacement distribution in the face area through 2-D finite element modelling of a typical longwall face in China. The results show: 1) most of the yielding occurs in the upper part of the face; 2) most vertical displacement occurs near the upper part of the coal face, while most horizontal displacement occurs around the centre part; 3) most face falls should, therefore, occur in the upper part of the coal face; 4) face stability is most affected by the load carrying characteristics of caved gob; 5) increased loading capacity of shields has limited effect on the stress distribution ahead of the face; 6) currently used face plate pressure distribution does not have a major effect on the redistribution of stress in the face area.

Microscopic mechanism for enhanced coal bed methane recovery and outburst elimination by hydraulic slotting: A case study in Yangliu mine, China

AbstractCoal bed methane (CBM) is a clean, efficient, and environmentally friendly energy, and its efficient exploitation can meet the growing demand for energy worldwide. Besides, methane (CH4) is a greenhouse gas (GHG) and is 25–72 times more powerful than carbon dioxide (CO2) in inducing global warming. With regard to the mining industry, CBM is regarded as a major hazard resource, as its inadequate management or control may lead to serious coalmine accidents. In China, gas drainage is the main measure adopted to resolve this problem. However, because the coal seams in China are characterized by low permeability, it is difficult to drain the gas contained in coal mass. Therefore, measures must be taken to reconstruct the coal reservoir to enhance coal seam permeability. In China, hydraulic slotting has been widely accepted and applied in coal mine engineering owing to its permeability‐enhancement effect. In this work, based on our previous research, we systematically elaborate the mechanisms of permeability enhancement and elimination of coal and gas outburst by hydraulic slotting from microscopic perspectives. To examine the rationality and practicability of the research results and the permeability‐enhancement effect of hydraulic slotting, a field experiment was conducted and the monitoring results were analyzed. Our results show that the gas flow rate and concentration in the slotted boreholes are significantly higher than those in the conventional boreholes. In addition, the time required by the slotted borehole to eliminate the outburst is 72.41% less than that needed by the conventional borehole. The practice in Yangliu coalmine could be extended to other coal mines with similar reservoir conditions. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd

A protective seam with nearly whole rock mining technology for controlling coal and gas outburst hazards: a case study

Coal seams with low gas permeability and high gas outburst hazards are becoming more serious as coal mines extend deeper, but there are no appropriate protective coal seams for this kind of coal seam in China. In this paper, mining technology using a protective seam with nearly whole rock (PSNWR) is used to improve gas drainage and ensure safety during production. The characteristics of the distribution and occurrence of PSNWRs and their mechanical properties are analyzed. A theoretical mechanics model and three-dimensional numerical model are established to study the controlling effect of PSNWR mining on pressure-relief gas drainage. In this context, the mining process, system and gas extraction design for PSNWRs are introduced. The results for Pingdingshan No. 12 Coal Mine show that mining with a PSNWR 2.0 m thick can effectively reduce the danger of coal and gas outbursts and improve gas drainage and utilization. The gas drainage rates are >80 %, which significantly increases the social, economic and environmental benefits of Pingdingshan No. 12 Coal Mine.

Study of characteristics and formation stages of macroscopic natural fractures in coal seam #3 for CBM development in the east Qinnan block, Southern Quishui Basin, China

Natural fractures including the cleat systems play a key role in the coalbed methane production. Therefore, understanding the natural fracture growth and distribution in coal seams is of great importance for modeling gas flow in coal reservoirs. It is also important for the understanding of hydraulic fracture growth during well stimulation. In this paper, an integrated approach using underground coal wall observations, core log analyses, and image logging interpretation was utilized to study the characteristics of the natural fractures generated under different stages of paleotectonic stress field. As a major coalbed methane production and coal mining seam in China, the coal seam #3 in the east Qinnan block, Southern Quishui Basin is studied for its natural fracture characteristics. The results show that macroscopic natural fractures are well developed in coal seam #3 in east Qinnan block and the nearby areas. Importantly, the fracture scale, densities, in-fillings, and connectivity display marked variations among the fractures developed in different geological age. It is found that there are three groups of exogenous fractures formed during the Yanshanian period in the study area, which are oriented at NE10-19°, NE60-79°, and NE40-49°, respectively. Meanwhile, three groups of endogenous fractures, which are orthogonal to the exogenous fractures, were formed during the same period. They are oriented at NW60-79°, NW10-29° and NW40-49°, respectively. These three sets of exogenous and endogenous fractures can be related to the three main episodes of the Yanshanian tectonic movement. Moreover, there are also three groups of exogenous fractures formed during the Himalayan period in the study area, which are oriented at NW60-79°, NW0-29° and NW40-49°, respectively. Meanwhile, three corresponding groups of endogenous fractures were formed from the Himalayan period, which are NE10-19°, NE60-79° and NE40-49°, respectively. These three sets of exogenous and endogenous fractures are also correspondent to the three main episodes in the Himalayan period. The main macroscopic fractures formed in this period were during the second episode, which had the strongest tectonic movement in the Himalayan period. Influenced by the multiple stages of tectonic movements, macroscopic natural fractures are complex and polydirectional in strike in the study area. The results of this study are of great significance for the understanding of coalbed methane production behavior in the east Qinnan block.

Coupled analysis about multi-factors to the effective influence radius of hydraulic flushing: Application of response surface methodology

Current coal seams in China are characterized by great depth, low permeability, and strong adsorption features, and thus, the extraction of methane is relatively difficult and gas disaster remains serious. To overcome this limitation, the hydraulic flushing and hydraulic slotting are adopted to improve the permeability of coal seams. A 2D model of gas seepage was established by using the solid mechanics and darcy module in Comsol Multiphysics, version 4.3b. The factors influencing pressure relief, such as the radius of borehole after hydraulic flushing, the initial gas pressure, and the permeability of coal seam, were studied. The coupling effects of these three factors on the effective influence radius were studied through the response surface methodology, and a quadratic polynomial equation between effective influence radius and three factors was derived. The results showed that Gas pressure distribution (e.g. gas full emission region, gas pressure transition region and original gas region) near the borehole, corresponded well with stress distribution (stress relief zone, stress concentrating range and original stress zone). The plastic zone formed around the borehole leaded to stress reduction, and the increasing porosity and permeability resulted in pressure decline. The effective influence radius increased with the borehole radius and the initial permeability, but decreased with original gas pressure. Besides, it was found that an increase in one factor would restrict the role of other two factors on the effective influence radius. This study is helpful to choose the most suitable parameters to achieve high extraction efficiency and lower gas content/pressure.

Improvement of drainage gas of steep gassy coal seam with underground hydraulic fracture stimulation: a case in Huainan, China

The paper introduced a new technique named underground hydraulic fracture stimulation to improve gas drainage efficiency. And the technique was successfully applied in a steep gassy coal seam in Huainan of China. Application results showed that water injection changed water/gas content distribution greatly and gas drainage efficiency was improved greatly. After the stimulation, central part of coal seam around fracturing borehole showed lower water content and much higher gas content than other parts. Average pure gas flow rate for single borehole of central part is 3~8 times higher than that of original part. The upper and below part showed higher water content and lower gas content than central part. The phenomenon is closely related to the steep structure of coal seam. In consideration of the wide distribution of steep gassy coal seam in China, the results have great significance for popularisation and application of the technique in Chinese coal mines. [Received: December 11, 2014; Accepted: July 4, 2015]

Distinguishing and controlling the key block structure of close-spaced coal seams in China

Distinguishing and controlling the key block structure of close-spaced coal seams in China

Novel integrated techniques of drilling–slotting–separation-sealing for enhanced coal bed methane recovery in underground coal mines

Coal bed Methane (CBM), a primary component of natural gas, is a relatively clean source of energy. Nevertheless, the impact of considerable coal mine methane emission on climate change in China has gained an increasing attention as coal production has powered the country's economic development. It is well-known that coal bed methane is a typical greenhouse gas, the greenhouse effect index of which is 30 times larger than that of carbon dioxide. Besides, gas disasters such as gas explosive and outburst, etc. pose a great threat to the safety of miners. Therefore, measures must be taken to capture coal mine methane before mining. This helps to enhance safety during mining and extract an environmentally friendly gas as well. However, as a majority of coal seams in China have low-permeability, it is difficult to achieve efficient methane drainage. Enhancing coal permeability is a good choice for high-efficiency drainage of coal mine methane. In this paper, a modified coal-methane co-exploitation model was established and a combination of drilling–slotting-separation–sealing was proposed to enhance coal permeability and CBM recovery. Firstly, rapid drilling assisted by water-jet and significant permeability enhancement via pressure relief were investigated, guiding the fracture network formation around borehole for high efficient gas flow. Secondly, based on the principle of swirl separation, the coal–water–gas separation instrument was developed to eliminate the risk of gas accumulation during slotting and reduce the gas emission from the ventilation air. Thirdly, to improve the performance of sealing material, we developed a novel cement-based composite sealing material based on the microcapsule technique. Additionally, a novel sealing–isolation combination technique was also proposed. Results of field test indicate that gas concentration in slotted boreholes is 1.05–1.91 times higher than that in conventional boreholes. Thus, the proposed novel integrated techniques achieve the goal of high-efficiency coal bed methane recovery.

The Capture, Utilization, Storage of CO<sub>2</sub> in Methane Recovery

According to the problems that the coalbed methane resource was rich in deep seam in China, but the economic and technology conditions were limited, it would be hard to mine with a conventional method. The CO2 capture, utilization and storage technology was provided (CO2-ECBM). The application of the technology would not only improve the methane recovery ratio from deep and unminable layer, but also put CO2 effectively in the deep layer for storage to reach a target of reducing emission. The study showed that a coal rank, coal seam pressure, coal seam permeability, injection time, injected gas types and others would affect to the recovery ratio of methane in a production mine. Therefore, before we use this technology, a rational evaluation should be conducted on the place location. So the capture and storage technology of CO2 has an important significance in protecting the natural environment.

High resolution processing of 3D seismic data for thin coal seam in Guqiao coal mine

Accurate identification of small faults for coal seams is very important for coal-field exploration, which can greatly improve mining efficiency and safety. However, coal seams in China are mostly thin layers, ranging from 2–5m. Moreover, the shallow coal seam with strong reflection forms a shield underneath thin coal seam which is only about 40m deeper. This causes great difficulty in seismic processing and interpretation. The primary concern is to obtain high-resolution seismic image of underneath thin coal seam for mining safety. In this paper, field data is carefully analyzed and fit-for-purpose solutions are adopted in order to improve the quality of reprocessed data and resolution of target coal seam. Identification of small faults has been enhanced significantly.

Present situation of fully mechanized mining technology for steeply inclined coal seams in China

Abundant and widely distributed in China, steeply inclined coal seams generally have diverse and complicated geological characteristics. Intrinsic problems in mining such seams, such as large dip angle, short length of mining face, ineffective sealing and protection of working space by support systems, poor stability of equipment, high labor intensity, and inadequate protection of the workers hinder the safe and efficient excavation. Recently, significant progress has been made in China in mining such coal seams. In some coal mines, fully mechanized mining technology has been successfully applied, and working face outputs were increased remarkably. This paper analyzes the main technical difficulties in applying fully mechanized mining technology, such as surrounding rock control, mining equipment stability control, skidding prevention, and flying gangue prevention. Based on the analysis, prosperous development directions for steeply inclined coal seam mining in China are proposed.

Study on Evaluation System for Coalbed Methane from Deep Coal Seams in China

In this study, data from 244 CBM production wells were collected and analyzed in China. Based on the data, coalbed methane (CBM) reservoir properties and geological factors were studied. Combined with previous study results, the current situation of CBM resource evaluation, the classification of evaluation indexes and evaluation standard were analyzed. Finally, the evaluation system aiming at deep coal seam CBM was suggested. This system tries to provide important theoretical and technical support to CBM exploration and geological target selection evaluation for deep coal seams in China. This will provide important guideline and technical support to CBM exploration and geological target selection evaluation in deep coal seam CBM in China.

Research and Application of Auger-air Drilling and Sieve Tube Borehole Protection in Soft Outburst-prone Coal Seams

Hole accidents during drilling and borehole collapse during extracting are bottlenecks restricting gas drainage efficiency in soft outburst-prone coal seams in China. The auger-air combined drilling technique and sieve tube mounting method are an alternative solution to these technology bottlenecks. The auger-air drilling technique combines the advantages of “dry style” auger drilling and air drilling. Specially designed blade in drill rod can stir up large particles of coal so that large particles can be brought to ground smoothly using compressed air and is efficient to prevent borehole accidents. After drilling is completed, the sieve tube is tripped in through the inner hole of drilling pipes, and then lifting up drilling pipes, the tube sieve will provide a complete tunnel for gas extraction. Field application proves that with proper drilling parameter selection and appropriate tube install control, it is more promising to double drilling depth and raise gas drainage efficiency.

Anthracite bio-degradation by methanogenic consortia in Qinshui basin

Studies have identified that the methanogenic consortia live in bituminous and lignite coal beds. The two main goals of this study are to verify the existence of the consortia in anthracite and to determine whether this high rank coal bed could provide nutrients for the methanogenic consortia. Research has confirmed that types of methanogenic consortia live in the Qinshui basin and use some compounds in anthracite as carbon sources to yield biogas. Maceral analysis confirmed that volatile matter is a controlling factor in the biological methane yield process. The CH4 and CO2 yield process showed that the methanogenic consortia can degrade volatile matter in anthracite into bio-methane in situ. This study confirmed that the methanogenic consortia bio-degradation process can work on anthracite to increase the CH4 concentration in the Qinshui basin, which is located in the Shanxi province and is an important methane-producing coal seam in China. It was estimated that approximately 3.0×10−2~4.0×10−2m3/kg of methane can be generated in the Qinshui basin by microbial bio-degradation.

Analysis on Sealing Method and Sealing Materials of Gas Drainage Borehole

With the increase of gas drainage volume and gas drainage requirement of coal seam in China, more and more attention to gas drainage effect of coal seam is paid. The gas flow law and borehole sealing mechanism are briefly described, the borehole sealing technologies of different materials are analyzed and the requirements are obtained as follows: 1Sealing materials shall have permeability. 2Sealing materials shall have expansion. 3The strength and hardness of sealing material can not be too high. 4Sealing materials are compact; sealing materials shall have certain strength. 5Sealing materials can be obtained easily and their price shall be low. 6Sealing process is simple and is constructed easily. Finally, the high polymer foam sealing technology, the mechanical elastic sealing technology, borehole packer sealing technology, the new pressure grouting hole sealing technology, yellow clay borehole sealing technology and rubber ring (or capsule)-sealing fluid sealing technology are briefly analyzed, the advantages and disadvantages of the borehole sealing method and the sealing materials used in coal mines in China are summed up. The results can provides the reference for selection of borehole sealing methods and sealing materials of gas drainage in coal mines.

Review on safe blasting technology in high-temperature coal seams in China

In order to define the development direction of safe blasting technology in high-temperature coal seams in China, this paper presents a systematic, state-of-the-art review on safe blasting technology. Information such as high-temperature coal fire extinguishment and cooling, anti-high-temperature blasting supplies, heat insulation blasting setting, operation procedures for high-temperature blasting, and so on, are presented. Analysis shows that the development of effective technology for coal seam fire extinguishing and cooling, the manufacture of anti-high-temperature blasting supplies, especially priming materials, the development of heat insulation blasting setting, and the establishment of technical specifications suit large-scale blasting design and construction work at high-temperature coal seams, are the essential methods for safe blasting at high temperature coal seams.

The Current Situation and Developing Trend of Extremely Thin Coal Seam Mining of China

Degree of mechanization of extremely thin coal seam mining is growing, with introducing the main mining method currently used by the different inclination of the extremely thin coal seams in China, focusing on the different mining technology and related ancillary mining equipment development status and existing problems, and discuss the extremely thin coal seam in the future to realize the remote control, automatic monitoring and unmanned mining development trend.

Origin of hydrogen sulfide in coal seams in China

In recent years, coal mines in China have seen accidents caused by sudden release of hydrogen sulfide (H2S), with higher severity. Based on recent local and overseas research on the origin of hydrogen sulfide and the comparative analysis of isotopes, the paper presents three formation models of hydrogen sulfide: bacterial sulfate reduction (BSR), thermo-chemical sulfate reduction (TSR) and igneous activity, as well as the characteristics for identification. This paper analyzes and categorizes the formation models of some hydrogen sulfide in coal seams in China. Some suggestions were put forward for future research of H2S in coal mines in China.

Rock Bearing Capacity-Based Stepwise Bolting for Deep Underground Coal Roadway in China

Complicated occurrence situation of deep underground coal seams in China determines absence of a national rock classification index. Take Huainan mining area as an example, this paper aims to provide a practical method. It begins with a geo-mechanical assessment to check bolting feasibility of related rock mass. Once this is settled, roof self-bearing capacity coefficient and coal hardness coefficient which can comprehensively indicate basic characteristics of surrounding rock are integrated to classify the surrounding rock into four levels. Accordingly, stepwise bolting are provided according to dynamic combination of four basic bolting forms: super bolt assembly, cable bolt beam, high-pretension roof truss and metal chock plus grouting. An application case with rock Level III in Guqiao Mine, Huainan was introduced. Statistics show that bolt-based supporting is widely being used in more than 80% of Chinese coal mines with sound efficiency and applicability.