Protective Coal Seam Research Articles

Dynamic permeability and porosity evolution of coal seam rich in CBM based on the flow-solid coupling theory

Based on poroelasticity theory and diffusion-seepage mechanism, the dual porosity and permeability evolution model was established by considering fracture gas pressure and pore gas pressure, respectively. By the solid and PDE modules of COMSOL Multiphysics, the protective seam mining was simulated on the base of the effective coupling of stress and gas fields. The results showed that with the mining of the protective layer, stress relief region (SRR) and stress concentration region (SCR) are formed in the protective seam, which correspond to the gas releasing region (GRR) and gas accumulative region (GAR), respectively, in the protected seam. The porosity and permeability increase in the SRR, whereas they decrease in the SCR. It is notable that the porosity cannot be restored to the original level due to the irreversibility of internal damage in coal mass. With further mining, the surrounding rock is tensioned to form the damage region, in which tensile fractures are formed, which eventually expand and connect with each other. When the destruction area gradually enlarges to link up the protected coal seam and the gob of the protective layer, the methane accumulated in the protected coal seam flows into the gob. The evolution of stress, porosity, and gas pressure is closely related. The variation of stress will cause change in porosity, which influences gas pressure, whereas the desorption of methane affects the stress of coal, all of which together form the coupling relation. This study is helpful to reveal the coupling mechanism of coalbed stress and methane, understand the dynamic porosity and permeability evolution, and learn tensile damage development.

Green coal mining technique integrating mining-dressing-gas draining-backfilling-mining

Aiming to address the following major engineering issues faced by the Pingdingshan No. 12 mine: (1) difficulty in implementing auxiliary lifting because of its depth (i.e., beyond 1000m); (2) highly gassy main coal seam with low permeability; (3) unstable overlying coal seam without suitable conditions for implementing conventional mining techniques for protective coal seam; and (4) predominant reliance on “under three” coal resources to ensure production output. This study proposes an integrated, closed-cycle mining-dressing-gas draining-backfilling-mining (MDGBM) technique. The proposed approach involves the mining of protective coal seam, underground dressing of coal and gangue (UDCG), pressure relief and gas drainage before extraction, and backfilling and mining of the protected coal seam. A system for draining gas and mining the protective seam in the rock stratum is designed and implemented based on the geological conditions. This system helps in realizing pressure relief and gas drainage from the protective seam before extraction. Accordingly, another system, which is connected to the existing production system, is established for the UDCG based on the dense medium-shallow trough process. The mixed mining workface is designed to accommodate both solid backfill and conventional fully mechanized coal mining, thereby facilitating coal mining, USCG, and backfilling. The results show that: The mixed mining workface length for the Ji15-31010 protected seam was 220m with coal production capacity 1.2million tons per year, while the backfill capacity of gangue was 0.5million tons per year. The gas pressure decreased from 1.78 to 0.35MPa, and the total amount of safely mined coal was 1.34million tons. The process of simultaneously exploiting coal and draining gas was found to be safe, efficient, and green. This process also yielded significant economic benefits.

Pressure-relief and methane production performance of pressure relief gas extraction technology in the longwall mining

Pressure relief gas extraction technology (PRGET) has been successfully implemented at many locations as a coal mine methane exploitation and outburst prevention technology. Comprehensive PRGET including gob gas venthole (GGV), crossing seam drilling hole (CSDH), large diameter horizontal long drilling hole (LDHLDH) and buried pipe for extraction (BPE) have been used to extract abundant pressure-relief methane (PRM) during protective coal seam mining; these techniques mitigated dangers associated with coal and gas outbursts in 13-1 coal seam mining in the Huainan coalfield. These extraction technologies can ensure safe protective seam mining and effectively extract coal and gas. This article analyses PRGET production performance and verifies it with the field measurement. The results showed that PRGET drilling to extract PRM from the protected coal seam significantly reduced methane emissions from a longwall ventilation system and produced highly efficient extraction. Material balance analyses indicated a significant decrease in gas content and pressure in the protected coal seam, from 8.78 m3 t-1 and 4.2 MPa to 2.34 m3 t-1 and 0.285 MPa, respectively. The field measurement results of the residual gas content in protected coal seam (13-1 coal seam) indicated the reliability of the material balance analyses and the pressure relief range of PRGET in the protected coal seam is obtained.

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.

Selection of suitable engineering modes for CBM development in zones with multiple coalbeds: A case study in western Guizhou Province, Southwest China

Recently, many types of coalbed methane (CBM) engineering practices have been applied in western Guizhou. However, most of these CBM practices (e.g., vertical well, horizontal well, cluster well.) have failed due to the complex geological conditions. To improve the success rate of CBM development projects, this paper proposes a CBM engineering mode selection method based on the geological conditions of the CBM resources found in western Guizhou. By analyzing the geology and considering the Upper Permian coal seams in western Guizhou as a case study, the geological conditions associated with CBM development were examined using measured data. In addition, the selective matching of existing CBM engineering methods to the geological conditions was examined and preliminarily demonstrated. The results indicated that the evolution of the geological conditions was primarily controlled by the vertical distribution of the coal strata, the hydrogeological conditions, and the tectonic stresses. The classification of the geological conditions into different types of CBM development geology was conducted according to the regional configuration of the geological parameters. A basic relationship exists between the type of geology and the engineering method for CBM development. To optimize the development engineering method and obtain high CBM production, the engineering methods of thin-layer staged fracturing of multiple coal seams, supplementary fracturing of sandstone reservoirs in coal deposits, and reservoir stimulation via protected coal seams during mining were preliminarily established. These methods represent a new direction for the CBM development of thin to moderate coal seams in western Guizhou. The technical feasibilities of the selected CBM development methods were verified by engineering practices, and certain CBM wells reported good production. This study describes the engineering effectiveness and adaptability of CBM engineering practices from the perspective of the development methods in the study area. In addition, this study emphasizes that the relationships between the geological conditions and the engineering methods might be broadly applicable when choosing the best CBM development engineering method for zones with multiple coalbeds in China.

Effect of protective coal seam mining and gas extraction on gas transport in a coal seam

A gas–solid coupling model involving coal seam deformation, gas diffusion and seepage, gas adsorption and desorption was built to study the gas transport rule under the effect of protective coal seam mining. The research results indicate: (1) The depressurization effect changes the stress state of an overlying coal seam and causes its permeability to increase, thus gas in the protected coal seam will be desorbed and transported under the effect of a gas pressure gradient, which will cause a decrease in gas pressure. (2) Gas pressure can be further decreased by setting out gas extraction boreholes in the overlying coal seam, which can effectively reduce the coal and gas outburst risk. The research is of important engineering significance for studying the gas transport rule in protected coal seam and providing important reference for controlling coal and gas outbursts in deep mining in China.

Quantitative evaluation of stress-relief and permeability-increasing effects of overlying coal seams for coal mine methane drainage in Wulan coal mine

To eliminate the gas outburst hazards at Wulan Coal Mine, China, a methane and coal exploitation strategy is proposed for highly outburst-prone coal seams, in which the exploitation of the lower protective coal seams and pressure-relief methane drainage using surface boreholes are combined. This study investigates the stress-relief and permeability-increasing effects of overlying coal seams to test the effectiveness of this coal and methane exploitation strategy at Wulan mine. According to a numerical simulation of the protected seam extraction process in the Wulan coal mine, the stress-relief effect of the overlying coal seams was analyzed from two aspects: evolution regularities of the relief angle and stress evolution regularities. Due to the exploitation of the lower protective seams, the stress of the overlying coal seams can be fully relieved, which is beneficial for gas drainage.A permeability evolution model of a damaged coal seam was proposed in this study, in which the effect of damage on the permeability was considered, and the reliability of this model was demonstrated through a comparison with the permeability model of undamaged coal and an experimental data matching. Then, based on this permeability model of an undamaged coal seam, along with the stress evolution characteristics of the protected coal seam, the permeability distribution characteristics of the overlying coal seams were obtained. After mining coal seam 7# and coal seam 8#, the permeability maximum of coal seam 3# is 2054 times the value of the initial permeability, and the permeability maximum of coal seam 2# is 1781 times the value of the initial permeability.A field test at Wulan mine showed that this methane and coal exploitation strategy effectively eliminates the outburst hazards of the overlying coal seams and thereby creates suitable conditions for coal mine methane drainage. Over more than 2 years of degasification, the methane content of coal seam 3# decreased from 9.81 m3/t to approximately 3.14 m3/t, and that of coal seam 2# decreased from 10.6 m3/t to approximately 3.63 m3/t. The outburst hazards of coal seams 2# and 3# are eliminated completely, and a safe mining condition could be achieved by executing this strategy at Wulan mine.

A field investigation of the deformation of protected coal and its application for CBM extraction in the Qinglong coalmine in China

Protective coal layer mining with pressure-relief gas extraction from the protected coal layer is an effective and economical way of controlling coal and gas outbursts in underground mines. According to a field test of the deformation of the protected coal layer in the Qinglong coal mine, during the mining of the protective coal layer, the floor of the protective coal layer can be divided into five areas in the horizontal direction: the original stress area, the stress concentration area, the stress relaxed area, the stress recovery area, and the recompaction area. From coal bed methane (CBM) extraction with pressure relief, the floor strata can also be divided into five areas: the original gas flow area, the gas flow reduction area, the initial pressure relief area with increasing permeability and flow, the full pressure relief area with high permeability and flow, and the pressure recovery area with decreasing permeability and flow. Mining of a protective coal layer will result in the breakage and movement of the overlying strata as well as floor heaves in the floor strata. In these unloaded coal strata, relieving the stress leads to the production of new cracks, which increases the permeability. The coal deformation and gas desorption are remarkable in the protected coal seam. The maximum dilation and dilation rate were 13 mm and 4.19‰, respectively. The gas flow and permeability increased significantly. The maximum natural emission flow and permeability reached 0.194 m³/min and 19.75 m2/MPa2.d respectively, There were 27- and 42-fold increases in permeability, which indicated that the pressure relief in this region was complete and that fractures in the coal body developed. This emission is conducive to CBM migration and extraction.

CBM drainage engineering challenges and the technology of mining protective coal seam in the Dalong Mine, Tiefa Basin, China

As mining depths increase, new challenges, such as gas drainage, gas outbursts and rock bursts induced by mining extraction, have emerged, particularly in gassy, multiple-seam coal mines. The primary reason for this phenomenon is that increased depths lead to higher stresses, higher gas pressures and lower permeabilities in coal seams. Stress and pressure relief require temporal and spatial pre-drainage protection. However, the pursuit of increased coal production, along with economic interests, has resulted in a lack of time and space for CBM pre-drainage engineering. To solve these challenges, protective seam mining is the best way to reduce stress, increase coal permeability and increase CBM extraction efficiency. The stress relief of the rock mass below the protective seam generated five zones and three belts, providing the time and space for CBM pre-drainage engineering. Based on simulations of the outburst risk and mining economics using LaModel, the gassy No. 12 coal seam was selected as the first-mined protective seam. The floor rock roadway and cross boreholes were designed to drain mining-induced stress-relief gas from the protected seam. Therefore, they should be constructed before the protective seam mining begins. The time and space allocation pattern of CBM drainage can be summarized as follows: CBM drainage before coal seam mining using long bedding boreholes, stress-relief gas drainage during mining using floor roadway crossing boreholes and CBM extraction after mining using the roof roadway. Finally, field applications indicate that the remnant gas pressure and content of the protected seam significantly decreased after mining the protective seam. The permeability coefficient increased 1465-fold. The simultaneous extraction of CBM and coal was realized. The gas drainage rate increased from 45% to 70%, and the CBM utilization rate improved from 23% to 90%. These CBM drainage practices could provide insight and guidelines for other coal mines under similar conditions.

Approach to increasing the quality of pressure-relieved gas drained from protected coal seam using surface borehole and its industrial application

During mining of lower protective coal seam, a surface borehole can efficiently extract not only the pressure-relieved gas from the protected layer, but also the gas from the mining layer gob. If the distance between the borehole and gob is too large, the quantity of gas drained from the protected layer decreases substantially. To solve this problem, a mathematical model for extracting pressure-relieved gas from a protected coal seam using a surface borehole was established, based on the radial gas flow theory and law of conservation of energy. The key factors influencing the quantity of gas and the drainage flow network using a surface borehole were presented. The results show that the quantity of pressure-relieved gas drained from the protected layer can be significantly increased by increasing the flow resistance of the borehole bottom. Application of this method in the Wulan Coal Mine of the Shenhua Group significantly increased the flow of pure gas and the gas concentration (by factors of 1.8 and 2.0, respectively), thus demonstrating the remarkable effects of this method.

A field investigation for overlying strata behaviour study during protective seam longwall overmining

Field experiments were conducted in a protected coal seam using three borehole methods: multi-point displacement measuring, double end seal with water injection method and borehole television photographing. These methods were used to study the movement law of overburden strata in the Sihe no. 2 Coal Mine, China. Results show that, parallel to the retreating direction, the overlying strata of the #9 coal seam can be divided into four sections: the coal wall support zone (0∼8.1 m), the roof separation zone (8.1∼23.0 m), the caving compaction zone (23.0∼43.0 m) and the analogous in situ zone (>43.0 m). In the vertical perpendicular retreating direction, the height of the boundary between the fracturing and caving zones is 4.3∼5.7 m, while the height between the fracturing and deformation zones is 27.5∼30.5 m, with an overburden strata bulking factor of 1.36∼1.54. After the lower protective seam (#9 coal seam) is mined, the overlying strata failure mainly develops in the weak rock layers of the fracturing and caving zones, rather than in the deformation zone. The number of mine-induced fractures is greatly increased, the fractures are broadened and mine interactivity fractures are mostly oriented horizontally.

Theoretical and experimental validation of mining-enhanced permeability for simultaneous exploitation of coal and gas

Simultaneous recovery of coal and methane is a promising solution for making reasonable use of energy resources and preventing gas disasters. Coal mining produces an effect that enhances the permeability of coal and thus paradoxically enhances the ability to collect methane. Hence, existing knowledge about permeability enhancement needs to be improved. Therefore, a new parameter, referred to as mining-enhanced permeability, is proposed to characterize the evolution of permeability for coal and other rocks. A theoretical expression for the new parameter is deduced from the Tablet Fluid model. To analyze the mining-enhanced permeability of coal mass quantitatively, a laboratory test was conducted to simulate the stress environment of the protected layers in protected coal seam mining. At the same time, a field test was conducted at a working face of Mine No.10 at the Pingdingshan Coal Mining Group in China. The evolution of the volumetric strain and mining-enhanced permeability for both protective and protected layers were analyzed quantitatively. The loading-induced volumetric strain and permeability enhancement in the laboratory showed improvements of 1.59 and 72.26 %, respectively. The mining-induced volumetric strain and permeability enhancement also showed improvements of 0.14 and 7.77 %, respectively. The evolution of enhanced-permeability agreed with gas flow data observed in both laboratory and in situ tests. Namely, a higher mining-enhanced permeability led to high gas flow, demonstrating the plausibility of mining-enhanced permeability theory based on the Tablet Fluid model. The theory describes the impact of mining on coal permeability and quantitatively captures the evolution of coal permeability. In conclusion, the proposed theory provides a strong foundation for designing systems to simultaneously recover coal and gas.

Three-dimensional numerical simulation of methane drainage by high-level drill holes in a lower protective coal seam with a “U” type face

Different drill-hole positions may produce different drainage results in low protective coal seams. To investigate this possibility, a 3D stope model is established, which covers three kinds of drill holes. The FLUENT computational fluid mechanics software is used to solve the mass, momentum and species conservation equations of the model. The spatial distributions of oxygen and methane was obtained by calculations and the drainage results of different drill-hole positions were compared. The results show that, from top to bottom, methane dilution by oxygen weakens gradually from the intake to the return side, and methane tends to float; methane and oxygen distribute horizontally. The high-level crossing holes contribute to better methane drainage and a greater level of control. Around these holes, the methane density decreases dramatically and a “half circle” distribution is formed. The methane density decreases on the whole, but a proportion of the methane moves back to deep into the goaf. The research findings provide theoretical grounds for methane drainage.

Permeability distribution characteristics of protected coal seams during unloading of the coal body

Based on the engineering practices used in mining the protective coal seam at Huainan coalfield in China, this study investigates the temporal evolution of three-dimensional stress of the protected coal seam by numerical simulation. The coal body of the protected coal seam should first be loaded and then unloaded with the three-dimensional stress being reduced simultaneously. In accordance with the forcing process around the coal body of the protected coal seam, permeability experiments were performed under fixed axial displacement with unloading confining pressure (FADUCP) and fixed deviatoric stress with unloading confining pressure (FDSUCP) stress paths. During the loading stage before the unloading point, the permeability and axial strain have a negative exponential relationship. However, during the unloading stage after the unloading point, the permeability and axial strain have a positive exponential relationship. Using the relationship between permeability and axial strain, along with the swelling and deformation of the protected coal seam, the permeability distribution characteristics for the protected coal seam was obtained. According to the permeability distribution characteristics of the protected coal seam, drilling designs for gas drainage can be optimized. Finally, the rationality of the drilling design was verified using gas data measured during mining of the protected coal seam.

The evolution of permeability and gas composition during remote protective longwall mining and stress-relief gas drainage: a case study of the underground Haishiwan Coal Mine

The mining of protective coal seams can cause changes in geostress, leading to changes in the permeability of coal rock and creating favorable conditions for gas extraction from coal seams. At the Haishiwan Coal Mine, field tests using remote protective coal seam mining were performed in the protected layer, which is rich in CO2 gas. In remote protective longwall mining, the permeability and composition of extracted stress-relief gas can vary. Under the conditions of remote protective longwall mining, the permeability of a protected coal seam can be generally described by the Liu model. During protective layer mining, the permeability of the protective layer increases rapidly with the release of stress, then decreases gradually with the recovery of the geostress. However, matrix shrinkage and decreased pore pressure caused by CO2 desorption from coal seams also cannot be ignored when considering the factors that affect the permeability. Thus, it is necessary to appropriately configure the cross-measure boreholes in advance to drain the stress-relief gas during remote protective layer mining. Stressrelief CO2 gas extraction presents multiple consecutive peaks. The No. 2 coal seam has different trap pressure systems as CO2 migrates into the coal seam. The protected seam experiences different effective stresses during protective layer mining, and the permeabilities appear to periodically increase due to differences in the original permeability. The various permeability and diffusion coefficients for CO2 and CH4 in coal induce CO2 and CH4 fractionation in the roof and floor of the No. 2 coal seam.

Research on Protective Area of Exploiting Lower Protective Coal Seam and its Feasibility

Based on the reasonable consideration of the role of the second main stress in D-P criterion, the distribution of stress and deformation with the method of finite difference was simulated. And the stress relief angles along incline and strike according to the rules of stress and deformation were obtained. Then the protective area resulted from exploiting the lower protective coal seam were acquired. The results show that the stress relief angles along incline are 75.5 and 77.7 degrees, the ones along strike are 78.4 and 83.5 degrees in the sense of protection when the advancing distance is 300m. But the real effective protective area is much smaller. The largest degree of stress relief locates in the protected coal seam corresponding to the upper and middle of the working face. According the simulation results and the water inrush coefficient method of effective water-resisting seam, the water inrush risk area of lower protective coal seam was divided, and the feasibility of exploiting protective coal seam was analysed and judged. Research results are of certain guidance and reference significance in the layout of mining and gas extraction for the similar coal mines.

Underground hydraulic mining of thin sub-layer as protective coal seam in coal mines

Underground hydraulic mining of thin sub-layer as protective coal seam in coal mines

3D reconstruction method and connectivity rules of fracture networks generated under different mining layouts

In current research, a series of triaxial tests, which were employed to simulate three typical mining layouts (i.e., top-coal caving, non-pillar mining and protected coal seam mining), were conducted on coal by using MTS815 Flex Test GT rock mechanics test system, and the fracture networks in the broken coal samples were qualitatively and quantitatively investigated by employing CT scanning and 3D reconstruction techniques. This work aimed at providing a detail description on the micro-structure and fracture-connectivity characteristics of rupture coal samples under different mining layouts. The results show that: (i) for protected coal seam mining layout, the coal specimens failure is in a compression-shear manner and oppositely, (ii) the tension–shear failure phenomenon is observed for top-coal caving and non-pillar mining layouts. By investigating the connectivity features of the generated fractures in the direction of σ1 under different mining layouts, it is found that the connectivity level of the fractures of the samples corresponding to non-pillar mining layout was the highest.

Study on Pressure Relief Scope of Underlying Coal Rock with Upper Protective Layer Mining

In this paper,in the case of upper protective layer mining of a mine for the engineering background, based on the model of semi-infinite plane body in elastic mechanics, the theoretical calculation model of verticalhorizontalshear stress of floor coal rock is established, and it is concluded that the stress expression of any point in the impact of distributed force. By introducing the rock strength index I and generalized Hooke's law, it is concluded that the discrimination criterion of guide gas fissure zone and pressure relief desorption belt of underlying coal rock with upper protective layer mining. The pressure relief scope of protection with the mining of upper protective layer is determined. Theoretical analysis shows that: the "pressure relief desorption zone" of the underlying protected coal seam that the vertical distance to the protective layer is 40m lags behind 22.2m.Through field test, the results show that the obtained pressure relief scope of protection is 26.98m,and the theoretical analysis results are basically similar.

An analysis on the effect of mining height and floor lithology on pressure relief of upper protective layers

In order to understand the effect of mining height and floor lithology at the upper protective layer face on the pressure relief of protected coal seams, this paper uses a numerical simulation method to model the pressure changes at protected coal seam during mining upper protective layer. The results show that the taller the mining height at the upper protective layer face, the greater the protection on protected coal seam due to the higher level of pressure release; the upper protective layer face with hard rock floor impedes the pressure release at the protected coal seam, which affects the overall effect of the pressure release at protected coal seam using the protective layer mining method.