Safety analysis of stability of surface gas drainage boreholes above goaf areas

As longwall caving mining method prevails rapidly in China coal mines, amount of gas emission from longwall faces and goaf area increased significantly. Using traditional gas drainage methods, such as drilling upward holes to roof strata in tailgate or drilling inseam and cross-measure boreholes, could not meet methane drainage requirements in a gassy mine. The alternative is to drill boreholes from surface down to the Iongwall goaf area to drain the gas out. As soon as a coal seam is extracted out, the upper rock strata above the goaf start to collapse or become fractured depending upon the rock characteristics and the height above the coal seam. During overlying rock strata being fractured, boreholes in the area may be damaged due to ground movement after the passage of the Iongwall face. The sudden damage of a borehole may cause a Iongwall production halt or even a serious mine accident. A theoretical calculation of the stability of surface boreholes in mining affected area is introduced along with an example of determination of borehole and casing diameters is given for demonstration. By using this method for the drilling design, the damage of surface boreholes caused by excessive mining induced displacement can be effectively reduced if not totally avoided. Borehole and casing diameters as well as characteristics of filling materials can be determined using the proposed method by calculating the horizontal movement and vertical stain at different borehole depths.

Numerical simulation on the movement law of overlying strata in the stope with a fault and analysis of its influence on the ground gas drainage boreholes

In order to study the influence of a fault on the movement law of the overlying strata as well as its effect on the gas drainage boreholes, based on the practical situation of 1242（1） panel at Xieqiao Mine in Huainan, the Finite Element Method （FEM） model was built up, and the distribution of the stress field and the displacement field of the overlying strata in the stope with a fault were simulated by using the FEM software ANSYS. The results indicate that because of the existence of the fault, the horizontal displacement of overlying strata near the gas drainage borehole becomes larger than that in the stope without a fault, and the distribution of the stress field of the overlying strata changes greatly. When the working face is far away from the fault, the distribution of the stress field is approximately symmetrical. As the working face advances to the place 50 m away from the fault, the stress range at the right side goaf area is as twice as that at the left side. Here, the stress distribution area of goaf area and the fault plane run through, the fracture-connected-zone is formed. It can be presumed that the gas adsorbed in the coal and rock will flow into the fault zone along the fracture-connected-zone, which causes the quantity of gas drainage reduce remarkably.

Experimental and measured research on three-dimensional deformation law of gas drainage borehole in coal seam

Using self-researched gas drainage borehole stability dynamic monitoring device, three-dimensional deformation characteristics of borehole under steady vertical load were researched experimentally and systematically. This research indicated that under the action of steady loading, the mechanical deformation path of the simulated gas drainage borehole is gradually complicated, and the decay of the borehole circumferential strain is an important characterization of the prediction and early warning of borehole instability and collapse. The horizontal position of borehole occurs compressive strain, and the vertical of which occurs tensile strain under the action of vertical stress. At the initial stage of loading, the vertical strain is more sensitive than that in the horizontal direction. After a certain period of time, the horizontal strain is gradually higher than the vertical one, and the intersection of the borehole horizontal diameter and the hole wall is the stress concentration point. With the increase of the depth of hole, the strain shows a gradual decay trend as a whole, and the vertical strain decays more observably, but there is no absolute position correlation between the amount of strain decay and the increase in borehole depth,and the area within 1.5 times the orifice size is the borehole stress concentration zone.

Dynamic leakage mechanism of gas drainage borehole and engineering application

Borehole leakage not only affects the gas drainage effect but also presents considerable risk to human security. For the research on the leakage mechanism of gas drainage borehole, the rheological and visco-elastic-plastic characteristics were considered to establish the mechanical model of coal mass around borehole, which is used to analyze the leakage mechanism and deduce the dynamic leakage model. On the basis of the real coal seam conditions, the variation rules of the stress, leakage ring, and air leakage amount were analyzed through numerical simulation, and the influence factors of air leakage amount were also investigated to provide the theoretical basis for the sealing technology. Results show that the air leakage amount of borehole is inversely proportional to the increase in supporting stress and sealing length, and directly correlated with the increase in borehole radius and softening modulus. Using theoretical analysis, we design a novel active supporting sealing technology that can use grouting material to seal the fractures to reduce the leakage channels and also provide supporting stress to prevent borehole deformation. The engineering test results indicate that the average gas concentration with the novel active supporting sealing technology is increased by 162.12% than that of traditional polyurethane sealing method. Therefore, this technology not only effectively resolves borehole leakage but also significantly improves the gas drainage effect.

Regional gas drainage techniques in Chinese coal mines

China's rapid economic development has increased the demand for coal.These results in Chinese coal mines being extended to deeper levels.The eastern Chinese,more economical developed,regions have a long history of coal mining and many coal mines have now started deep mining at a depth from 800 to 1500 m.This increase in mining depth,geostresses,pressures,and gas content of the coal seam complicates geologic construction conditions.Lower permeability and softer coal contribute to increasing numbers of coal and gas outburst,and gas explosion,disasters.A search on effective methods of preventing gas disasters has been provided funds from the Chinese government since 1998.The National Engineering Research Center of Coal Gas Control and the Huainan and Huaibei Mining Group have conducted theoretical and experimental research on a regional gas extraction technology.The results included two important findings.First,grouped coal seams allow adoption of a method where a first,key protective layer is mined to protect upper and lower coal seams by increasing permeability from 400 to 3000 times.Desorption of gas and gas extraction in the protected coal seam of up to 60%,or more,may be achieved in this way.Second,a single seam may be protected by using a dense network of extraction boreholes consisting of cross and along-bed holes.Combined with this is increased use of water that increases extraction of coal seam gas by up to 50%.Engineering practice showed that regional gas drainage technology eliminates regional coal and gas outburst and also enables mining under low gas conditions.These research results have been adopted into the national safety codes of production technology.This paper systematically introduces the principles of the technology,the engineering methods and techniques,and the parameters of regional gas drainage.Engineering applications are discussed.

Fracture evolution and pressure relief gas drainage from distant protected coal seams under an extremely thick key stratum

When an extremely thick rock bed exists above a protected coal seam in the bending zone given the condition of a mining protective seam, this extremely thick rock bed controls the movement of the entire overlying stratum. This extremely thick rock bed, called a ＂main key stratum＂, will not subside nor break for a long time, causing lower fractures and bed separations not to close and gas can migrate to the bed separation areas along the fractures. These bed separations become gas enrichment areas. By analyzing the rule of fracture evolution and gas migration under the main key stratum after the deep protective coal seam has been mined, we propose a new gas drainage method which uses bore holes, drilled through rock and coal seams at great depths for draining pressure relief gas. In this method, the bores are located at a high level suction roadway （we can also drill them in the drilling field located high in an air gateway）. Given the practice in the Haizi mine, the gas drainage rate can reach 73% in the middie coal group, with a gas drainage radius over 100 m.

Gas emission quantity prediction and drainage technology of steeply inclined and extremely thick coal seams

Gas emissions of workfaces in steeply inclined and extremely thick coal seams differ from those under normal geological conditions, which usually feature a high gas concentration and a large emission quantity. This study took the Wudong coal mine in Xinjiang province of China as a typical case. The gas occurrence of the coal seam and the pressure-relief range of the surrounding rock(coal) were studied by experiments and numerical simulations. Then, a new method to calculate the gas emission quantity for this special geological condition was provided. Based on the calculated quantity, a further gas drainage plan, as well as the evaluation of it with field drainage data, was finally given. The results are important for engineers to reasonably plan the gas drainage boreholes of steeply inclined and extremely thick coal seams.

CFD simulations for longwall gas drainage design optimisation

Computational fluid dynamics(CFD) simulation is an effective approach to develop and optimise gas drainage design for underground longwall coal mining. As part of the project supported by the Australian Government Coal Mining Abatement Technology Support Package(CMATSP), threedimensional CFD simulations were conducted to test and optimise a conceptual design which proposes using horizontal boreholes to replace vertical boreholes at an underground coal mine in Australia.Drainage performance between a vertical borehole and a horizontal borehole was first carried out to compare their capacity and effectiveness. Then a series of cases with different horizontal borehole designs were simulated to optimise borehole configuration parameters such as location, diameter, and number of boreholes. The study shows that the horizontal borehole is able to create low pressure sinks that protect the workings from goaf gas ingresses by changing goaf gas flow directions, and that it has the advantage to continuously maintain such low pressure sinks near the tailgate as the longwall advances. An example of optimising horizontal borehole locations in the longwall lateral direction is also given in this paper.

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.

Prediction of coal structure using particle size characteristics of coalbed methane well cuttings

Severely deformed coal seams barely deliver satisfactory gas production. This research was undertaken to develop a new method to predict the positions of deformed coals for a horizontal CBM well. Firstly, the drilling cuttings of different structure coals were collected from a coal mine and compared. In light of the varying cuttings characteristics for different structure coals, the coal structure of the horizontally drilled coal seam was predicted. And the feasibility of this prediction method was discussed. The result shows that exogenetic fractures have an important influence on the deformation of coal seams. The hardness coefficient of coal decreases with the deformation degree in the order of primary structural, cataclastic and fragmented coal. And the expanding-ratio of gas drainage holes and the average particle size of cuttings increase with the increase of the deformation degree. The particle size distribution of coal cuttings for the three types of coals is distinctive from each other. Based on the particle size distribution of cuttings from X-2 well in a coal seam, six sections of fragmented coal which are unsuitable for perforating are predicted. This method may benefit the optimization of perforation and fracturing of a horizontal CBM well in the study area.