Effects of stress lower limit during cyclic loading and unloading on deformation characteristics of sandstones

During the multiseam mining process, the overlying strata were damaged due to different mining disturbances, where a large number of fractures were produced which were the main seepage paths of gas and water, easy to cause gas or water inrush accidents. Thus, it is necessary to explore the development characteristics of the fractured zone in the overlying strata from multiseam mining. In this paper, laboratory triaxial compression tests were firstly conducted on the rock specimens subjected to different mining disturbances. In these tests, the specimen was loaded to different stress before a regular test to simulate the different damage effects on the rock strata caused by different mining disturbances. And the corresponding values of m and s could be obtained through a calculation based on the test results. Subsequently, the numerical simulation was conducted to simulate the field situation, in which the m and s values were used in the numerical model combined with the Hoek‐Brown–modified criterion, and the results show that the overburden failure height was increased with the mining disturbances from 48 to 120 m, and then, the effects of mining height and interval thickness on the development height of fracture zone were studied by numerical simulation. Finally, the leakage of roof drilling at different stages was tested and calculated in the field. The results showed that the loss of leakage drilling can directly reflect the development degree of roof cracks, and the detection results were close to the development height of the numerical simulation fracture zone. It is verified that the simulation method used in this paper is feasible and can be used as a reference for similar research.

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Development characteristics and field detection of overburden fracture zone in multiseam mining: A case study

Tian

Liu

Yang

et al. 2019

“…Therefore, some dynamic disasters such as coal and gas outbursts become increasingly serious. With the increase of mining depth, the number and intensity of coal and gas outbursts occurring in heading face grows and the difficult of preventing outbursts also increases . At present, predraining gas for pressure relief in deep coal seams is the main measure to reduce the risk of gas disasters; however, owing to the permeability of coal seams being low, many drainage boreholes generally need to be drilled with a small drainage radius, resulting in delays to mining.…”

Section: Introductionmentioning

confidence: 99%

“…With the increase of mining depth, the number and intensity of coal and gas outbursts occurring in heading face grows and the difficult of preventing outbursts also increases. 7,8 At present, predraining gas for pressure relief in deep coal seams is the main measure to reduce the risk of gas disasters 9 ; however, owing to the permeability of coal seams being low, many drainage boreholes generally need to be drilled with a small drainage radius, resulting in delays to mining. To solve this problem, many solutions are available, including deep-hole prefracturing blasting, hydraulic flushing, water injection in coal seams, hydraulic slotting, and hydraulic fracturing.…”

Section: Introductionmentioning

confidence: 99%

Elimination of coal and gas outburst risk of low‐permeability coal seam using high‐pressure water jet slotting technology: A case study in Shihuatian Coal Mine in Guizhou Province, China

Xie

Zou

et al. 2019

Coal seams in China are mainly characterizsed by low permeability and high risk of coal and gas outburst. Therefore, the elimination of coal and gas outburst risk of low‐permeability coal seam remains a hard challenge. Taking Shihuatian Coal Mine in Guizhou Province, China as an example, boreholes were slotted using high‐pressure water jet to relieve the gas pressure inside the coal seam and increase the permeability of coal mass, which thus improves the gas drainage efficiency and eliminate the outburst risk. The results show that the gas drainage efficiency of the slotted boreholes significantly increases. Compared with the conventional boreholes, the average gas concentration and flow velocity improved by 1.6 and 7.5 times, respectively. Moreover, the prediction index of outburst risk is less than the critical value after high‐pressure water jet slotting, which indicate that the coal and gas outburst risk has been effectively eliminated. The measured values of gas desorption index of drilling cuttings are all lower than the critical value after the application of the high‐pressure water jet slotting technology. The research achievements could provide a practical reference for the effective gas disaster prevention and control in low‐permeability coal seams.

“…[1][2][3][4][5][6][7] Coal and gas outburst threatens the production safety seriously in underground coal mines. [1][2][3][4][5][6][7] Coal and gas outburst threatens the production safety seriously in underground coal mines.…”

Section: Introductionmentioning

confidence: 99%

Permeability enhancement mechanism of sand‐carrying hydraulic fracturing in deep mining: A case study of uncovering coal in cross‐cut

Yin

Cheng

et al. 2019

Coal and gas outburst disasters are prone to occur within rock cross‐cut coal uncovering in deep underground coal mines. To reduce the risk of coal and gas burst under rock cross‐cut coal uncovering, a technique of permeability enhancement using hydraulic fracturing sand‐carrying is proposed. Field test was conducted using HF sand‐carrying approach, and the effect of permeability enhancement was investigated. The results show that the evolution of cracks has been subjected three stages: energy slowly increasing and crack initiation, damage localization of coal seam and gradually failure, and crack instability expansion and formation of a complex fracture network system. In addition, it is found that “water‐sand” type injection of high‐pressure fluid carried sand into HF cracks, which creates a support force on crack surface and prevents crack closure. A fully developed fracture network system with high flow conductivity capacity is created and thus increases gas permeability largely. Furthermore, a clearly increasing trend associated with the HF sand‐carrying is found for the treated coal seam area. The permeability coefficient is about 21.5‐30.5 times higher than that from the raw coal seam. A good prevention effect of coal and gas outburst is achieved using HF sand‐carrying method.