Abstract
The gas desorption index of drill cuttings is a basic index that measures the initial desorption capacities of coal seams and predicts coal seam gas outbursts. Following a long period of gas drainage in the No.7 coal seam of the No.86 mining area in the Haizi coal mine, the gas desorption index of drill cuttings was still found to be much higher than the threshold value for outburst risks. This abnormal phenomenon led to the present study of the rational selection of test methods and objects in this context. In this study, particle size distribution, pore structure and gas desorption characteristics of coal samples in anomalous areas were analyzed. We found that desorption characteristics are related to particle size and particle size varies in relation to tectonic stress and magmatic intrusions. It appears that the anomalous readings are related to particle size of the coal, resulting from tectonic/magmatic pulverization. Furthermore, measured particle size of drill cuttings is not actually reflective of coal particle size – larger particles may be comprised of multiple smaller particles. The results show that coal samples with particle size <1 mm accounts for 76.3% of total samples and coal samples with particle size >1 mm only accounts for 23.7% of total samples. The porosity and total pore volume increase as the particle size decreases. The specific surface area increases with decreasing pore diameter. Transitional pores and micropores increase the specific surface area of the coal sample considerably. The desorption capacity increases with decreasing particle size. The additional tectonic stress caused by magmatic intrusion has a crushing effect, and 1–3 mm particles used in test were composed of a large amount of smaller particles, eventually resulting in abnormal gas desorption index phenomena. Therefore, we proposed an improved method for measuring the gas desorption index of pulverized coal.
Highlights
Coal and gas outburst is a very complicated gas dynamics phenomenon in coal mines
To study the pulverization characteristics of the coal samples obtained from the field, we measured the particle size distribution
Three groups of screening results of the pulverized coal samples were found to be similar, so we used the average value of the three groups of screening results as the mass distribution of coal samples with different particle sizes
Summary
Coal and gas outburst is a very complicated gas dynamics phenomenon in coal mines. An outburst is the failure of coal and its ejection by stored potential energy being converted to kinetic form. The Coal Research Institute (China) proposed gas desorption index of drill cuttings (e.g. Áh2 and K1) as an outburst prediction index that reflects the initial desorption capacities of coalbed methane, which are closely related to the damage degree to the coal structure and adsorption/desorption capacity (Cheng et al, 2010; Gui et al, 2009; SACMS, 2009). Tectonic coal under the magmatic sill in the No 7 coal seam of the No 86 mining area has been subjected to extremely high degrees of pulverization, and abnormally high risks of coal and gas outbursts have been detected by in situ stress distribution and outburst indexes measurements (Wang et al, 2017). The gas desorption index of drill cuttings (Áh2) exceeded the critical value (200 Pa) in many areas of conveyor roadways along the 764 working face during mining and tunneling, as shown, and values reached up to 350 Pa, far exceeding the critical value, at some sites. In order to study the characteristics of the pulverized coal, we obtained the particle size distribution by screening the coal samples in the laboratory and measured the pore distribution and desorption characteristics
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