Desorption Effects and Laws of Multiscale Gas-Bearing Coal with Different Degrees of Metamorphism

Abstract

Understanding gas desorption effects and laws of coal mass under different conditions is essential for the effective exploration of gas emission in underground coal mines, prediction and prevention of coal and gas outburst, accurate detection of gas [coal methane (CBM)] content in coal seams, and prediction of CBM productivity. Using a self-developed test platform, we simulated gas adsorption and desorption and performed physical simulation tests. Based on these tests, we investigated the differences in the total amount of gas desorbed, desorption rate, and initial amount of gas desorbed by long-flame coal, coking coal, meager-lean coal, and anthracite on different scales under different gas pressures. Two methods are used for compensating gas loss, namely, the method and the power function method, as stipulated in the current Standards for Determination of Gas Content in Coal Seams in China. By combining these two methods, we analyzed the applicability of these two compensation methods in coal on different scales with varying degrees of metamorphism under gas pressures. The results demonstrated that (1) under the same gas adsorption pressure, the cumulative total amount of gas desorbed per unit mass within 90 min for the four kinds of coal samples increases with the degree of metamorphism. Changes in the cumulative amount of gas desorbed per unit mass and the desorption rate with the degree of metamorphism vary with stages. Notably, a higher adsorption pressure leads to a more obvious stage change. (2) Under the same gas adsorption pressure, the cumulative total amount of gas desorbed per unit mass and the desorption rate of coal with the same degree of metamorphism are inversely proportional to the size of the coal sample. This indicates significant scale effects. The larger the degree of metamorphism and gas adsorption pressure, the more significant are the scale effects of gas desorption. (3) For coal with the same degree of metamorphism, the higher gas adsorption pressure leads to a larger cumulative total amount of gas desorbed and a higher desorption rate throughout the desorption process and a larger proportion of the cumulative amount of gas desorbed in the initial stage. The smaller the size of the coal sample, the more obvious the pressure effects of gas desorption are. (4) For coal samples with the same degree of metamorphism, when the gas content in coal seams is kept constant, the larger the size of the coal sample, the smaller the actual gas loss is. Moreover, a higher gas content in coal seams results in a greater gas loss and a larger calculation error for gas loss. Compared with the method, the power function method reveals a smaller deviation between the calculated gas loss and the actual gas loss, which is found to be more accurate. A larger size coal sample results in higher accuracy in the calculated gas loss.

Characteristics and Influence Factors of Natural Desorption in Coal Bodies from Fukang Mining Area, Xinjiang, China

“…This indicates that open pores feature a larger aperture. Desorption is a contrary procedure to adsorption; cumulative desorption volume should theoretically be positively correlated with methane adsorption in the coal body . Additionally, in the coal body, methane adsorption mainly depends on microporosity .…”

“…Desorption is a contrary procedure to adsorption; cumulative desorption volume should theoretically be positively correlated with methane adsorption in the coal body. 61 Additionally, in the coal body, methane adsorption mainly depends on microporosity. 55 Therefore, with an increasing number of open pores, methane adsorption and accumulated desorption both decrease.…”

Effective Approach with Extra Desorption Time to Estimate the Gas Content of Deep-Buried Coalbed Methane Reservoirs: A Case Study from the Panji Deep Area in Huainan Coalfield, China

“…17,18 The direct method was first proposed in 1970, 19 and later the United States Bureau of Mines (USBM) adopted this method in an accelerated methane emission project. 15,20 The basic principle of this method is that CBM has three occurrence states, namely adsorption, free, and water-soluble states, among which the adsorbed state is the main occurrence state of CBM. 21−23 In addition, the coal seam has a triple structure of coal matrix, pores, and fractures.…”

“…The direct method was first proposed in 1970, and later the United States Bureau of Mines (USBM) adopted this method in an accelerated methane emission project. , The basic principle of this method is that CBM has three occurrence states, namely adsorption, free, and water-soluble states, among which the adsorbed state is the main occurrence state of CBM. − In addition, the coal seam has a triple structure of coal matrix, pores, and fractures. − The adsorbed gas dissociates from the coal matrix to fracture through a desorption–diffusion process and escapes outward under the action of differential pressure. ,, The gas desorption process is regarded as gas diffusion from a collection of spherical coal particles with a zero surface concentration and a constant initial gas concentration. ,− On the basis of related research and guidelines, , the cumulative desorption gas volume at the initial time is directly proportional to the square root of time, which is used to calculate the lost gas content. With a long exposure time, the lost gas content and relative error exhibit great numerical values .…”