Behavior and mechanism of the adsorption/desorption of tectonically deformed coals

Abstract

The physical and chemical texture of tectonically deformed coals produced by various formational mechanisms are different from those of primary coals, thus resulting in major differences among the physical properties of the reservoirs of these coals. We have studied the adsorption/desorption behavior of tectonically deformed coals by the use of isothermal adsorption/desorption experiments under equilibrium moisture condition. Experiments of isothermal adsorption/desorption of methane or multi-component gases have indicated that, the adsorption curves of coals with a low degree of tectonic deformation conform to the type of isothermal adsorption curve described by the Langmuir equation; the methane adsorption curves of coals with strong tectonic deformation cannot be described by the Langmuir equation. The adsorption/desorption process of methane and multi-component gases in the deformed coals is not consistent with primary coals, which form an effect of hysteresis in different kinds of tectonically deformed coals. With the change of pore structure of tectonically deformed coals at reservoir condition, the added adsorbed CH 4 in the experiments is desorbed on the pore surface of coals during the pressure reduction process. Thus, the result shows that the adsorption volume in the process of desorbing is greater than that in adsorbing. Because of the deformation, structural change, and transformation of the adsorption potential field of coals, it is essential to form a new kind of isothermal adsorption curve and the hysteresis effect of the desorption process.tectonically deformed coal, adsorption/desorption behavior, methane, multi-component gases, hysteresis effectThe physical and chemical texture of coals and their properties have been comprehensively investigated and discussed by scientists throughout the world [1][2][3][4] . Further, scientists have intensively studied the coal and gas outbursts experienced in nearly all the major coal-producing countries [5,6] . Of the extensive research on coal and gas outbursts, the study of tectonically deformed coals is of vital importance [7][8][9] . Within these deformed coals, the adsorption/desorption behavior and higher gas content are the major scientific problems. In fact, tectonically deformed coal is a type of coals in which, under mono-or multi-phases tectonic stress fields, the primary texture and structure were deformed, broken or destroyed, even the chemical composition was changed. Tectonically deformed coals formed by different deformational mechanisms are divided into three series of deformation and ten classes: the series of brittle deformation include cataclastic structure coal, mortar structure coal, granulitic structure coal, mealy structure coal, schistose structure coal, and thin-layer structure coal; the series of ductile deformation include wrinkle structure coal, mylonitic structure coal and ductile structure coal; the series of brittle-ductile include scale structure coal [9] .

Nanoscale Pores in Coal Related to Coal Rank and Deformation Structures

“…After doing mercury intrusion experiments on 12 coal samples, Zhou and Wu (2012) discovered the "U"-shaped change of porosity and pore volume with the degree of metamorphism, with R O,ran = 2.0 % as the dividing point. Tectonic deformation can cause significant changes in the physical and chemical structures of coal Ju et al 2009;Li et al 2012;Hou et al 2012), resulting in subsequent changes in pore characteristics and even nanoscale pore structures (Ju et al 2005). Through mercury intrusion experiments on Pingdingshan tectonically deformed coal in China, Ogawa 2001, 2003) noted that the porosity in the tectonically deformed coal was 3-8 times higher than the normal structure coal in the same location and the specific surface area of pore is 2-10 times higher.…”

Experimental Development of Coal-Like Material with Solid-Gas Coupling for Quantitative Simulation Tests of Coal and Gas Outburst Occurred in Soft Coal Seams

“…It can be seen that the gas adsorption and desorption properties of coal-like materials were consistent with those of the natural coal and could meet the test requirements [6]. Coal has a strong gas adsorption capacity mainly because of its good porosity and relatively large specific surface area [36,37]. Therefore, under the same molding pressure conditions, the coal mass ratio had the most impact on these characteristics.…”

“…This is mainly because TDC is formed under mono-or multi-phase strong tectonic movements. Its original structure has been deformed or damaged, resulting in larger adsorption surface area and higher gas content than the primary coal [37]. In this paper, the natural coal samples were selected from a typical tectonically deformed coal seam from No.…”

“…In addition, the occurrence of coal and gas outbursts must be affected by the deformation and failure mechanism of the coal [13,37]. According to the different deformation mechanisms, different types of TDC are divided into three series of deformation and ten classes, that is, the brittle, the ductile and the brittle-ductile deformation [38].…”

Experimental Study on Damage and Gas Migration Characteristics of Gas-Bearing Coal with Different Pore Structures under Sorption-Sudden Unloading of Methane

“…St. George and Barakat [26] studied the coal deformation during sorption and desorption of different gases under uniaxial pressure. Ju et al [27] found through experiments that the desorption curve conforms to the Langmuir equation when the degree of coal deformation is low, but not when the degree of coal deformation is high. Liu et al [28,29] experimentally studied the swelling deformation of coal induced by gas sorption using a self-developed testing apparatus based on coal solid-gas coupled mesomechanics and discussed the dynamic process of expansion deformation due to sorption of gas under different pressures, the anisotropy of sorption deformation, and the relationship between sorption deformation value and gas sorption amount.…”