•The preferential adsorption of CO2 over CH4 improves with coal rank.•Pore structure of coal affects the thermodynamics of adsorption of CO2 and CH4.•Adsorbed CO2 has a more ordered configuration ...than CH4 on different rank coals.
In this paper, the pore structures of three different rank coals sampled from China (anthracite, bituminous coal and lignite) were characterized by CO2 and N2 adsorption. The isothermal adsorption curves of CO2 and CH4 on three samples were measured by gravimetric method and fitted by Langmuir model. The preferential selectivity (αCO2/CH4) was calculated using the Langmuir parameters of CO2 and CH4, and the Henry’s coefficient (KH) was obtained with the help of virial equation. More importantly, a comparative analysis of adsorption thermodynamics of CO2 and CH4 on three different rank coals, including surface potential (Ω), Gibbs free energy change (ΔG) and entropy loss (ΔS), was presented according to the adsorption data. It is found that the uptakes of CO2 and CH4 on anthracite are the largest, followed by lignite and bituminous coal in sequence. αCO2/CH4 increases with the increase of coal rank. Low temperature helps injected CO2 to displace pre-adsorbed CH4. The KH values on anthracite are the biggest, while KH values on bituminous coal are the smallest. Ω, ΔG and ΔS of CO2 and CH4 all exhibit a U-shaped function with maturity. Anthracite has the highest Ω, ΔG and ΔS, while bituminous coal has the lowest Ω, ΔG and ΔS. The thermodynamics parameters of Ω, ΔG and ΔS are affected by pore size distributions of three coals. Ω, ΔG and ΔS of CH4 are smaller than those of CO2. CO2 adsorption on coal is more favorable and spontaneous, and adsorbed CO2 molecules form a more efficient packing on coal.
The isothermal sorption experiments of CH4, CO2 and CH4/CO2 mixture gases on shale were performed. The sorption affinities of CH4, CO2 and CH4/CO2 mixture gases were compared using Henry’s ...coefficient (KH). The thermodynamics of sorption of CH4, CO2 and their mixture were discussed based on surface potential (Ω), entropy loss (ΔS) and change of Gibbs free energy (ΔG). It is shown that the uptakes of CO2 is largest, followed by CH4/CO2 mixture gases and CH4 in sequence. The introduction of a small amount of CO2 into mixture gases can significantly enhance the adsorption quantity of mixture gases and suppress the adsorption of CH4. The lower pressure and the smaller CO2 mole fraction in bulk phase (yCO2) improve separation factor and boost the displacement of adsorbed CH4 by CO2. KH, Ω and ΔS for CO2 are higher than those for CH4, and KH, Ω and ΔS for CH4/CO2 mixture gases are between those for CH4 and CO2. Increasing yCO2 increases KH, Ω and ΔS. ΔG of CO2 and CH4/CO2 mixture gases are bigger than that of CH4. CO2 adsorption is more spontaneous and the introduction of CO2 enlarges the spontaneity degree of adsorption of mixture gases.
Dynamic events, such as rib spalling and coal burst, occurs frequently on the coal walls and crossheadings of deep working faces in water-rich areas. This paper adopts the methods of case analysis, ...theoretical analysis and numerical analysis to reveal the mechanism of rock burst on deep working faces induced by drainage in water-rich areas and makes the following conclusions: (1) drainage results in the damage of rock stratum and uneven stress distribution with the stress within the damage zone decreased and the stress on the edge of damage zone increased; (2) in the mining process towards water-rich areas, the stress peaks in different positions from lowest to highest are the stress on the edge of water-rich areas, outside water-rich areas and under water-rich areas respectively; (3) drainage brings about stress concentration on the working face. It is likely to suffer from rock burst when the sum of concentrated stress and abutment pressure on the working face is larger than the critical value of rock burst occurrence. Rock burst is the most likely to take place during the mining process towards the edge of water-rich areas. On this basis, rock burst prevention measures are proposed, including optimizing the arrangement of drainage holes, slowing the mining speed, releasing large diameter pressure, increasing the initial support force of hydraulic support, selecting reasonable roadway positions and strengthening roadway support. Specifically for B Mine in the Nalinhe mining area, large-energy microseismic events can be found during the mining process towards the edge of water-rich areas. No large-energy microseismic events are monitored outside and within water-rich areas because of taking prevention measures in advance. In spite of coal burst and rib spalling, rock burst cannot be found, which ensures the face safe mining.
With increasing coal mining depths in China, the number of mines experiencing rock burst is increasing. The conditions in which a mine can reach the critical impact-mining depth remain theoretically ...unknown, which hampers the determination of the depth at which rock burst prevention can begin in coal mines. Based on different mining boundary conditions of rock bursts occurring in coal mines, this study proposes the concepts and calculation methods of three critical depths of rock bursts in coal mining. The strength criterion for rock mass impact of medium-hard and hard coal is used to evaluate the failure strength and stress state of the coal rock mass under dynamic loading conditions. Then, the value of the dynamic stress coefficient of the coal rock mass is determined. The research team applied this method to numerous mines in China to test rock bursts under different types of working face conditions with a coincidence rate of 100%. Results verify the rationality of the calculation formula of the critical impact depth and the value of the dynamic stress coefficient. The findings of this research can provide a basis for evaluating rock bursts.
The greater adsorption ability of shale to CO2 offers an alternative method to enhance shale gas recovery, where a potential solution for CO2 sequestration into a deep shale reservoir can also be ...realized. Clay minerals are the important storage space for shale gas. In this work, the isotherm adsorption curves of CH4 and CO2 on montmorillonite at 288 K, 308 K and 328 K were obtained using gravimetric apparatus. The selectivity factor of CO2 over CH4 (SCO2/CH4) and Henry's constants (KH) of CH4 and CO2 were calculated. More importantly, the thermodynamics parameters of CH4 and CO2, including Gibbs free energy change (ΔG), surface potential (Ω), isosteric heat of adsorption (Qst) and entropy change (ΔS), were thoroughly analysed. The results indicate that all SCO2/CH4 values are obviously greater than one, and injecting CO2 into a clay-rich shale reservoir to enhance the shale gas recovery is feasible. CH4 has smaller KH than CO2, which shows that CH4 has a weaker affinity on the montmorillonite surface. Low temperature helps to enlarge KH and promote the adsorption of CH4 and CO2. Increasing the pressure causes the increase in ΔG and Ω of CH4 and CO2. The higher ΔG and Ω of CO2 compared to CH4 reveal that the CO2 adsorption on montmorillonite is more favourable and spontaneous. When the loading increases, the Qst values of CH4 and CO2 reduce. CH4 has a smaller Qst than CO2, which suggests the stronger interaction of CO2 with montmorillonite. CH4 and CO2 have decreasing ΔS with the loading. Under larger surface coverage conditions, the disorder of the adsorbed molecules is larger, and most of the injected molecules are trapped as a free state in the pore space. The higher ΔS of CO2 illustrates that CO2 molecules constitute a much more stable rearrangement than CH4 molecules. The temperature has a slight effect on the ΔS of CH4 and CO2.
•Montmorillonite has a stronger adsorption affinity for CO2 than CH4.•Displacing CH4 by CO2 on montmorillonite-bearing shale is feasible.•Adsorbed CO2 has more orderly arrangement than CH4 on montmorillonite.
Understanding the adsorption behaviors of CH4, N2, CO2, and their mixture in clay minerals is crucial to the approach of enhanced hydrocarbon recovery using gas injection with application in ...clay-rich gas-shale reservoirs. In this study, the adsorption measurements of CH4, N2, CO2, and their mixture on montmorillonite were conducted using a gravimetric technique. The underlying competitive adsorption mechanism of CH4, N2, and CO2 on montmorillonite was discussed using Henry's constant, adsorption selectivity, and thermodynamic variables. The adsorption selectivity of CO2 over N2 (αCO2/N2) was the highest, followed by the selectivity of CO2 over CH4 (αCO2/CH4) and that of CH4 over N2 (αCH4/N2). The αCO2/CH4 value was greater than one, confirming that CO2 can stimulate CH4 desorption through molecular swapping. Reducing the temperature increased the selectivity and the desorption of the pre-adsorbed gas was more easily triggered by the injection of the favorably adsorbing gas at lower temperatures. The uptake and affinity of CO2 were the highest, whereas N2 exhibited the smallest uptake and affinity. In addition, CO2 adsorption was more spontaneous than CH4 and N2 adsorption. Although N2 adsorption was less favorable than CH4 adsorption, injecting N2 could lower the spontaneity of CH4 adsorption by decreasing the CH4 partial pressure. The adsorbed CO2 molecules were the most ordered, and the freedom of adsorbed N2 molecules was the highest. The adsorption isotherm of the mixed gases was closer to that of the strong adsorption component. Thus, a strong adsorption gas plays a leading role in the adsorption of mixed gases.
•The order of affinities of gases on montmorillonite is as follows: CO2 > CH4 > N2.•Adsorbed CO2 molecules are more orderly than adsorbed CH4 and N2 molecules.•Strong adsorption gas plays a dominant role in adsorption process of mixed gases.
Aiming at the phenomenon that many dynamics occur frequently in working face or roadway when the first mining face of mine A, 2-2upper201 is driven to the edge of water-rich area, the effects on ...original rock stress by draining water-rich area and rockburst risks in two different conditions of drainage and no-drainage are studied through some methods such as theoretical analysis and numerical analysis, based on working face 2-2upper201A(the second working face of Mine A). The findings are as follows: ①The drainage in water-rich area can damage aquifer, resulting in stress decrease inside damaged area and stress concentration in the edge; ②Considering the effect by draining water-rich area, rockbust scope and degree in damaged area will change. In detail, the range below water-rich with risk will shrink and degree of risk will decline, while the edge and outside of water-rich area will enlarge as risk rises. The field monitoring indicates that in the process of mining, stress increments on measure spots rank as the order of from large to small as follows: edge > outside > inside. When working face is driven to the edge of water-rich area, many drastic dynamics like coal blasting, wall carving will occur on the wall, which suggests that draining water-rich area has a great impact on rockburst risk of working face. The research result has very important guiding significance to rockburst prevention in water-rich area.
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