Pore structure of low-permeability coal and its deformation characteristics during the adsorption–desorption of CH4/N2

Abstract

The pore structure of coal plays a key role in controlling the storage and migration of CH4/N2. The pore structure of coal is an important indicator to measure the gas extraction capability and the gas displacement effect of N2 injection. The deformation characteristic of coal during adsorption–desorption of CH4/N2 is an important factor affecting CH4 pumpability and N2 injectability. The pore structure characteristics of low-permeability coal were obtained by fluid intrusion method and photoelectric radiation technology. The multistage and connectivity of coal pores were analyzed. Subsequently, a simultaneous test experiment of CH4/N2 adsorption–desorption and coal deformation was carried out. The deformation characteristics of coal were clarified and a coal strain model was constructed. Finally, the applicability of low-permeability coal to N2 injection for CH4 displacement technology was investigated. The results show that the micropores and transition pores of coal samples are relatively developed. The pore morphology of coal is dominated by semi-open pores. The pore structure of coal is highly complex and heterogeneous. Transition pores, mesopores and macropores of coal have good connectivity, while micropores have poor connectivity. Under constant triaxial stress, the adsorption capacity of the coal for CH4 is greater than that for N2, and the deformation capacity of the coal for CH4 adsorption is greater than that for N2 adsorption. The axial strain, circumferential strain, and volumetric strain during the entire process of CH4 and N2 adsorption/desorption in the coal can be divided into three stages. Coal adsorption–desorption deformation has the characteristics of anisotropy and gas-difference. A strain model for the adsorption–desorption of CH4/N2 from coal was established by considering the expansion stress of adsorbed gas on the coal matrix, the compression stress of free gas on the coal matrix, and the expansion stress of free gas on micropore fractures. N2 has good injectability in low-permeability coal seams and has the dual functions of improving coal seam permeability and enhancing gas flow, which can significantly improve the effectiveness of low-permeability coal seam gas control and promote the efficient utilization of gas resources.

Review on Carbon Capture, Utilization, and Storage for Enhancing Gas Recovery

“…The study by ref thoroughly examined the pore structure and deformation characteristics of low-permeability coal during CH 4 /N 2 adsorption–desorption processes. The study revealed important advancements in micropores and transition pores in the coal samples, highlighting a prevalent occurrence of semiopen pores in the pore structure.…”

“…Ji et al 51 6 2023 Explore the impact of coal's pore structure on methane storage, focusing on the importance of coal quality. Li et al 52 7 2022 Injecting CO 2 into gas reservoirs to reduce emissions and boost output.…”

The Physical Behavior of Protected Coal Seams Based on Triaxial Unloading Conditions

“…Since the self-weight of the subgrade rock was not considered after simplifying the bottom into a semi-infinite plane body, the stress distribution in the actual bottom should be based on Equation (15), plus the stress level due to the self-weight of the bottom rock layer.…”

“…Protective seam mining can realize both the stress unloading of the coal body of the protected seam, which reduces the possibility of high-energy disasters, and the resolution of the original adsorbed state gas in the coal body of the protected seam [12], which provides conditions for the extraction of gas from the protected seam. Gas threatens the safe production of coal mines [13][14][15], but it is also a clean energy source [16]. It is economical (and indeed sensible) to conduct coal seam mining while also achieving the extraction and utilization of unburdened gas [17].…”

Fine Structure Characterization of Representative Coal-Bearing Rocks of the Late Carboniferous Taiyuan Formation in Huainan Coalfield: Combined SEM, FESEM, and μCT Techniques

“…Extracting CBM as a high-quality clean energy source does not only realize waste to treasure but also reduces the occurrence of gas disasters in the mining process and achieve the purpose of turning harm into benefit. In recent years, with the accelerated development of China’s CBM industry and gas extraction technology, the universal macroscopic characteristics of China’s CBM reservoirs, such as complex geological structure, low permeability and strong adsorption, have been gradually highlighted, especially for some low-permeability coal seams, where both the management and utilization of coalbed gas disasters are more difficult. , In order to solve this problem and take corresponding measures, it is necessary to have a comprehensive understanding of the microscopic pore structure of the coal seam or coalbed strata, where pores and fissures are both storage spaces and important channels for seepage and output of CBM. − As an important storage space for unconventional natural gas, coal pore structure not only affects the macroscopic physical properties of coal, but also influences the adsorption, diffusion and percolation of gas in coal seams. − The degree of development, microstructure and distribution characteristics of different pore fractures will directly affect the processes of adsorption, desorption and seepage characteristics of CBM, which to some extent restrict the development of CBM and the effectiveness of gas extraction. , …”