Experimental Assessment of Combined Contribution of Coal Mass Diffusion and Seepage to Methane Migration under In Situ Stress Conditions

Abstract

Gas diffusion and seepage characteristics can reflect the transport capacity of coal mass pores and fracture channels and are also important parameters to determine the mechanism of methane migration. Currently, research on the contribution of diffusion and seepage to methane migration is carried out mainly by fixing one parameter and discussing the effect of the other. However, few studies are focused on the dynamic combined effect of diffusion and seepage on methane migration under in situ stress conditions, which is mainly attributed to difficulties in obtaining the parameters of diffusion characteristics of coal mass under stress. This paper presents a study on the dynamic combined effect of coal mass diffusion and seepage on methane migration under different in situ stress conditions. First, the physical structure model of coal mass under high and low stresses was constructed based on the variations of coal mass porosity, permeability, and computed tomography under different stresses. Meanwhile, based on the methane desorption and diffusion curves of coal mass under different stresses, the diffusion parameters of coal mass were obtained according to the corresponding diffusion model. Furthermore, the coupled multifield model was used to calculate the methane migration in coal mass during the process of borehole extraction with different stresses. The results suggest that the difficulty in methane extraction is directly proportional to the stress, but a huge discrepancy exists in methane migration under high and low stresses principally due to the change of dominant factors in methane migration of coal mass under different stresses. Additionally, given the essential factors controlling the overall methane migration of coal mass under high and low stresses, a joint technical method for promoting methane flow rate under multicoal seam conditions in the same mining area was proposed, i.e., hydraulic fracturing and sand injection (increasing permeability) in coal seams under low stress and simultaneous application of hydraulic fracturing and sand injection and multistage cavitation pressure relief (increasing diffusion) in coal seams under high stress. The research results obtained in this paper provide theoretical support for understanding the methane migration mechanism in coal seams under in situ stress and stimulated gas production.

Simulation Study of Coal Seam Gas Extraction Characteristics Based on Coal Permeability Evolution Model under Thermal Effect

“… diffusion field governing equation: 1 governing equation of seepage field: 35 coal constitutive equation: 25 temperature field governing equation: 22 , 27 permeability model: …”

“…At present, more than half of deep coal mines in China are high-gas mines . In a deep high-gas mine, the gas concentration and pressure are greater in the coal seam, and the problem of gas outburst is becoming increasingly obvious .…”

“…At present, more than half of deep coal mines in China are high-gas mines. 1 In a deep high-gas mine, the gas concentration and pressure are greater in the coal seam, and the problem of gas outburst is becoming increasingly obvious. 2 The gas extraction of low-permeability coal seam in high-gas mine is faced with such problems as difficult extraction, small range, and high decay rate.…”

Comparison of elastic properties of tectonic coals along the face and butt cleat directions

“…One of the key issues for the CBM reservoir modelling is the characterization of the pore structure of coals. The pore structure of coals is very complex, which can be considered a dual pore structure consisting of matrix pores and fractures (Guo et al, 2021;Tian et al, 2023). Assuming that all fractures are closed at the highest pressure, the matrix pores are added by the differential equivalent medium (DEM) model (Bandyopadhyay, 2009;Hornby et al, 1994).…”

Constructing a Microdiffusion–Seepage–Stress Multifield Coupling Model for Nanopore Gas

“…For the transport process of methane in carbon nanopores, Zhang 27 and others simulated the arrangement, slip, and transition flow of wall atoms in the supercritical state and the application of different driving forces on the basis of MD theory. Facing the problem that the characteristic parameters of coal diffusion under stress conditions are hard to obtain, Tian 28 and other scholars used a multifield coupling model to calculate the methane transport in the coal body in the process of borehole extraction under different stresses. In order to conduct experiments on the diffusion behavior of shale gas under high temperature and pressure conditions, Sun 29 proposed a new approach for calculating the multifield coupling diffusion coefficient by using molecular membrane samples with uniform pore sizes instead of shale cores.…”