Abstract
This paper puts forward a new mathematical model, which is a coal damage-heat-fluid-solid multi-field coupling theory, in order to reveal the mechanical mechanism of the increase of coal-bed methane recovery through thermal stimulation, and to evaluate its effect. The strain field is introduced to define the damage of coal by considering of the effects of temperature, gas pressure, and mining stress of the coal seam. It is used to quantitatively describe the degree of coal rupture and damage. Additionally, the elastic and damage constitutive equation of coal and rock mass, the governing equation of the temperature field, and the coupling equation of gas diffusion and seepage are established. Based on these equations, the finite element source program is redeveloped by using the FORTRAN language, and a multi-field coupling analysis program is compiled. This program takes the temperature, the gas seepage, and the damage and deformation of coal and rock mass into consideration. The effect of heat injection temperature on gas production efficiency, gas pressure distribution, and effective extraction radius during coal-bed methane mining process is analyzed. The results show that the injection of heat can significantly improve the desorption and diffusion of gas, as well as the gas production rate and the production efficiency of coal-bed methane.
Highlights
The coal seam in China has a strong adsorption, low permeability, and slow desorption rate.many coal-bed methane wells have experienced a significant decline in gas production after a period of extraction
Kang [1] suggested the use of liquid CO2 gasification blasting technology to fracture the coal seam, and results showed that this technology could effectively improve the coal seam permeability, and enhance coal-bed methane recovery
When compared with the traditional extraction technology, Salmachi [8] found that the gas extraction volume of coal seam was increased by 58% during a period of 12 years after the geothermal water at 80 ◦ C was continuously injected into coal seam, and the maximum gas production rate was 6.8 times higher
Summary
The coal seam in China has a strong adsorption, low permeability, and slow desorption rate. Zhu et al [13] comprehensively studied the influence of gas pressure, matrix thermal expansion and deformation, the temperature change on coal porosity, and established a permeability model. The mechanism of the thermal stimulation of production improvement can be explained, as follows, the increase of temperature can reduce the adsorption amount of coal-bed methane, changing the microstructure of coal body and improving the permeability of coal seam, thereby increasing the production of coal-bed methane per unit time and achieving the goal of enhancing the extraction of coal-bed methane. Thermal stimulation for the mining of coal-bed methane is a typical heat-flow-solid multi-field coupling process. The influence of different heating temperatures on the dynamic evolution process of gas pressure and the mining efficiency of coal-bed methane is deeply studied
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