Meitan xuebao (Jun 2023)
Differential heat transfer characteristics and mechanism of different coal composition and distribution
Abstract
The reservoir stimulation method of thermal induction is effective for improving the production of coalbed methane well, in which the heat transfer difference of coal composition and distribution is the key and fundamental problem to be solved urgently in coal reservoir thermal stimulation project. Taking the bituminous coal in the Wangjialing Coal Mine of Baode as an example, this paper accurately identified and characterized the spatial distribution characteristics of maceral based on the multi-scale observation method combining macro-description, microscopy-CT imaging of coal sample. The dynamic heat transfer process of coal with different lithotypes were characterized by uniform plate heating and thermal imaging acquisition experiment system. The thermal properties of macerals were quantitatively characterized by experimental data combining with the laws of thermodynamics to reveal the differential heat transfer mechanisms of maceral composition and distribution. The results show that in the heat transfer process, the temperature of semi-bright coal presents a bimodal state, while that of dull coal presents a unimodal state. The temperature span of both decreases first and then increases, and the temperature span of semi-bright coal is significantly wider than that of dull coal. The closer the proportion of vitrain + clarain and durain + fusain is, the more significant the temperature difference is. According to the evolutionary characteristics of temperature difference, the heat transfer process can be divided into four stages: rapid temperature rise, slow growth, fluctuating warming and dynamic equilibrium. The correspondence between the distribution of maceral components and the temperature spreading shows that the heat transfer is controlled by maceral composition and distribution. The inertinite filled with mineral is heated up the fastest to form a high-temperature zone, the inertinite generally forms a sub-high-temperature zone, and the vitrinite is heated up the slowest to a low-temperature state. In addition, the heating rate of the vitrinite and the temperature at equilibrium are inversely proportional to the distance of the inert group. The quantitative analysis of thermal properties shows that the mineral-filled inertinite has larger thermal conductivity and diffusion coefficient, thus its speed of transferring thermal perturbation is the fastest and takes the least time to reach thermal equilibrium, the inertinite is the second and the vitrinite is the slowest. The new fissure in coal after the thermal process mainly develops in vitrinite, and cracks mostly along the cross linking of inertinite and vitrinite. The thermal effect of crack reform in semi-bright coal is obviously stronger than that in dull coal, therefore the semi-bright coal is the high quality target of thermal reform.
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