A numerical analysis of radio frequency heating of coal with different ranks

Abstract

Radio-frequency (RF) heating is a novel thermal stimulation method in developing coalbed methane (CBM). Various research has been conducted on the effect of electromagnetic (EM) heating on the physical properties of coal. However, few studies considered the working conditions of underground coal seam heating. This paper calculates the coal seam temperature distribution based on the coupling between electromagnetic wave propagation and heat transfer in a vertical well to study the influence of coal seam metamorphism and thermoelectric characteristics on temperature distribution. The reservoir thermophysical parameters related to temperature are considered in the heat transfer and wave equations, respectively. Numerical simulations reveal the influence of coal ranks and thermo-electrical properties on heating efficacy. Results indicate that the temperature in the vicinity of the RF heater is relatively high, and the whole heated zone forms an elliptical shape. Low-metamorphism coal, such as lignite, is more functional for RF heating and has a broad heating range, leading to a uniform diffusion coefficient enhancement and good thermal homogeneity. Higher thermal conductivity, lower specific heat capacity, and water saturation can expand the heating area and reduce the temperature near the borehole, benefiting the maintenance of wellbore integrity. The coal seam relative permittivity has little effect on the reservoir temperature when its value is between 2.4 and 6.4.