Abstract
Heat injection is an important artificial technique, which can significantly enhance the extraction efficiency of coal seam gas (CSG) and reduce the outburst risk caused by CSG. Although heat injection has been comprehensively investigated, the effect of temperature on the coal–gas interactions in CSG extraction is still not clear. In this study, a thermo-hydro-mechanical model was developed considering the expansion of coal mass and the change of adsorption capacity induced by heat injection. Subsequently, the reliability of the model was verified through a comparison with other theoretical models and field data. Finally, a numerical simulation and parameter analysis of the heat injection process were performed and compared with the traditional gas extraction method. The simulation results show that heat injection can significantly increase the gas production rate and cumulative gas production through the gas desorption and the permeability increase. The gas content in the coal seam dramatically decreases in the vicinity of the production and heat injection wells under the condition of heat injection, which greatly accelerates the gas drainage. The coal deformation caused by thermal-induced gas desorption has a more dominant effect on the porosity than other factors, i.e., pore pressure, thermal strain and compressive strain.
Highlights
Coal seam gas (CSG) is recognized as a fuel and chemical raw material for clean energy recovery.Simultaneously, it is a hazardous gas in coal mines that can cause explosions, worker suffocation, and gas outbursts
Where Qs is the gas source, and mass content m is calculated as [44]: m = ρ g φ f + ρ ga ρc Vsg where m is the gas mass content, ρ ga is the gas density at standard temperature and pressure (STP), ρc is the coal density, and Vsg is the content of the absorbed gas (m3 ·kg−1 )
The evolution of the gas pressure calculated by the numerical model and analytical model is compared in Figure 2a, which verifies the validity of the numerical model
Summary
Coal seam gas (CSG) is recognized as a fuel and chemical raw material for clean energy recovery. The principle of thermal stimulation is to heat the coal seam by injecting heated gas or heated water or to adopt the electromagnetic technology to heat the coal seam, and to achieve efficient gas extraction from the production well [19,34]. It can effectively increase the recovery rate by increasing the reservoir temperature. In previous thermo-hydro-mechanical models [40,42], non-Darcy effect and Klinkenberg effect of the fluid were rarely considered, which may cause underestimates in the analyses of heat and gas transfer characteristics during heat injection process. The effect of heat injection on coal seam gas recovery was analyzed quantitatively
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