A fully coupled thermal-hydraulic-mechanical model with two-phase flow for coalbed methane extraction

Abstract

Although the interaction of gas and coal has been comprehensively investigated in coalbed methane (CBM) extraction process, fewer scholars have taken the effect of temperature and groundwater into account, which brought a large deviation for CBM extraction design. In this study, a fully coupled thermal–hydraulic–mechanical model (THM) including coal deformation, gas seepage, water seepage, and thermal transport governing equations is developed and solved using the finite element (FE) method. The coal mass is simplified as a dual-porosity and single-permeability media while CBM migration is considered as a tandem process of desorption, diffusion and seepage. The dynamic evolution model of permeability serving as the coupled term for THM model is developed under the combined impact of stress, water pressure, gas pressure, gas adsorption/desorption and temperature. The proposed model is first verified by showing that the modeled gas production rate and water production rate match reasonably with the in-situ measured ones. Different coupled models for CBM extraction were comparatively analyzed by accomplishing a series of simulations. It is found that the gas production rate of models ignored water effect monotonously reduces over time; while the model considered water effect rises at beginning and then gradually reduces. The model ignored water effect will overestimate gas production, and the model ignored thermal effect will underestimate gas production, particularly coal seam contains considerable amount of water. The evolution of permeability is the competition result of two opposite effects: the matrix shrinkage effect caused by temperature reduction, the matrix swelling effect caused by gas pressure decrease and methane adsorption increase. A rising permeability resulted from the integrative action of both lower reservoir temperature and pressure during CBM extraction is observed. The impact of initial water saturation on gas production can not be ignored in the whole extraction process, especially during the water drainage period. Gas production rate of CBM well decreases with initial reservoir temperature, initial water saturation and Langmuir volume constant, while increases with the Klinkenberg factor. Permeability rate increases with initial water saturation and Klinkenberg factor, however decreases with initial reservoir temperature. As Langmuir volume constant increases, the peaking value of gas production rate increases and delays. The Klinkenberg effect promotes coalbed methane migration significantly, ignoring which will underestimate the gas production, and the impact of Klinkenberg effect gradually increases with the drop of gas pressure.