Numerical simulation of CO₂-ECBM based on fluid-solid-thermal coupled model

In order to analyze the effect of coal reservoir characteristic parameters on CO₂-ECBM project, based on the non-isothermal competitive adsorption and seepage characteristics of binary gas, a fluid-solid-thermal coupled model of CO₂-ECBM was established. The effects of initial reservoir temperature, permeability, and pressure on permeability, CH₄ production rate, and CO₂ storage rate were studied by numerical simulation. Results show that: CH4 production rate drops rapidly in the initial stage of pumping, after that, the production rate slowly decreases with the increase of the extraction time; the CO₂ storage rate can be divided into three stages: initial rapid reduction, almost stable in the middle period, and slowly decreasing in the later period; in the gas extraction process without CO₂ injection, the variation of permeability is competition result of two types of factors: the coal matrix shrinkage caused by temperature reduction and gas desorption increase, the other is the coal matrix expansion caused by gas pressure decrease, so the permeability follows the rule of first decreasing and then increasing with the extraction time, the injection of CO₂ has a great influence on the permeability of coalbed, adsorption of CO₂ by the coal matrix causes the permeability to drop rapidly; under the same conditions, the CH₄ production rate and CO₂ storage rate are inversely proportional to the initial reservoir temperature and are proportional to the initial reservoir permeability and initial reservoir pressure.