Numerical simulation study and engineering practice of in-situ heat injection mining of coalbed methane

The low permeability of coal seams and the strong adsorption characteristics of methane are the key problems restricting the exploitation of coalbed methane (CBM). And how to exploit CBM quickly and efficiently is an important subject in the research of CBM mining. Firstly, based on the water- gas-heat-solid coupling mathematical model, a physical model of heat injection enhanced CBM extraction was established and the correctness of the model was verified. Then, the migration mechanism of water-gas two phase in the process of heat injection was studied through numerical simulation software. Finally, it was applied in the coal mine. The numerical simulation results show that the gas-water migration in the process of heat injection enhanced CBM drainage follows the changing law of “drainage in early stage and gas extraction in later stage”. The water in the coal seam has an important influence on the migration of methane. During heat injection, water occupies the migration channel of methane, and the inhibition of water on methane is greater than the promotion of temperature, resulting in the reduction of CBM production. After heat injection, high temperature promotes methane desorption and relieves the inhibition of water on methane, resulting in a significant increase in CBM production. Through the heat injection test in coal mine, two mature heat injection methods have been formed, namely intermittent heat injection method and alternate heat injection method. The two heat injection methods have similar gas production laws, that is, the daily gas production and methane concentration in the heat injection stage are generally low, while in the extraction stage after heat injection they are significantly increased. The field test results show that compared with the traditional negative pressure extraction method, the intermittent heat injection method can increase the methane concentration and daily gas production by more than 10 times and 100 times respectively. Also, the alternative heat injection method can increase the methane concentration and daily gas production by more than 10 times and 50 times respectively. Finally, the results of both field test and numerical simulation show that the effect of heat injection enhancing CBM extraction is significant, and the extraction stage after heat injection is a high yield period of CBM. The research results can provide a theoretical basis and technical guidance for the engineering practice of CBM extraction by heat injection.