Numerical study on the movement of the decomposition front of natural gas hydrate under depressurization

In the process of hydrate decompression, there occurs a decomposition front between the decomposed and undecomposed regions of gas hydrate reservoir. Studying the movement of the decomposition front may help to understand the hydrate decomposition characteristics and further predict the gas volume, which will provide a scientific reference for the actual exploitation potential. In this paper, a one-dimensional and three-phase mathematical model is established. After analyzing the parameter magnitude, the movement of gas and water in hydrate reservoir is regarded as steady flow, and the decomposition front is calculated. Meanwhile, the temperature field equations were dimensionless trans-formed to obtain the transcendental equations for calculating temperature. Combined with the model example, it is considered that the movement of the hydrate decomposition front is linear with the square root of time, and the gas production rate rapidly decreases to a stable value after reaching the peak in the early period. In addition, based on the results of the first trial production in Shen Hu area of the South China Sea, it is found that the total gas production calculated by the model is higher than the actual trial production value, and the relative error is within the acceptable range. Therefore, this paper provides a new simple calculation method for hydrate exploitation characteristics, and gives an optimistic prediction for the exploitation potential. Finally, through sensitivity analyses of the initial temperature, absolute permeability and porosity, it is found that with the increase of the initial temperature and permeability of the formation, the moving distance of the hydrate decomposition front will increase, and the initial formation temperature has a significant effect on the decomposition of hydrate. As the porosity of the formation gets greater, the movement rate of the decomposition front decreases, the moving distance decreases, and the pressure difference between the wellhead and the decomposition front decreases. At this time, the movement of the decomposition front is determined by the thermal physical parameters of the reservoir.