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在深海条件下采用单井降压法开采的天然气水合物矿藏中,利用TOUGH+HYDRATE软件对其开采过程和甲烷气体扩散过程进行数值模拟。物理模型由上至下依次为上盖层、水合物沉积层和下盖层。将上、下盖层外边界的温度设为恒定,与含水合物沉积层之间有热量和质量交换,数值模型采用二维圆柱坐标网格。模拟结果表明开采过程中井口产气速率是一个升高—降低—波动升高的过程,水合物分解产生的气体有一部分通过上盖层溢出,能在一定程度上增加大气中温室气体的量。开采初期水合物分解速率降低的主要原因是水合物分解产生的甲烷气体在地层中大量累积,开采后期水合物分解速率产生波动的主要原因是发生“气穴现象”。井口附近由于压力变化较快水合物分解最为剧烈,其附近有个低温区存在。上、下盖层附近水合物分解速率也较快。
In natural gas hydrate deposits exploited by single well depressurization method under deep sea conditions, the TOUGH + HYDRATE software is used to simulate the mining process and methane gas diffusion process. The physical model from top to bottom are the cap layer, the hydrate sedimentary layer and the lower cap layer. The temperature of the outer boundary of the upper and lower capping layers is set to be constant, and there is exchange of heat and mass with the hydrate-bearing sedimentary layer. The numerical model uses a two-dimensional cylindrical coordinate grid. The simulation results show that the gas production rate at the wellhead is a process of increasing, decreasing and fluctuating during the mining process. Part of the gas generated by the hydrate decomposition overflows the upper capping layer and can increase the amount of greenhouse gases in the atmosphere to a certain extent. The main reason for the decrease of hydrate decomposition rate in the early stage of mining is that methane gas generated by hydrate decomposition accumulates in the strata. The main reason for the fluctuation of hydrate decomposition rate in the late stage of mining is “cavitation”. Near the wellhead, the hydrate decomposes most rapidly due to the pressure changes, and there is a low temperature zone nearby. The rate of hydrate decomposition is also higher near the upper and lower cap rocks.