Numerical study on the movement of the decomposition front of natural gas hydrate under depressurization
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摘要:
在水合物分解过程中，已经分解的区域与未分解的区域之间存在一个过渡带，即分解前缘，研究分解前缘移动规律有助于认识水合物分解特征和进一步预测分解气量，对实际开采以及开采潜力评价提供大方向的科学参考。本文依据Stefan边界理论建立水合物分解前缘一维三相数学模型，通过参数量级分析，将水合物分解渗流场作为拟定常场，解析计算得到分解前缘随时间移动函数，同时将温度场方程无维化转换后，得到计算温度变化的超越方程。据分解前缘移动函数进一步计算总产气量以及井口产气速率。结合模型算例，认为水合物分解前缘移动与时间的平方根呈线性关系，移动速率随时间推移而减小，产气速率在开采前期达到峰值后快速下降达到稳定值。另外，以南海神狐海域第一次试采结果为依据，对比发现模型计算总气量高于实际试采值，两者相对误差在可接受范围内。因此，本文对水合物开采特征评价提供了一种新的简单计算方法，并对开采潜力给出了乐观的预测。最后，通过对地层初始温度、绝对渗透率以及孔隙度敏感性分析发现，地层初始温度和渗透率增大，水合物分解前缘移动距离随之增大，初始地层温度对水合物分解影响显著。而地层孔隙度越大，分解前缘移动速率反而降低，移动距离减小，井口与分解前缘压差减小，此时分解前缘移动由储层热物理参数决定。
Abstract: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.
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Key words:
- gas hydrate /
- decomposition front /
- depressurization /
- theoretical model
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表 1 初始条件与边界条件
Table 1. Initial conditions and boundary conditions
初边值条件 值 初始地层压力( 
初始地层温度( 
井底压力( 
)
外边界压力( 
)
外边界温度( 
)
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表 2 相关物性参数
Table 2. The correlated parameters used for calculation
参数 数值 水合物密度 
（kg/m3）910 岩石密度 
（kg/m3）2 650 水合物层厚度h（m） 40 水合物饱和度 
0.3 水合物层水饱和度 
0.7 孔隙度 
0.3 初始地层压力 
（MPa）14.0 初始地层温度 
（K）287.15 井底压力 
（MPa）3.0 单位质量水合物中气体体积分数 
0.129 气体运动黏滞系数 
（Pas）1.e-5 水相运动黏滞系数 
（Pas）1.e-3 水合物层绝对渗透率K（md） 2.9 井筒地层绝对渗透率K（md） 150 水合物分解吸收热 
（kJ/kg）430 水合物导热系数 
（W/mK）2.11 岩石导热系数 
（W/mK）2.0 水导热系数 
（W/mK）0.58 水合物比热容 
（KJ/kg K）2.22 水比热容 
（KJ/kg K）4.2 岩石比热容 
（KJ/kg K）1.0
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