天然气水合物降压开采数值模拟研究

Numerical simulation on depressurization production of natural gas hydrate

为揭示天然气水合物降压开采过程中水合物分解规律,建立了柱坐标系下水合物降压开采的物理模型和数学模型,应用有限差分法进行求解,并应用神狐海域试采数据进行了验证,进而分析了降压开采过程中压力、水合物饱和度、渗透率的变化规律以及多种边界条件下分解过渡带的移动规律,研究了不同降压开采参数对水合物分解过程的影响.结果表明:降压开始后,井眼周围迅速形成压降漏斗,随着开采时间的增加,压降漏斗向储层远处扩散;井眼处的水合物最先分解,随着开采时间的推进,水合物饱和度逐渐降低,储层渗透率逐渐增大;水合物饱和度降低的区域沿径向向外扩散,渗透率增大的区域也与之相应;过渡带外沿、内沿移动速度不同步,开采后期移动速度都变慢,分解过渡带宽度随着开采时间逐渐增大,到一定天数后趋于稳定;水合物降压开采的主要控制参数包括开采井压力、水合物初始饱和度、储层绝对渗透率、水合物分解动力学常数等;模拟水合物降压开采时,如果选择封闭型边界且半径较小,则所得出的模拟结果与实际开采情况会有较大的差别,甚至相悖.

In order to study the decomposition process of gas hydrate depressurization production,a mathematical model of gas hydrate reservoir is established in cylindrical coordinate system and it is solved by finite difference method.With this modeling program,we studied the variation of pressure,hydrate saturation,permeability and the movement of the decomposition transition zone during the depressurization production process.The main control parameters are also analyzed for the depressurization production.Modeling results reveals the following facts:(1)After the pressure reduction starts,a pressure drop funnel is rapidly formed around the wellbore,as time goes,the pressure drop funnel spreads farther into the reservoir,the hydrate near the wellbore decomposes first,the hydrate saturation decreases,while the permeability increases.(2)The decomposition transition zone moves outwards along the radial direction,with the outer and inner edge of the transition zone moving at different speed,and the moving slows down in the later stage of the production.The width of the decomposition transition zone increases gradually with the production time and tends to be stable after a certain number of days.(3)The main control parameters of hydrate decompression production include production well pressure,initial hydrate saturation,absolute permeability,hydrate decomposition kinetic constants.(4)The radius of the model boundary plays an important role in the simulation of the hydrate decomposition.Improperly small radius may lead to results contradicting to the real production process.