页岩气藏多级压裂水平井压力动态分析

摘要

基于非结构Voronoi网格技术生成压裂水平井非结构网格，引入尘气模型(DGM)建立考虑页岩气藏吸附解吸、扩散和达西流的运移数学模型，并结合控制体有限差分法和全隐式法推导得到页岩气藏压裂水平井压力动态分析数学模型离散格式，编程绘制页岩气藏压裂水平井不稳定压力动态分析典型曲线。研究结果表明：页岩气藏压裂水平井压力动态分析典型曲线可划分为井筒储集、过渡流、裂缝线性流、早期径向流、复合线性流、系统径向流和边界流7个流动阶段；吸附解吸作用越强，试井典型曲线的位置越低，同时生产时井底附近压力降落最大，近井带解吸气对气井供给量最大，使得过渡流出现一个“凹子”，但受吸附解吸强度的影响，过渡流的“凹子”可能被掩盖掉；扩散系数能提高极低页岩渗透率储层的气体流动能力，当扩散系数达到一定值时，试井曲线中对应影响的流动阶段位置偏低；渗透率、裂缝数量、裂缝半长和裂缝间距对页岩气藏压裂水平井压力动态的影响与对常规气藏的影响一致。%The multi-fractured horizontal well(MFHW) unstructured grids were generated by using the Voronoi grid technology and the transport mathematical model which considers adsorption/desorption, diffusion and Darcy flow in shale gas reservoirs was established by introducing Dusty Gas Model. Furthermore, the numerical mathematical model for MFHW in shale gas reservoirs was derived based on control volume finite element method and fully implicit method, and the typical curves of transient pressure dynamic analysis for MFHW were drawn by computer programming. The results show that typical curves of pressure dynamic analysis for MFHW in shale gas reservoirs can be divided into seven flow stages which include wellbore storage, transient flow, fracture linear flow, early time radial flow around, compound linear flow, system radial flow and boundary flow. When the adsorption/desorption is stronger, the position of typical curves is lower, meanwhile there is a characteristic dip in the pressure derivative during transient flow stage because of the biggest pressure drop and the largest gas supply amount of desorbed gas near the well bottom. However, the dip of the pressure derivative may be masked off due to influence of desorption strength. And the flow capacity in ultra-low permeability shale gas reservoir can be improved by diffusion coefficient, and when the diffusion coefficient reaches a certain value, the position of the curves’ flow stages which is influenced goes lower. Besides, the influences of permeability, fracture number, half length of fracture and fracture space on shale gas MFHW well pressure danamic behaviors are consistent with that on conventional gas reservoirs.