双重介质致密分形气藏水平井压力动态分析

Transient pressure analysis of the horizontal well in dual-medium tight fractal gas reservoirs

经典渗流理论描述双重介质致密气藏的非均质性与多尺度特征具有局限性。应用分形理论描述裂缝系统的应力敏感性及分形特征,考虑边界层及屈服应力的影响描述致密基质块的渗流特性,建立存在启动压力梯度的双重介质致密分形气藏水平井非线性渗流模型。利用等价压力点处理内边界条件,应用有限元方法求解渗流模型;绘制井底压力动态曲线,将渗流过程划分7个流动阶段。对比不同模型的渗流特点,对非线性参数、水平井长度及分形指数进行敏感性分析。结果表明:分形非线性模型综合考虑储层非均质性对渗流的影响,更贴近实际储层。非线性增强导致窜流延迟且强度减弱;水平井增长使窜流出现时间延迟;分形指数增大影响曲线在中后期流动阶段的上翘程度。该结果对双重介质致密气藏的试井参数解释及水平井井底压力动态特征评估具有实际意义。

The classical seepage theory has limitations in describing the heterogeneity and multi-scale characteristics of dual-media tight gas reservoirs.This paper applies the fractal theory to characterize the stress sensitivity and fractal characteristics of the fracture system.The influence of boundary layer and yield stress are taken into account to describe the seepage characteristics of dense matrix blocks,so that the nonlinear seepage model of the horizontal well with start pressure gradient in dual-medium tight fractal gas reservoirs is established.Then the inner boundary condition is processed by the equivalent pressure point and the finite element method is used to solve the seepage model,so that the dynamic curves of the bottom hole pressure is drawn and the seepage process is divided into seven flow stages.Finally the seepage characteristics of different models are compared and the sensitivity analysis of nonlinear parameter,horizontal well length and fractal index are conducted.The results show that the fractal nonlinear model which takes the influence of the heterogeneity on seepage into account is flexible and comprehensive,and the model is closer to the actual reservoir.Nonlinear enhancement makes the channeling delay and weakens the channeling intensity.The increase of horizontal well length makes the channeling delay but it does not affect the channeling duration and intensity.The increase of fractal index does not affect the start time and intensity of the channeling while it can changes the upward warping degree of the curve in the middle and late flow stages.