考虑启动压力梯度的复合气藏不稳定压力动态分析

低渗透气藏的渗流过程中存在启动压力梯度,导致其渗流机理比常规气藏更加复杂。其次,在低渗透气藏开发过程中由于酸化、压裂或气藏本身的特点,会使得气藏近井区和远井区的岩石及流体性质存在差异,形成复合气藏。因此,建立了考虑启动压力梯度的复合气藏水平井不稳定渗流模型,使用Laplace变换、正交变换等方法获得了该模型的无量纲解析解。在此基础上,研究了启动压力梯度、水平段长度、内外区流度比和内区半径对复合气藏水平井不稳定压力动态特征的影响。结果表明,考虑启动压力梯度的复合气藏水平井的压力动态曲线可划分为纯井储阶段、井储后的过渡阶段、早期垂向径向流阶段、中期线性流阶段、内区拟径向流阶段、内外区拟径向流过渡阶段和外区拟径向流阶段。启动压力梯度越大,压力动态曲线后期上翘幅度越大;水平段长度越大,压力动态曲线位置越低;内外区流度比越大,压力动态曲线位置越高;内区半径越大,内区拟径向流阶段持续时间越长。研究结果丰富了低渗透复合气藏不稳定渗流的相关理论,为更好地认识气藏渗流规律和气藏的科学高效开发提供了理论依据。

Dynamic Analysis on Unsteady Pressure of Composite Gas Reservoirs With Consideration of Threshold Pressure Gradient

The existence of threshold pressure gradient in the flow process of low permeability gas reservoirs makes its flow mechanism more complicated than that of the conventional gas reservoirs.Moreover,due to acidizing,fracturing and some inherent features of gas reservoirs during the development of low-permeability gas reservoirs,there are some differences in the rock and fluid properties between the near wellbore areas and far-from-wellbore areas,resulting in the formation of composite gas reservoirs.Therefore,the paper establishes a unsteady flow model considering the threshold pressure gradient for horizontal wells in composite reservoirs,and obtains dimensionless analytical solutions of the model by using complex mathematical methods such as Laplace transformation and orthogonal transformation.Based on which,the influences of threshold pressure gradient,horizontal section length,mobility ratio and inner radius on the dynamic characteristics of the unsteady pressure in horizontal wells in the composite reservoirs are also studied.The results show that the pressure response type curves of the proposed model can be divided into 7 stages such as wellbore storage stage,transitional flow stage after wellbore storage,early vertical radial flow stage,mid-term linear flow stage,inner-zone pseudo-radial flow stage,transitional flow stage from inner zone to outer zone and outer-zone pseudo-radial flow stage.The higher the threshold pressure gradient,the larger the up-warping amplitude of the pressure response type curve;the longer the horizontal section length,the lower the position of the type curve;the bigger the mobility ratio,the higher the position of the type curve;the larger the inner zone radius,the longer the duration of inner-zone pseudo-radial flow stage will be.This research enriches the theories related to unsteady flow in low-permeability composite gas reservoirs,and provides a theoretical basis for a better understanding of the flow characteristics and for efficient development of gas reservoirs.