基于随机分形网络模型研究近井地带凝析油气非达西微观渗流机理

The immediate vicinity of the wellbore where condensate flows involves a complex seepage mechanism due to the impacts of the high capillary number and non-Darcy effect. It is hard to accurately characterize by using a conventional experimental method. By means of random fractal network modeling, we quantitatively characterized the critical saturation of the static or quasi-static condensate to flow. This has great significance for characterizing and studying the microscopic gas condensate seepage mechanism of near-wellbore. Seeing that the near-wellbore multi-phase condensate seepage mechanism is not yet identified, we intended to quantitatively characterize the involved important parameters on the basis of the fractal network modeling. So, in the study, a random fractal pore network model was firstly created for the microscopic numerical simulation according to the microscopic reservoir features by using Forchheimer's equation rather than the conventional Darcy equation, then microscopic seepage characteristics of the two-phase condensate in the porous medium were studied in terms of static parameters (pore size fractal ), condensate properties (condensate oil content and interfacial tension), and dynamic parameters (differential pressure and velocity), and finally, a mathematical model featuring the fractal characterization of high-velocity non-Darcy flow coefficient and oil and gas relative permeabilities was created. The findings of the study can quantitatively interpret the impact of the high-velocity non-Darcy flow effect and thoroughly reveal the seepage capability variation law of the near-wellbore condensate flow region, thus providing a theoretical basis for gas condensate well production prediction and determination.

近井地带凝析油流动区受高毛管数和非达西效应影响的渗流机理复杂，难以用常规实验方法准确描述和表征。我们前期运用随机分形网络模型表征了静（拟静）态凝析油临界流动饱和度，对研究微观渗流机理起重要作用。针对近井地带多相渗流影响机制尚不明确的现状，本课题拟提出对重要参数分形定量化表征的设想。为了验证这个设想，课题首先基于微观储层特征建立随机分形孔隙网络模型，结合凝析油气相态实验结果，用Forchheimer方程取代常规的达西方程进行微观数值模拟，从微观储层静态参数（孔隙尺寸的分形表征）、凝析油气属性（凝析油含量、界面张力）、动态参数（压差）等层面研究多孔介质凝析油气两相微观渗流机理，建立近井地带临界流动饱和度、油气相对渗透率分形表征的数学模型。本研究成果可定量化地诠释高速非达西效应影响程度，深层次揭示近井地带凝析油气渗流能力的变化规律，为凝析气藏的经济高效开发提供理论指导和技术支持。

近井地带凝析体系受高毛管数和非达西效应影响的渗流机理复杂。本课题首先基于恒速压汞实验提取真实岩心微观特征参数建立随机分形网络模型，结合凝析油气衰竭过程中相态变化的CT扫描实验，建立了考虑界面张力、毛管力等因素的确定临界凝析油饱和度新方法，引入Forchheimer方程建立了微观非线性渗流数值模拟方法，从微观储层静态参数、凝析油气属性、动态参数等层面研究多孔介质凝析油气两相微观渗流机理，建立临界流动饱和度、非达西系数分形表征的数学模型。.研究结果表明配置的二元凝析体系在真实岩心中露点压力有别于常规PVT筒观察，由于CT扫描标定区间有限无法具体量化含水凝析油气体系中随孔隙半径不同的露点压力值。运用凝析油临界饱和度确定新方法可以获得多孔介质中的凝析油微观分布规律。凝析油临界饱和度随平均孔隙半径的增大而减小，随着分形维数的增大而增大；随油气系统界面张力增大临界含油饱和度增大，高于临界界面张力值时含油饱和度增幅不大；流速越大临界含油饱和度越小。运用微观非线性渗流数值模拟方法，结合高速非达西实验结果分析了非达西系数的影响因素，并推导了含有束缚水的非达西系数分形表征方程。非达西系数是孔隙平均半径、分形维数、束缚水饱和度、迂曲度等反映储层微观属性参数的函数，其值随着孔隙平均半径的增大而减小，随分形维数的增大而增大，随着束缚水饱和度的增大而增大，随着迂曲度的增大而增大。.本微观数值模拟方法在胜利油田剩余油微观研究中起到一定的作用。该研究成果可定量化地诠释高速非达西效应影响程度，深层次揭示近井地带凝析油气渗流能力的变化规律，为凝析气藏的经济高效开发提供理论指导和技术支持。