考虑非达西效应的酸蚀裂缝流场数值模拟

酸压改造后的酸蚀裂缝是储层流体的主要流动通道，研究流体在酸蚀裂缝中的流动规律是十分必要的。本文介绍了目前裂隙流动研究中的4种常用控制方程及各自的适用条件，确定了Navier-Stokes方程为酸蚀裂缝流场数值模拟的控制方程。为了分析酸蚀裂缝流场分布规律以及非达西效应对裂缝导流能力的影响，利用逆向工程技术对两种不同刻蚀形态的酸蚀岩样壁面进行了实体重构，并采用有限元数值模拟方法进行了不同流量下的酸蚀裂缝流动试验。结果表明：酸刻蚀形态和裂缝接触关系对流场分布的影响较大。隙宽分布平滑的均匀刻蚀裂缝，其流态稳定、曲折度低，但由于裂缝开度较窄且比表面较高，边界层效应明显，流体流动时产生的黏滞阻力较高；沟槽裂缝曲面粗糙，隙宽分布复杂，流体流动时流态不稳定，曲折度高，在高流量下会产生明显的涡流流动，增大惯性阻力。裂缝通道存在缩颈现象会引起流体绕流和多次加减速，产生额外的压力损耗。随着模拟流量的增大，压降与流量会逐渐偏离线性关系而呈现出非达西流动现象。酸蚀裂缝壁面越粗糙，产生非达西效应时的临界流量和临界雷诺数就越小，在相同流量下非达西效应就越强烈，导流能力下降速率更快。

Numerical simulation of flow field of acid etched fractures considering non-Darcy effect

Acid etched fracture caused by acid fracturing is the main flow channel for fluid, thus, it is necessary to study the flowing rules of fluid in acid etched fracture. Four governing equations commonly used in fracture flow research and their applied conditions are introduced, among which Navier-Stokes equation is chosen as the governing equation for numerical simulation on acid etched fracture. In order to analyze the behavior of fluid flow through acid etched fractures and high-speed non-Darcy effect on fracture conductivity capacity, two kinds of fractures with different acid etching morphologies are reconstructed to solid models by using reverse engineering techniques, and flow experiments are carried out under different flow rates by using finite element numerical simulation. It indicates that, the acidized surface morphology and contact area of acid etched fracture have a great impact on the flow field distribution. The uniform etching fracture with smooth aperture distribution leads to stable flow pattern and low tortuosity, however, with the presence of obvious boundary layer effect caused by narrow fracture width and wide surface area, high viscous resistance will be generated while flowing. The channeling fracture with rough surface and complex aperture distribution results in unstable flow pattern and high tortuosity, and there is an obvious vortex under high flow rate, hence, increasing the inertial resistance loss. The necking phenomenon in contact area leads to circumfluence, and multiple acceleration-decelerations cause additional pressure loss. Meanwhile, with the increase in simulating flow rate, pressure drop and flow rate will gradually deviate from the linear relationship and present the non-Darcy flow phenomenon. The rougher the fracture surface is, the smaller the critical flow and critical Reynolds number are, the stronger the non-Darcy effect of fluid is under the same flow rate, the faster the conductivity capacity decreases.