基于水平集方法的低渗砂岩数字岩心气水两相渗流模拟

低渗致密砂岩气藏孔喉结构复杂,气水流动及分布规律表征难,常规岩心实验精细刻画困难。基于真实低渗砂岩岩心,利用Micro-CT扫描技术构建储层三维数字岩心,提取连通孔隙结构并建立非结构化四面体网格模型。结合水平集法与N-S方程,建立气水两相流数学模型并利用有限元方法进行求解,研究低渗砂岩气水两相流动中的水驱气过程、残余气分布特征、岩石润湿性对两相流的影响以及并联通道中的窜流特征。结果表明：利用水平集方法能清晰地观察到气水两相分布特征与驱替前缘的运移规律;岩石润湿性对两相流过程影响较大,水湿条件下采出程度更高;并联通道中的窜流现象明显,大通道中水相优先突破并形成优势流动通道,狭小通道中的流动受毛管现象影响,存在较大的附加阻力。     

基于VOF方法的水驱油藏孔隙尺度剩余油分布状态研究

准确认识多孔介质内水驱剩余油的微观赋存状态对于改善高含水油田水驱开发效果,提高水驱采收率具有重要意义。在对Volume of Fluid (VOF)方法验证的基础上,充分利用其追踪两相界面动态变化、再现微观渗流物理过程的优势,开展特高含水期砂岩油藏物性条件、驱替方式对剩余油微观赋存特征和采收率的影响研究。通过分析典型孔隙结构的微观渗流特征和剩余油受力情况,揭示不同类型微观剩余油的动用机制和规律：水湿条件下驱替速度的增大和驱替方向的改变会使得微观剩余油分布较为分散且采收率得到不同程度的提高;油湿及高黏度比条件下毛管阻力、黏滞力较大,剩余油多以簇状和多孔状聚集分布,采出程度相对较低。     

水驱油过程的核磁共振二维谱研究

岩心水驱油过程中油水分布状况是岩心多孔介质的重要性质. 水驱油过程的研究是进一步进行提高采收率研究的基础. 核磁共振扩散-弛豫二维谱提供了岩心中流体性质的多方面信息,与核磁共振一维弛豫谱相比极大地提高了区分油水的能力. 该文通过2组岩心水驱油实验,从不同含油饱和度的扩散-弛豫二维谱中提取出水的一维弛豫谱,在原油粘度比较高的情况下获得了驱替过程中油水在不同孔隙中的分布状况以及润湿性等信息, 解决了单独用一维弛豫谱方法难以区分油水的问题. 该文的研究方法对油田提高采收率的研究有比较大的参考价值.     

应用核磁共振进行聚驱后泡沫驱油特性研究

采用并联岩心进行聚驱后泡沫驱油实验,利用核磁共振技术,对驱替后岩心的不同直径孔隙内的流体分布进行了研究,得到了水驱、聚合物驱、泡沫驱替阶段的驱出油的孔径范围以及剩余油分布.实验结果表明,与水驱和聚合物驱相比,泡沫驱增大了波及的孔径的范围.泡沫可以在水驱和聚驱易发生窜流的大孔径通道形成封堵,从而波及到了水驱和聚驱未波及到的孔径,不但大幅度的提高了低渗岩心的采收率,也驱出了部分高渗岩心的小孔径的油.     

混合润湿对多孔介质内驱替压差和采收率的影响

特低渗透油藏具有注水井压力高,注水效果差,采收率低等特点,调控油藏润湿性是该类油藏降压增注一种有效手段,本文基于格子Boltzmann方法,研究驱替过程中混合润湿性对驱替压差和采收率的影响规律.结果表明,在均匀润湿条件下,接触角越小,驱替压差越低;毛管数越大,被驱替相与驱替相黏度比越小,采收率越高;在混合润湿条件下,接触角越小,驱替压差降低,但是降低接触角不一定提高采收率,平均接触角和混合润湿结构对提高采收率均具有重要影响。     

多相非牛顿流体驱替过程的格子Boltzmann模拟

将非牛顿流体模型引入多相格子Boltzmann方法（LBM）,研究多相非牛顿流体的驱替过程.首先以牛顿流体驱替过程作对照,比较非牛顿流体的驱替特性.然后基于简单结构模型,分析不同界面张力下,非牛顿流体的驱替能力和驱替过程中被驱替相的形态变化规律,揭示界面张力的影响机理.结果表明：界面张力在驱替过程中起阻碍作用,其它参数相同情况下,界面张力越低,流体的驱替能力越强.最后,结合四参数随机生长（QSGS）结构生成方法,模拟不同压差、不同界面张力以及不同润湿性条件下多孔复杂结构内的非牛顿多相驱替过程,分析这些参数对过程的影响.     

低渗透介质启动压力梯度的介观机理研究

对于具有孔隙结构和微尺度效应的低渗透储层,其渗流现象的介观机理逐渐受到人们的重视。本文采用微流控技术在硅片上加工出微通道作为微观模型,模拟岩心内部的孔隙结构,对微通道中单相和油水两相的渗流规律进行了实验研究。研究结果表明,微观模型中带有喉道段的微通道存在启动压力梯度,这揭示了孔隙中的喉道是低渗透储层中产生启动压力梯度的主要介观因素。启动压力与孔隙结构和驱替介质的性质有关,在此基础上建立了适用于单相和油水两相渗流的启动压力预测模型。     

单一微孔隙中油水相对渗透率特性研究

孔隙渗流是多孔介质渗流的微观基础。本文对孔隙中的两相流应用分相模型,结合达西渗流定律,提出了适用于油水两相的相对渗透率模型,得到相对渗透率与孔隙度无关、相对渗透率只是微孔隙中相态分布的几何尺度以及流体饱和度的函数等结论。设计了微流动实验装置,以去离子水和白油的乳化液为两相流体,在不同含油率下研究了微孔隙中油水两相的渗流特性,并与相应条件下的模型计算结果进行对比。结果表明,对于低渗透尺度等级的微孔隙,实验与计算结果吻合较好,对于高渗透尺度等级的微孔隙,实验与计算结果存在偏差。在某些条件下,油与水的相对渗透率之和存在大于1的情形。     

微孔道聚合物溶液驱替残余油变形研究

为了进一步研究聚合物溶液驱替岩心微孔道中残余油的流动规律,探索驱替液弹性对残余油变形的影响规律,进一步研究黏弹性驱替液对岩心微孔道中残余油的驱替机理,运用非牛顿流体力学理论建立流动方程,采用数值计算方法求解残余油的变形。计算结果表明:随着We数的增大,残余油的前进角逐渐增大,后退角逐渐减小,变形指数逐渐增大,起始阶段变形指数按照线性规律增加,后成抛物线型增加。较高的浓度或分子量的聚合物溶液都会显著地增加其弹性,使得残余油产生显著的变形,更有利于残余油的运移或剥离,从而验证了数值计算所得的结果——弹性可增大残余油的变形,提高驱油效率,进一步提高原油采收率,这就是聚驱提高原油采收率的水动力学机理。     

规则多边形孔隙毛细管压力理论模型

真实油气储层中的孔隙形状极为复杂,Yang-Laplace方程无法模拟多边形孔隙的毛细管行为。为了研究润湿性及孔隙几何形状对规则多边形孔隙毛细管行为的影响,基于MS-P理论及多边形孔隙两相驱替法则,在均匀润湿状态下推导出等边三角形、正方形、五角星形孔隙的排驱与自吸毛细管压力理论模型,在相同条件下,对不同润湿性及形状因子孔隙的毛细管行为进行分析。结果表明:相同润湿性的排驱与自吸渗流特征在孔隙角隅处相同,而在中央存在明显差异;随着接触角的增加,油相排驱阻力与水相自吸动力减小,油相自吸动力与水相强制自吸阻力增大;在特定润湿状态下,非润湿相在突破压力下完全饱和孔隙;均匀水湿体系的Snap-off程度随着接触角的增加而减小,而均匀油湿体系相反;形状因子对油相排驱过程的突破压力无明显影响;随着形状因子的增加,水相与油相自吸动力、水相强制自吸阻力减小,且非润湿相进入孔隙角隅时的非润湿相饱和度增大;五角星形孔隙的Snap-off现象最为明显。     

Displacement front behavior of near miscible CO2 flooding in decane saturated synthetic sandstone cores revealed by magnetic resonance imaging

It is of great importance to study the CO₂-oil two-phase flow characteristic and displacement front behavior in porous media, for understanding the mechanisms of CO₂ enhanced oil recovery. In this work, we carried out near miscible CO₂ flooding experiments in decane saturated synthetic sandstone cores to investigate the displacement front characteristic by using magnetic resonance imaging technique. Experiments were done in three consolidated sandstone cores with the permeabilities ranging from 80 to 450 mD. The oil saturation maps and the overall oil saturation during CO₂ injections were obtained from the intensity of magnetic resonance imaging. Finally the parameters of the piston-like displacement fronts, including the front velocity and the front geometry factor (the length to width ratio) were analyzed. Experimental results showed that the near miscible vertical upward displacement is instable above the minimum miscible pressure in the synthetic sandstone cores. However, low permeability can restrain the instability to some extent.     

Laboratory observations of altered porous fluid flow behavior in Berea sandstone induced by low-frequency dynamic stress stimulation

It has been observed repeatedly that low-frequency (1–500 Hz) seismic stress waves can enhance oil production from depleted reservoirs and contaminant extraction from groundwater aquifers. The physics coupling stress waves to fluid flow behavior in porous media is not understood, although numerous physical mechanisms have been proposed to explain the observations. To quantify the effects of low-frequency, dynamic-stress stimulation on multiphase fluid flow and in situ particle behavior in porous media, laboratory experiments were conducted with a core flow stimulation apparatus that allows for precise control and measurement of applied stress and strain, static confinement, and fluid flow parameters. Results are reported for experiments on stimulated single-phase and two-phase fluid flow behavior in 2.54-cm-diameter Berea sandstone cores. For all experiments, stimulation was applied to the cores in the form of sinusoidal, axial, mechanical stress coupled to the solid porous matrix at frequencies of 25 to 75 Hz. Applied stress RMS amplitudes ranged from 300 to 1200 kPa and, at these levels, produced coupled, pore-pressure fluctuations of much less than 1.2 to 4.8 kPa, respectively. During single-phase brine flow, stimulation increased the absolute permeability of the rock by 10–20%. This was caused by mobilizing in situ clay particles that were partially plugging the pore throats. During two-phase, steady-state, constant-rate flow of oil-brine and decane-brine mixtures, stimulation caused significant changes in the bulk fluid pressure drop across the core. The pressure changes showed a strong dependence on the viscosity of the nonwetting fluid phase (oil or decane) relative to the wetting phase (brine). This may indicate that relative changes in the mobility of wetting versus nonwetting fluid phases were induced by the dynamic stress. Under the specific experimental conditions used, pore-scale particle perturbation and altered wettability are possible physical mechanisms that can explain the results.     

Behavior of CO2/water flow in porous media for CO2 geological storage

A clear understanding of two-phase fluid flow properties in porous media is of importance to CO₂ geological storage. The study visually measured the immiscible and miscible displacement of water by CO₂ using MRI (magnetic resonance imaging), and investigated the factor influencing the displacement process in porous media which were filled with quartz glass beads. For immiscible displacement at slow flow rates, the MR signal intensity of images increased because of CO₂ dissolution; before the dissolution phenomenon became inconspicuous at flow rate of 0.8 mL min^− 1. For miscible displacement, the MR signal intensity decreased gradually independent of flow rates, because supercritical CO₂ and water became miscible in the beginning of CO₂ injection. CO₂ channeling or fingering phenomena were more obviously observed with lower permeable porous media. Capillary force decreases with increasing particle size, which would increase permeability and allow CO₂ and water to invade into small pore spaces more easily. The study also showed CO₂ flow patterns were dominated by dimensionless capillary number, changing from capillary finger to stable flow. The relative permeability curve was calculated using Brooks-Corey model, while the results showed the relative permeability of CO₂ slightly decreases with the increase of capillary number.     

多孔介质中卡森流体的分形分析

研究了非牛顿流体中的卡森流体在多孔介质中的流动特性.基于服从分形分布的弯曲毛细管束模型,运用分形几何理论推导出了该流体在多孔介质中流动的流量、流速、启动压力梯度和有效渗透率的分形解析解.模型中的每一个参数都有明确的物理意义,它将卡森流体在多孔介质中的流动特性与多孔介质的微结构参数有机联系起来.文中给出了卡森流体的流速、启动压力梯度和有效渗透率随着各影响因素的变化趋势,并进行了讨论.所得分形模型可以更深刻地理解卡森流体在多孔介质中流动的内在物理机理. 关键词： 多孔介质 卡森流体 分形     

流体/准饱和多孔介质中伪Scholte波的传播特性

研究流体/多孔介质界面Scholte波的传播特性对于水下勘探、地震工程等领域具有重要意义.本文基于Biot理论和等效流体模型,采用势函数方法,推导了描述有限厚度流体/准饱和多孔半空间远场界面波的特征方程和位移、孔压计算公式.在此基础上,分别以砂岩和松散沉积土为例,研究了流体/硬多孔介质和流体/软多孔介质两种情况下,可压缩流体层厚度和多孔介质饱和度对伪Scholte波传播特性的影响.结果表明：多孔介质软硬程度显著影响界面波的种类、相速度、位移和水压力分布;有限厚度流体/饱和多孔半空间界面处伪Scholte波相速度与界面波波长和流体厚度的比值有关;孔隙水中溶解的少量气体对剪切波的相速度的影响不大,对压缩波相速度、伪Scholte波相速度和孔隙水压力分布影响显著.     

Quantitative evaluation of porous media wettability using NMR relaxometry

We propose a new method to determine wettability indices from NMR relaxometry. The new method uses the sensitivity of low field NMR relaxometry to the fluid distribution in oil-water saturated porous media. The model is based on the existence of a surface relaxivity for both oil and water, allowing the determination of the amount of surface wetted either by oil or by water. The proposed NMR wettability index requires the measurement of relaxation time distribution at four different saturation states. At the irreducible water saturation, we determine the dominant relaxation time of oil in the presence of a small amount of water, and at the oil residual saturation, we determine the dominant relaxation time of water in the presence of a small amount of oil. At 100% water and 100% oil saturation, we determine the surface relaxivity ratio. The interaction of oil with the surface is also evidenced by the comparison of the spin-lattice (T1) and spin-locking (T1rho) relaxation times. The new NMR index agrees with standard wettability measurements based on drainage-imbibition capillary pressure curves (USBM test) in the range [-0.3-1].