Streamline simulation of counter-current imbibition in naturally fractured reservoirs

Describing fluid transport in naturally fractured reservoirs entails additional challenge because of the complicated physics arising from matrix–fracture interactions. In this paper, the streamline-based simulation is generalized to describe fluid transport in naturally fractured reservoirs through a dual-media approach. The fractures and matrix are treated as separate continua that are connected through a transfer function, as in conventional finite difference simulators for modeling fractured systems. The transfer functions that describe fluid exchange between the fracture and matrix system can be implemented easily within the framework of the current single-porosity streamline models. In particular, the streamline time of flight concept is utilized to develop a general dual-porosity, dual-permeability system of equations for water injection in naturally fractured reservoirs. The saturations equations are solved using an operator splitting approach that involves ‘convection’ along streamline followed ‘matrix–fracture’ exchange calculations on the grid. The proposed formulation reduces to the commonly used dual-porosity model when the flow in the matrix is considered negligible. For modeling matrix–fracture interactions, two different transfer functions are examined: an empirical transfer function (ETF) and a conventional transfer function (CTF). The ETF allows for analytical solution of the saturation equation for dual porosity systems and is used to validate numerical implementation. Results obtained using the CTF are compared with a commercial finite-difference simulator (ECLIPSE) for waterflooding in five-spot and nine-spot patterns. The streamline approach shows close agreement in terms of recovery histories and saturation profiles with a marked reduction in numerical dispersion and grid orientation effects. Finally, an examination of the scaling behavior of the computation time indicates that the streamline approach is likely to result in significant savings for large-scale field applications.     

Pressures acting in counter-current spontaneous imbibition

Four water/oil and four oil/air linear counter-current spontaneous imbibition experiments were performed on Berea sandstone cores with permeabilities ranging from 0.065 to 1.094 μm². The cores were initially 100% saturated with non-wetting phase and all faces except one end were sealed. The experiments showed a clear frontal displacement mechanism. Capillary pressure was the driving force of the imbibition process. As well as viscous drag in both phases between the imbibition front and the open face, there is a significant opposing capillary back pressure associated with production of non-wetting phase at the open face. The location of the imbibition front, the overall changes in core saturation, and the pressure in the nonwetting phase in the dead end space ahead of the imbibition front, were monitored during the course of imbibition. The dead end pressure was essentially constant after a short start-up period. The distance advanced by the imbibition front was proportional to the square root of time. Based on the assumption that the properties of Berea sandstone of different permeabilities can be scaled, the experimental data were matched by numerical simulation to predict the saturation and pressure profiles, the saturation and capillary pressure at the imbibition front, and the capillary back pressure at the open face. The ratio of the capillary back pressure to the estimated capillary pressure at the imbibition front ranged from approximately 1/3 to 2/5, for oil displacing air, to approximately 1/9 to 1/4, for water displacing oil.     

Generalized scaling of spontaneous imbibition data for strongly water-wet systems

Mass transfer between fractures and matrix blocks is critical to oil recovery by waterflooding in fractured reservoirs. A scaling equation has been used for rate of oil recovery by spontaneous imbibition and presented their results as oil recovery vs. dimensionless time. Many conditions apply to this scaling equation, including identical core sample shapes and fluid viscosity ratios. Recent investigation by experiment of these two factors has resulted in a more generalized scaling equation for strongly water-wet systems with a general definition of characteristic length and a viscosity ratio term included in the definition of dimensionless time. In this paper, published data on oil recovery by imbibition have been analyzed and correlated through application of the new definition. These data sets were for different porous media, core dimensions, boundary conditions, and oil and water viscosities. All of the systems were strongly water-wet. The generalized correlation was fitted closely by an empirical mass transfer function with the new definition of dimensionless time as the only parameter.     

无机盐对亲水砂岩化学渗吸的影响

利用国产露头砂岩岩心,研究了亲水条件下,无机盐对岩心采用表面活性剂溶液进行自发渗吸驱油的规律和采收率的影响。研究表明,无机盐对亲水砂岩的化学渗吸行为和最终渗吸采收率存在一定影响,具体表现为表面活性剂溶液中添加C2＋或SO4^2-后,随着2价离子浓度增加,采收率先小幅下降后快速增加,采收率与2价离子浓度呈类似“u”型变化特征,较高浓度的2价离子对化学渗吸具有一定促进作用。添加适量的Ca2＋或SO4^2-,可以促进岩心的化学渗吸,从而提高岩心渗吸采收率。     

低渗透裂缝性油藏自发渗吸渗流作用

裂缝和基质中流体渗吸作用是低渗透裂缝性油藏注水开采依据的重要机理。通过自发渗吸实验研究低渗透裂缝性油藏在不同渗透率级别下岩心的渗吸驱油机理,结果表明,该类油藏的油层渗吸体系因毛细管力较高,其渗吸过程为毛细管力支配下的逆向渗吸,毛细管力在吸渗过程中可作为驱油的动力;渗吸早期产油量高,约50 h后产油量明显降低,最后基本不产油。低渗透裂缝性油藏岩心自发渗吸采出程度平均为12%,其自发渗吸采出程度随渗透率的增大而增大,当渗透率大于2×10-3μm2时,自发渗吸采出程度的增加并不明显,孔隙结构越好越有利于自发渗吸作用发生。由于岩心和油藏储层尺寸存在差异,因此对实验结果进行等比例关系处理,使实验值可以用来预测实际油田开发指标。     

Similarity solution for linear counter-current spontaneous imbibition

Recovery of nonwetting phase (NWP) by counter-current spontaneous imbibition (COUCSI) of a wetting phase (WP) has been investigated. Experimental results cover a range of viscosity ratios of over seven orders of magnitude. An analytical solution for linear counter-current spontaneous imbibition based on similarity is presented. The mathematical model is based on: (1) Darcy's law for each phase, (2) the imbibition capillary pressures acting as the only driving force, (3) a back pressure given by the drainage interfaces associated with production of NWP at the open face, and (4) continuity of counter-current flow. An important consequence of the similarity solution is that the ratio of the WP flow rate at any location to the WP flow rate at the inlet of the sample is a function only of saturation.     

基于核磁共振测试的低渗亲水岩心静态渗吸特征

低渗透储层岩性致密、孔喉细小,导致毛管力引起的压裂液渗吸作用强,有助于提高原油采收率.基于室内静态渗吸实验,结合核磁共振仪定量刻画了低渗亲水岩心内的油水分布,并进一步分析了不同尺度孔隙渗吸速率和渗吸采出程度.结果表明:低渗亲水岩心渗吸首先发生在小孔隙中,其渗吸采出程度达到47.92％;大孔隙的渗吸采出程度仅为小孔隙的1...     

裂缝性低渗透油气藏压裂井压力动态特征

裂缝性低渗透油气藏往往存在明显的应力敏感性,利用椭圆流动模型和平均质量守恒方法,建立考虑介质变形影响的双重介质油气藏有限导流垂直裂缝井不稳定渗流的数学模型.给出数学模型的试探解,绘制并分析了压力动态特征的典型曲线.分析结果表明,介质变形对典型曲线的中晚期形态存在影响,由于渗透率应力敏感性的存在,使得压力导数曲线偏离0.5水平线.后期拟径向流特征段消失,并且渗透率模数越大,压力及压力导数曲线上翘越大.     

Stress sensitivity of fractured reservoirs

A fractured reservoir has a rock mechanical character which can be described by considering it to be built up from intact rock bounded by mechanical discontinuities. These discontinuities have been formed by natural processes and comprise fractures which normally make a steep angle to bedding, and the frequently ignored bedding-plane parallel discontinuities caused by, for example, weak clay-rich layers in clastics, or stylolites in carbonates. Both the intact rock and the discontinuities exhibit sensitivity to stress. In the case of the intact rock, this expresses itself as a pore geometry sensitivity which influences permeabilities and capillary pressures, with the changes being influenced by the stress-state. In the case of the discontinuities, three types of permeability changes are proposed, depending upon the stress and strain which develops across the discontinuity. Importantly, all the constitutive laws for permeability stress-sensitivity can now be incorporated in simulations. While allowing for these effects complicates the simulation of fractured reservoirs, the enhanced realism brought to the simulation should improve the efficacy of the reservoir management.     

低渗裂缝性油藏渗吸注水实验研究

由于裂缝性油藏储集空间物理性质的复杂性和特殊性,渗吸效果控制着低渗透裂缝性水驱开发动态与开发效果.通过室内实验研究,获得了低渗裂缝性油藏基质岩块自然渗吸动态规律和脉冲渗吸动态规律,为制定低渗裂缝性油藏合理的渗吸注水开发方式提供了理论依据.     

鄂尔多斯盆地裂缝性低渗透油藏渗吸驱油研究

裂缝性低渗透油藏储层岩性致密,裂缝发育,非均质性强,注水开发效果差,利用水的自发渗吸作用驱油是一种经济有效的开发手段。文中利用鄂尔多斯盆地延长油田西区采油厂的天然露头岩心,通过自发渗吸实验,研究了边界条件、润湿性、温度、原油黏度、界面张力及渗透率等因素对渗吸驱油作用的影响。实验结果表明:润湿性、黏度、界面张力及渗透率是影响渗吸驱油的主要因素,岩石越亲水,原油黏度越低,渗吸驱油效果越好。对于亲水岩心,渗透率相近时,界面张力为0.04 m N/m时渗吸效果最佳;岩石渗透率差异明显时,渗透率为2.94×10~(-3)μm~2时渗吸效果最佳。实验结果为鄂尔多斯盆地裂缝性低渗透油藏渗吸驱油提供了重要的指导作用。     

AVO技术在裂缝性储层中的应用

在油气勘探开发过程中,裂缝性油气藏所占的比例越来越大.裂缝性地层表现出很强的各向异性,勘探难度很大.而AVO技术从早先所认识到的一种现象已发展成一种完善的地球物理勘探技术,被广泛应用到油气勘探领域.当地层出现一定方位的裂缝时,地层的弹性参数会表现出方向性差异,这种差异能被AVO方法有效地检测.理论上讲,裂缝对地震属性参数的影响在垂直裂缝走向的方向上表现为最强,在水平裂缝走向的方向上表现为最弱,对AVO属性参数的影响也是如此.     

天然裂缝性油藏渗吸规律

为了研究渗吸作用对天然裂缝性油藏的影响,改善该类油藏的开发效果,以数值模拟为手段,通过建立概念地质模型,以静态和动态2种方式研究了毛细管压力、孔隙均匀程度、相渗曲线、基质和裂缝渗透率、原油黏度和基质含油饱和度对基质渗吸速度的影响,并在静态下对相关参数进行了公式拟合.静态下,渗吸速度与毛细管压力、基质渗透率、残余油饱和度、束缚水饱和度下的油相相对渗透率呈直线关系,与束缚水饱和度呈指数关系,与原油黏度呈幂指数关系;动态下,对于不同润湿性的油藏提出了不同的开发方式.     

裂缝性致密油藏多级压裂水平井试井模型

致密油藏由于渗透率低,表现出很强的非线性渗流特征,因此多采用水平井多级压裂技术提高单井产量.基于此,考虑了应力敏感和启动压力梯度的影响,根据物理模型建立裂缝性致密油藏多级压裂水平井试井模型,通过Laplace变换和有限余弦Fourier变换对数学模型进行求解,离散压裂裂缝并结合压降叠加原理,求得有限导流多级压裂水平井井...     

裂缝油藏AVO及AVA反演

地震波的反射幅度被用来探测油藏中的流体及裂缝特性。文章通过分析具有变化入射角及方位角的P波幅度来研究充满流体的垂直裂缝油藏特性。在考虑了P波线性反射系数的同时，该油气藏可以通过嵌在各向同性介质中的水平横向介质来模拟。裂缝强度可以作为最好的修正系数，用以对反演做出校正。在Bom近似条件下使用衍射层析技术时，建议使用k—ω变换来探测油藏的裂缝强度变化情况。将地震波数据按入射及反射平面波配对处理每次加亮一个裂缝强度域内光谱成分，仅是可见光谱成分反相处理。另外，变挟内部拾取反射波的幅度是不必要的。文章列出了一个应用于合成数据集的反演的例子。由于震源及接收器的数量及各波段谐波带宽的局限性会使分辨率有所损失。但总的来说，裂缝强度的变化可被正确的恢复出来。     

裂缝性油藏低渗透岩心自发渗吸实验研究

裂缝性低渗透油藏注水开发时,注入水沿裂缝窜流,油井含水率高,地下注水波及效果差,油层水淹后仍有大量原油滞留在基质岩块中.渗吸排油是裂缝性低渗透油藏重要的采油机理,为研究各种因素对渗吸效果的影响,采用胜利油区纯梁采油厂天然低渗透岩心,通过在地层水和表面活性剂溶液中的自发渗吸实验,研究了润湿性、温度、粘度、界面张力等因素对渗吸的影响规律.实验结果表明:温度不是影响渗吸的直接因素,而是通过改变模拟油的粘度来间接影响渗吸;润湿性、模拟油粘度以及界面张力是影响自发渗吸的主要因素,岩石越亲水,模拟油粘度越低,渗吸采收率越高;对于亲水岩心,渗透率和界面张力控制着渗吸发生的方式;不同渗透率级别对应一个最佳的界面张力范围,在该范围内,渗吸的采收率最高;对于亲水—弱亲水岩心,岩心渗透率越大,所对应的最佳界面张力越低.     

裂缝性油气藏采收率：100个裂缝性油气田实例的经验总结

通过对世界上100个裂缝性油气藏的综合评价，研究储集层及流体本身的性质(包括孔隙度、渗透率、黏度、可动油比例、含水饱和度、润湿性及裂缝分布特征等)和驱动机制及油藏管理战略(优化日产量和采用不同类型的提高采收率技术)对其最终采收率的影响。将裂缝性油气藏分为4类：I类的基质几乎没有孔隙度和渗透率，裂缝是储存空间和流体流动的通道；Ⅱ类的基质有较低的孔隙度和渗透率，基质提供储存空间，裂缝提供流动通道；Ⅲ类(微孔隙)的基质具有高孔隙度和低渗透率，基质提供储存空间，裂缝提供流动通道；Ⅳ类(大孔隙)的基质具有高的孔隙度和渗透率，基质提供储存空间和流动通道，裂缝仅增加渗透率。对26个Ⅱ类油气藏和20个Ⅲ类油气藏的开采历史的研究表明：Ⅱ类油气藏的采收率受水驱强度和最优日产量控制，日产量过高会很容易破坏Ⅱ类油气藏，一些Ⅱ类油气藏如果管理得当，采收率可以很高，不需要二次或三次采油；Ⅲ类油气藏的采收率主要受岩石和流体本身性质的影响，特别是基质渗透率、流体重度、润湿性以及裂缝强度等，不进行二次或三次采油不可能完全开采，往往需要采用一些提高采收率的专门技术。以往将Ⅱ类和Ⅲ类裂缝性油气藏归为一类，认识它们的区别将有助于选择更好的开发策略。     

辽河盆地裂缝性储层流体类型识别

裂缝性储层流体类型识别一直是国际测井界急待解决的难题.本次研究中,通过对流体类型影响因素的分析认为,不同流体类型与电阻率比值的高低、粘土含量的多少以及孔隙度的大小密切相关;提出了综合考虑上述3项因素的流体类型识别参数FTI,FTI接近于0为干层,FTI接近于1为油层,低产油层的FTI值介于油层和干层之间.此方法应用于辽河盆地大民屯凹陷变质岩和碳酸盐岩古潜山地层流体类型识别,符合率达到88%.