沉积储层相对渗透率的各向异性特征及其对渗流的影响

Anisotropic characteristics of relative permeability of sedimentary reservoirs and their influence on seepage

沉积过程和成岩过程通常会使储层具有各向异性特征,现有各向异性研究主要针对储层绝对渗透率,缺乏对相对渗透率各向异性的研究,无法反映各相之间干扰程度在不同方向上的差异。为研究各向异性结构对油水两相渗流的影响,本文设计了立体交错取心方法,以降低常规立方体岩心各向异性相渗实验中末端效应的影响,建立了各向异性相渗曲线的获取方法;在此基础上开展天然砂岩油水两相相对渗透率各向异性对比实验,证明了砂岩储层相对渗透率各向异性的存在;分析了各向异性相对渗透率的产生机理及张量表示;研究了相对渗透率各向异性对渗流过程的影响。结果表明,微观层状结构是造成沉积储层相对渗透率各向异性的主要原因;相对渗透率各向异性量化表征了不同方向上水相突进能力、水油流度比的差异;高渗方向的相渗曲线具有残余油饱和度较高、两相渗流区较窄、等渗点含水饱和度较小、等渗点相对渗透率较低的特征;相对渗透率各向异性会导致油水分向渗流,对油藏开发产生显著影响,主要表现为,随开发过程的进行,油水渗流方向会逐渐呈现出明显的差异性,相继出现单向死油区和平面死油区,导致剩余油分布更加复杂。

Sedimentary and diagenetic processes usually make reservoirs anisotropic.The existing anisotropy studies focus on the absolute permeability of reservoirs,but lack of studies on relative permeability anisotropy,which cannot reflect the difference of interference degree between fluid phases in different directions.In order to study the effect of an anisotropic structure on oil-water two-phase seepage,a three-dimensional staggered coring method was designed to reduce the influence of end effects in conventional cubic core anisotropy relative permeability experiments,and a method to obtain an anisotropic relative permeability curve was established.On this basis,an anisotropy experiment of oil-water relative permeability of natural sandstone is carried out,which proves the existence of anisotropy of relative permeability of sandstone reservoirs.The generation mechanism and tensor expression of anisotropic relative permeability are analyzed.The effect of relative permeability anisotropy on seepage processes is studied.The results show that bedding structure is the main cause of relative permeability anisotropy of sedimentary reservoirs.Relative permeability anisotropy quantifies the difference of water-phase coning ability and water-oil fluidity ratio in different directions.The relative permeability curve in the high permeability direction is characterized by high residual oil saturation,a narrow two-phase seepage zone,low water saturation and low relative permeability at the isotonic point.Relative permeability anisotropy will lead to oil and water seepage,which will have a significant impact on reservoir development.As the development process progresses,oil and water seepage directions will gradually show obvious differences,and one-way dead oil zones and plane dead oil zones appear successively,resulting in a more complex distribution of remaining oil.