致密砂岩储层孔隙结构及其对渗流的影响--以鄂尔多斯盆地马岭油田长8储层为例

摘要

致密砂岩油藏岩性致密、孔喉细小，贾敏效应及应力敏感性强，导致油气渗流规律不同于常规储层。为研究致密储层孔隙结构对渗流的影响，首先通过岩心观察、铸体薄片、扫描电镜及高压压汞等实验方法，研究了鄂尔多斯盆地马岭长8致密砂岩储层微观孔隙结构特征。结果表明，该储层平均面孔率较低，孔隙类型复杂，非均质性较强；渗透率小于1×10－3μm2的岩心纳米级与亚微米级孔喉占总孔喉的比例均较高（30％～55％），渗透率大于1×10－3μm2的岩心微米级孔喉占总孔喉的比例增大。应用毛细管渗流模型分析了不同尺度喉道对渗透率的贡献，指出研究储层中亚微米级孔喉对渗流起主导作用。通过岩心驱替实验发现，油相（ Swc ）最小启动压力梯度与岩心最大喉道半径之间呈幂函数负相关，最大喉道半径小于1．0μm时，油相（ Swc ）最小启动压力梯度随喉道半径的降低迅速增加；随岩心渗透率的降低，喉道分布曲线左移，喉道半径减小，对应岩心的流速－压差曲线非线性段增长。%The percolation of oil and gas in tight sandstone reservoirs is different from the conventional reservoirs because of its tight lithology,tiny pore and throat,Jamin effect and strong stress sensitivity.In order to understand the influences of pore structures on the percolation of oil and gas,the structural characteristics of micro pores of the Chang 8 reservoir were analyzed based on core and flake observation,SEM (scanning electron microscope) and mercury intrusion porosimetry. The results indicate that the reservoir has low surface porosity,complicated pore types and strong heterogeneity.The per-centage of nanopore throats and submicron pore throats in the total pore throats is high ( 30%~55%) in the cores with permeability lower than 1 ×10 -3μm2 ,while the percentage of micron pore throats in the total pore throats is higher in the cores with permeability higher than 1 ×10 -3μm2 .The contributions of the pores with different throat sizes to the permea-bility were analyzed with the capillary’ s model.The result shows that the submicron size pore throats play a major role in the permeability of reservoir in this study.The result of core displacement experiments shows that the minimum starting pressure gradient of oil(Swc)is negatively correlated with the largest pore sizes.The minimum starting pressure gradient of oil(Swc)increases rapidly along with the decreasing of pore throat size when the largest pore throat radius is less than 1.0μm.As the core permeability decreases,the throat distribution curve is shifted to the left,the pore throat radius lowers, and the nonlinearity section of the velocity-pressure gradient curve becomes longer.