黏土矿物和孔喉特征对致密砂岩可动流体分布的影响——以鄂尔多斯盆地东部山西组储层为例

Effects of clay minerals and pore-throat characteristics on the movable fluid of tight sandstones:A case study of the Shanxi Formation in eastern Ordos Basin

开展致密砂岩可动流体分布的影响因素研究可有效提高储层开发潜力评价的准确性。选取鄂尔多斯盆地东部山西组典型致密砂岩样品,联合铸体薄片观察、扫描电镜观测、X射线衍射分析、恒速压汞及核磁共振测试,分析了黏土矿物及孔喉特征对可动流体分布的影响。根据致密砂岩的孔隙类型,将其划分为“粒间孔—溶孔—晶间孔型”、“溶孔—晶间孔型”及“晶间孔型”储层。砂岩样品的可动流体百分数介于9.39%~78.79%之间,平均为41.63%,且与渗透率呈中等正相关性。研究发现,黏土矿物的存在不利于流体的可动性,半径>200μm区间的孔隙与半径>1μm区间的喉道有利于流体的可动性。伊利石含量较高,呈搭桥式分割孔隙,对流体的可动性具有较强的抑制作用。“粒间孔—溶孔—晶间孔型”砂岩中半径>200μm区间的孔隙占比较高,同时半径>1μm区间的喉道占比大于50%,可动流体百分数较高。“溶孔—晶间孔型”砂岩的可动流体百分数变化较大,但随着溶孔的增多、晶间孔的减少,孔喉对流体的束缚性减弱,可动流体百分数增高。“晶间孔型”砂岩以半径<200μm区间的孔隙为主,同时半径<0.5μm区间的喉道占比大于50%,可动流体百分数极低。

The study of the factors affecting the movable fluid of tight sandstones can effectively improve the ac⁃curacy of reservoir development potential evaluation.The typical tight sandstone samples of Shanxi Formation in the eastern Ordos Basin were collected,and the effects of clay minerals and pore-throat characteristics on the movable fluid were analyzed by using multiple tests,including thin section petrography,scanning electron mi⁃croscopy,X-ray diffraction,rate-controlled porosimetry,and nuclear magnetic resonance.Based on the pore types,the tight sandstones were divided into three types as follows:(a)Intergranular pore-dissolution pore-in⁃tercrystalline pore type(type-I);(b)Dissolution pore-intercrystalline pore type(type-II);and(c)Intercrystal⁃line pore type(type-III).The contents of movable fluid saturation(MFS)of sandstones ranged from 9.39%to 78.79%,with an average of 41.63%,and had a moderate positive correlation with permeability.The study showed that the clay minerals were not conducive to movable fluid,and the pores larger than 200μm and throats larger than 1μm were conducive to fluid mobility.The high content of illite occurred as pore-bridging phases,which played a significant role in inhibiting fluid mobility.The type-I sandstones had high MFS values,of which the pores greater than 200μm occupied a relatively high proportion,and the throats greater than 1μm occupied more than 50%.The contents of MFS in type-II sandstones varied greatly.However,with the in⁃crease of dissolution pores and the decrease of intercrystalline pores,the binding on the fluid caused by the porethroats was weakened,resulting in the increase of the MFS values.The type-III sandstones were dominated by pores less than 200μm,and throats less than 0.5μm accounted for more than 50%.Thus,the contents of MFS in type-III sandstones were extremely low.