鄂尔多斯盆地超低渗透油藏渗吸特征及其影响因素——以渭北油田三叠系延长组三段储层为例

鄂尔多斯盆地渭北油田属于超低渗透油藏，具有孔隙细小、孔喉结构复杂的特点。在前期的注水开发中存在明显的渗吸现象，对油田开发效果有一定的提高，但渗吸作用特征以及通过渗吸作用提高采收率的幅度认识不清。选取渭北油田三叠系延长组三段(长3)储层样品，通过物性测试、铸体薄片分析、扫描电镜、高压压汞和各类渗吸等实验，分析了研究区储层物性和孔喉结构特征，进行了不同介质类型以及不同含油饱和度下的渗吸实验。结果表明研究区储层溶蚀孔、粒间孔以及晶间孔三类孔隙比较发育，不同孔隙类型为主储层对应的孔喉结构特征差异较大，溶蚀孔、晶间孔为主的储层其孔喉结构逐渐变差。直接渗吸实验表明，裂缝型储层渗吸驱油效率均高于基质型储层，平均渗吸驱油效率分别为34.8%和23.2%；残余油下渗吸作用可以提高储层驱油效率，但幅度有限(5.3%~6.7%)。储层物性、介质类型、孔喉结构、渗吸时机等是影响渗吸驱油效率的主要因素，当储层物性越高、孔喉结构越好、含油饱和度相对较低时，储层渗吸作用相对较弱，渗吸驱油效率相对较低。 

Imbibition characteristics and influencing factors of reservoirs with ultra-low permeability of Ordos Basin: a case study of third member of Triassic Yanchang Formation in Weibei Oil Field

The Weibei Oil Field of the Ordos Basin has reservoirs of ultra-low permeability with small pores and complex pore throat structures. Imbibition is obvious at the early stage of water injection, which improved the effect of oilfield development. However, the characteristics of imbibition and the extent of EOR by imbibition are not clear. By collecting core samples from the third member of the Triassic Yanchang Formation in the Weibei Oil Field, physical property test, cast thin section observation, scanning electron microscope test, high pressure mercury injection and various imbibition experiments were carried out to study the reservoir physical properties and pore throat structure characteristics of the study samples, and to make imbibition experiments under different media types and different oil saturation. Three types of pores were discovered in the study area, namely, dissolution pore, intergranular pore and intergranular pore. The pore throat structure corresponding to different pore types are largely different. Pore throat structures of the reservoirs dominated by dissolution pores and intergranular pores gradually deteriorates. The oil displacement efficiency of fractured reservoir is higher than that of matrix reservoir with the values of 34.8% and 23.2%, respectively. The imbibition experiment under residual oil saturation shows that imbibition can improve the displacement efficiency of reservoir, but the increase is limited (5.3%-6.7%). The main factors affecting oil displacement efficiency are physical property, pore throat structure, medium type and oil saturation. When the reservoir physical property is higher, the pore throat structure is better and the oil saturation is relatively lower, the reservoir imbibition is relatively weak and the imbibition displacement efficiency is relatively low.