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.