Abstract:

The different micro pore throat structure characteristics of tight reservoirs lead to different permeability and corresponding development strategies. Taking the Chang 6 reservoir in the Jiyuan area of the Ordos basin as an example, this study comprehensively analyzes the pore throat structure, connectivity, and fluid mobility of the tight reservoir using analysis and testing methods such as casting thin sections, scanning electron microscopy, constant velocity mercury injection, nuclear magnetic resonance, and computed tomography (CT). The study also uses core multiphase permeability experiments and real core visualization microscopic permeability experiments to study the permeability characteristics of different types of pore throat structures. Research has shown that the average porosity of the Chang 6 reservoir in the Jiyuan area is 12.1%, with an average permeability of 0.48×10-3μm2. The reservoir pore types are mainly intergranular pores, followed by feldspar dissolution pores, and are accompanied by a small amount of intercrystal pores and rock debris dissolution pores. According to the physical properties and pore throat structure characteristics of the reservoir, the pore throat structure types of the Chang 6 reservoir can be divided into three categories. Among them, Type Ⅰ is mainly composed of intergranular pores, supplemented by feldspar dissolution pores, with well-developed pores and good connectivity between throats. The movable space is developed, and the microseepage is mainly uniform. Type Ⅱ reservoirs are still dominated by intergranular pores, followed by feldspar dissolution pores. However, the proportion of intergranular pores in Type Ⅱ reservoirs decreases, while the proportion of dissolution pores increases, and the pore throat radius decreases compared to Type Ⅰ reservoirs. The connected pore structures are sparsely clustered and connected, with more developed movable spaces. Microseepage is mainly network-like. The pore types of Type Ⅲ reservoirs are mainly feldspar dissolution pores, followed by intergranular pores. The proportion of intercrystal pores increases, and the throat connectivity between effective pore reservoirs is poor. The three-dimensional pore structure space shows strong heterogeneity, and the movable space is not developed. Microseepage is mainly finger-shaped. The comprehensive analysis between macroseepage methods and microscopic pore throat structures shows that reservoir physical properties, pore throat combination methods, and degree of throat development determine different types of seepage methods. Reservoirs with good physical properties and well-developed throats have better water flooding effects, thereby improving the final recovery efficiency.

Key words: tight reservoir, pore throat structure type, seepage characteristics, Jiyuan area, Ordos basin