EXPERIMENTAL STUDY ON PHYSICAL, ELECTRICAL AND FRACTAL CHARACTERISTICS OF TIGHT OIL SANDSTONE RESERVOIR

Physical properties, resistivity and pore structure parameters are indirect reflection of underground reservoir quality. In this paper, taking the Chang 6 tight oil sandstone reservoir of the Yanchang Formation in L area, southeastern Ordos Basin as an example, the physical, electrical and fractal characteristics of the tight oil reservoirs were systematically studied based on thin section, oil displacement, physical property, electrical property, mercury injection and relative permeability experiments. The results show that compaction is common in the Chang 6 Member, and the strong compaction results in linear and concave-convex contacts between particles. The high plastic particle content is also an important reason for the low porosity and low permeability of the target layer. It is found that feldspar dissolution pore is the main pore type in the target layer. The content of carbonate cements in the target layer is not high, but the metasomatism and filling of calcite minerals are common. Secondary dissolution occurs in cements, but the strength of dissolution is not large. The reservoir of the Chang 6 Member has strong heterogeneity. The water displacement tests show that the final oil displacement efficiency of the tight oil sandstones is 44.6%. When the water saturation is less than 45.5%, the reservoir produces pure oil. The water saturation calculation formula of tight oil sandstone reservoir is constructed based on the electrical experiments and the Archie formula. Finally, the fractal dimension of the samples was obtained based on the mercury injection experiments, and the fractal dimensions range from 2.2 to 2.8. It is found that the higher the quartz content, the smaller the fractal dimension. However, the higher the clay mineral content, the greater the fractal dimension. With the increase of fractal dimension, the pore throat structures in the rock tend to be complicated, and the drainage pressure of the reservoir increases, which is not conducive to oil and gas charging. Therefore, fractal dimensions can be used to indirectly evaluate the quality of tight oil sandstone reservoirs.