Effect of different pore throat combinations on fluids flow of tight reservoirs in the Ordos Basin, China

Abstract

Clarifying the size, morphology, connectivity of pores and throat, and pore throat structure is crucial for studying the CO2 storage efficiency in tight sandstone reservoirs. In this study, a series of experiments were carried out on tight sandstone samples from the Yanchang Formation of the Ordos Basin, China to study pore throat structure, including cast thin section analysis, scanning electron microscopy, pressure mercury intrusion and nuclear magnetic resonance test. The results show that the major reservoir space is typically formed by the combination of intergranular pores and dissolution pores. Pore and throat combination types mainly include large pore-narrow throat, conduit-like pore throat combination, and tree-like network combination. The pore-throat fractal curve has one segment, while the throat has two segments. The fractal dimensions of narrow throat (Dt1) is larger than that the fractal dimensions of wide throats (Dt2), indicating that small throat has higher self-similarity and more homogeneous distribution. The large pore-narrow throat type and conduit-like pore throat combination type contribute the major permeability, the tree-like network combination type has good connectivity of pore throats and contributes the major storage space. The wide throats connecting the intergranular pores constrain the flow of fluids in the tight sandstone. The study of the fluid movability based on bound water modeling shows that the higher proportion of the conduit-like pore throat combination type, the more homogeneous the pore throat structure is, bound water is redistributed on the pore-throat surface to form a thin water film after centrifugation, leading to a higher proportion of movable fluid, and the overall seepage capacity was stronger. A large number of laumontite dissolved pores as well as intercrystalline clay pores exit from the tree-like network combination type, and the strong capillary pressure causes a low proportion of moveable fluid.