Effects of pore-throat structures on the fluid mobility in chang 7 tight sandstone reservoirs of longdong area, Ordos Basin

Abstract

Pore structure and fluid mobility are important factors affecting reservoir quality evaluation and effective hydrocarbon resource assessment in tight sandstone reservoirs. In this research, 14 tight sandstone samples from the Chang 7 member of the Upper Triassic Yanchang Formation in the Longdong area of the Ordos Basin were analyzed through high-pressure mercury injection (HPMI), low-temperature nitrogen adsorption (LTNA) and nuclear magnetic resonance (NMR) experiments to clarify the pore-throat structure characteristics of tight sandstone reservoirs and their influence on the occurrence characteristics and mobility of reservoir fluids. The results suggest that the different distributions and combinations of pore and throat types and sizes resulted in three pore-throat structures and four pore-throat spaces in the study area, corresponding to different occurrence characteristics of movable fluid. Among them, the type I pore-throat structure is characterized by large pore throats, high pore-throat connectivity and weak heterogeneity, with the highest corresponding average movable fluid saturation (MFS) and movable fluid porosity (MFP). The pore-throat size distribution (PSD) of type II pore-throat structure has bimodal characteristics with a low pore-throat sorting degree but a good pores and throats configuration relationship with the higher average MFS and MFP. The type III pore-throat structure has developed intercrystalline pores with a smaller average pore-throat radius, the highest fractal dimension and the lowest fluid mobility. Micropores and mesopores, as the main pore-throat spaces in the three types of pore-throat structures, are the main contributors to the petrophysical properties and the main occurrence place for movable fluids. Nanopores with strong heterogeneity and macropores with high permeability both occupy small pore-throat spaces and have little influence on reservoir fluid mobility. It is thus cleat that the pore-throat structure of tight sandstone reservoirs is the main factor affecting reservoir fluid mobility. More specifically, a pore-throat structure with a large pore-throat radius, good pore-throat connectivity and weak heterogeneity is favorable to the occurrence and seepage of movable fluids. Moreover, reservoir fluid mobility is comprehensively influenced by reservoir petrophysical properties and various clay minerals. Lower clay mineral contents and higher reservoir quality are conducive to the high flowability of tight sandstone reservoir fluids.