Quantifying the Microscopic Distribution of Different Fluid Types in the Multiscale Pore-throat Structure of the Chang 7 Reservoir in the Longdong Area, Ordos Basin

Fluid type and content directly control fluid mobility in tight reservoirs. At present, there are two ways to classify fluid types. One is to classify fluids into movable fluids (MF), capillary-bound fluids (CAF), and clay-bound fluids (CLF). The other is to classify fluids into free fluids and adsorbed fluids. However, the intrinsic relationship between the two fluid classification schemes is still unclear. In order to investigate the pore structure and fluid type characteristics, a series of experiments were performed on the Chang 7 tight sandstone in the Longdong area. The full-scale pore size distribution (PSD) can be obtained by combining low-temperature nitrogen adsorption (LTNA) with nuclear magnetic resonance (NMR). The PSD of Chang 7 tight sandstones primarily ranges from 1 nm to 20 μm. Based on the fractal characteristics, pore system is divided into macropores (mainly >150.8 nm), and micropores (mainly <150.8 nm). MF, CLF, and CAF constitute 11.1-49.7% (avg. 34.6%), 17.3-40.1% (avg. 26.7%), and 32.9-48.8% (avg. 38.7%) of total fluid, respectively. Additionally, macropores are positively correlated with MF and negatively correlated with CAF and CLF, whereas micropores show the opposite trend. By integrating NMR data with theoretical modeling, a clear correspondence between the two fluid classification approaches was established: MF and CAF closely correspond to free fluids, while CLF is strongly associated with adsorbed fluids. Notably, in samples with low porosity and extremely fine pore throats, the combination of centrifugation and theoretical modeling may underestimate the actual free fluid content. MF shows a weak negative correlation with quartz and clay minerals; CAF and CLF are weakly positively correlated with quartz and clay but negatively correlated with feldspar. The occurrence patterns of different fluid types within various pore-throat structures were established, revealing the relationships among mineral composition, pore size, pore-throat structure, and fluid distribution. These findings provide valuable insights into the pore structure and fluid distribution of tight sandstone reservoirs, enhancing the understanding of fluid behavior in unconventional systems.