Characterization of the pore-throat size of tight oil reservoirs and its control on reservoir physical properties: A case study of the Triassic tight sandstone of the sediment gravity flow in the Ordos Basin, China

Abstract

Tight oil is a significant component of unconventional oil and gas resources. The pore structures of tight oil reservoirs determine the seepage law, which is a significant factor in evaluating reservoir quality. However, because of the strong heterogeneity of the pore structure in tight oil reservoirs, it is difficult to characterize the pore structure by conventional methods. Therefore, on the basis of conventional reservoirs research methods, combined with high-pressure mercury intrusion (HPMI) and constant-pressure mercury intrusion (CPMI), the pore-throat size distribution of tight sandstone reservoirs and its control on reservoir physical properties were studied. The main pore types in the study area were residual intergranular pores, secondary intragranular pores, intercrystalline micropores and microfractures, and throat types were sheet-like and tube throats. The pore-throat size distribution characterized by HPMI ranges from 0.0025 to 2.79 μm, and pore-throats larger than 10 μm occurred less frequently. The pore size distribution characterized by CPMI was between 90 and 350 μm, and the peak value distribution of the pore radius of the different samples was relatively concentrated, while the throat size distribution was between 0.12 and 6 μm, indicating that the throat radius of the different samples showed a strong heterogeneity, which also led to a strong heterogeneity of pore-throat radius ratio. Although HPMI and CPMI had their own limitations in characterizing the pore-throat size distribution, their combination could effectively characterize the whole pore-throat size distribution of tight reservoirs, which ranged from 0.0025 to 350 μm. The permeability of the tight sandstone reservoirs is primarily determined by the relatively wider pore-throats with small proportions, and the permeability decreased as the proportion of narrower pore-throats increased. The nanoscale pore throats (radius < 0.1 μm) were widely developed in the tight reservoirs, which had little influence on permeability, but these pore throats could be used as oil and gas storage spaces and played an important role in improving reservoir porosity.