Pore throat structure characteristics and its effect on flow behavior in Gaotaizi tight siltstone reservoir, Northern Songliao Basin

Abstract

The features of pore and throat structure of an unconventional reservoir are very complex due to multiscale pore-throat size and various pore types and thus hard to characterize. In this study, thin sections, field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), granulometry analysis, physical properties measurements, mercury intrusion capillary pressure (MICP) and nitrogen gas adsorption analysis (N2GA) were combined to investigate the pore-throat structures of the Gaotaizi tight siltstone reservoir in the northern Songliao Basin. The results of observation suggest that the pores are composed of intergranular pores, dissolution pores, intercrystalline pores within clay minerals and micro-fractures. The content of calcite cement has some negative effect on porosity. The content and types of clay minerals have some negative influence on petrophysical properties of reservoir as well. MICP and N2GA are combined to get the full-range of pore-throat size distribution. Based on previous studies and the actual situation of the Gaotaizi reservoir, a new classification for pore-throat size has been proposed, including micropore (<10 nm), transition pore (10 100 nm), mesopore (100 nm 0.625 μm) and macropore (>0.625 μm). On this basis, the contents of pore-throat in different scales have been calculated. The Gaotaizi tight siltstone reservoir has been divided into three types based on the pore-throat structure. There are big differences between the three types of reservoir with respect to the grains size, content of matrix, types of diagenetic minerals, the dominant pore types, pore-throat size distribution and the percentage of pore-throats in different scales. Accordingly, a pore structure model has been constructed for each type of reservoir. In addtion, it has been found that the pore-throat structure has an important effect on the percolation capacity of the tight siltstone reservoir. Porosity and pore-throat radius have a positive correlation with the permeability. Pore-throats in different scales have different impact on the flow characteristics of tight reservoir. The mesopores and macropores make great contribution to the flow in the reservoir while the micropores and transition pores do not contribute and even have an inhibitional effect. In general, the permeability or the flow characteristics of the tight siltstone reservoir is mostly controlled by the pore-throats larger than 0.1 μm.