Pore throat characteristics of tight sandstone of Yanchang Formation in eastern Gansu, Ordos Basin

Abstract

An important factor to evaluate reservoir quality is the pore-throat size. However, the strong heterogeneity makes it difficult to characterize the pore-throat distribution in tight reservoirs. The field emission scanning electron microscope (FESEM), high pressure mercury injection and rate-controlled mercury injection are used to investigate the pore-throat size distribution in tight sandstone reservoirs of Member 7 of the Yanchang Formation in eastern Gansu, Ordos Basin, and studies of the pore throat size controlling on physical property of the tight sandstone reservoirs are also carried out. The result shows that the pore type is mainly dominated by the residual intergranular pore, dissolution pore, micropore and a few micro-fractures; the high-pressure mercury injection experiment indicates that the pore-throat size ranges from 0.0148 μm to 40 μm, the pore throat more than 1 μm is less; the rate-controlled mercury injection experiment reveals that for samples with different physical properties, the pore radius mainly varies from 80 μm to 350 μm; the throat radius exhibits the strong heterogeneity, and is from 0.12 μm to 30 μm; the pore-throat size can be effectively characterized by combination of high-pressure and rate-controlled mercury injections, and it varies from 0.0148 μm to 350 μm. The permeability is mainly controlled by the large pore throat (>R50) which accounts for a small proportion; in the tight sandstone with the permeability greater than 0.1 mD, the permeability is mainly controlled by the micropore and mesopore; in the tight sandstone with the permeability smaller than 0.1 mD, the permeability is mainly controlled by the nanopore and micropore; the proportion of small pore throat increases with reduction of permeability, it is important that the small pore throat influences the reservoir storage property though its effect on permeability are small. Keywords: Pore-throat size, High-pressure and rate-controlled mercury injections, Reservoir physical property, Tight sandstone, Member 7 of the Yanchang Formation, Eastern Gansu