Micro-scale pore-throat distributions in tight sandstone reservoirs and its constrain to movable fluid

To understand the constrains of multi-scale microscopic pore-throat distributions to the movable fluid in tight sandstone reservoirs, high-pressure mercury injection and nuclear magnetic resonance (NMR) were employed to obtain the distribution characteristics of micro-scale pore-throat distributions of Chang 6, Chang 7 and Chang 8 oil-bearing sections of the Yanchang Formation in the southeastern part of the Yishan slope, Ordos Basin. The occurrences feature of movable fluid in tight sandstone reservoirs were also investigated by centrifugal experiment and the T2 spectrum analysis of NMR. Results show that the microscopic pore-throat radius distribution in the tight sandstone reservoirs of Yanchang Formation has a wide distribution (ranging from 1. 13 to 3 050. 80 nm), mainly distributed from 10 to 500 nm, referring to micro- and nano-scale pore-throats and mainly were nano-scale. The movable fluid saturation in tight sandstone reservoirs ranged from 9. 83% to 25. 64% with an average value of 17.53%, indicating a low content in tight sandstone reservoirs. The porosity and permeability of studied reservoirs were positively correlated to the pore-throats having movable fluid, indicating the physical properties of reservoirs playing a role in controlling the distribution of movable fluid in tight sandstone reservoirs.The ratio of pore-throat which greater than 50 nm, and ratio of pore-throat greater than 100 nm, the maximum pore-throat radius, peak pore throat radius etc., showed a positive correlation with the porosity of movable fluid in tight sandstone reservoirs. It can be indicated that the distribution of relative larger pore-throasts, especially those larger than 100 nm, have strong controlling affection to the relative content of movable fluid in tight sand- stone reservoirs. The sorting coefficient of pore-throat is positively correlated with the content of flexible fluid due to the well-sorted tight sandstones with a wide pore-throat radius distribution in tight sandstone reservoirs are dominated by fine pore-throats.