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Pore‐Throat Combination Types and Gas‐Water Relative Permeability Responses of Tight Gas Sandstone Reservoirs in the Zizhou Area of East Ordos Basin, China

With the aim of better understanding the tight gas reservoirs in the Zizhou area of east Ordos Basin, a total of 222 samples were collected from 50 wells for a series of experiments. In this study, three pore‐throat combination types in sandstones were revealed and confirmed to play a controlling role in the distribution of throat size and the characteristics of gas‐water relative permeability. The type‐I sandstones are dominated by intercrystalline micropores connected by cluster throats, of which the distribution curves of throat size are narrow and have a strong single peak (peak ratio >30%). The pores in the type‐II sandstones dominantly consist of secondary dissolution pores and intercrystalline micropores, and throats mainly occur as slice‐shaped throats along cleavages between rigid grain margins and cluster throats in clay cement. The distribution curves of throat size for the type‐II sandstones show a bimodal distribution with a substantial low‐value region between the peaks (peak ratio <15%). Primary intergranular pores and secondary intergranular pores are mainly found in type‐III samples, which are connected by various throats. The throat size distribution curves of type‐III sandstones show a nearly normal distribution with low kurtosis (peak ratio <10%), and the micro‐scale throat radii (>0.5 (μm) constitute a large proportion. From type‐I to type‐III sandstones, the irreducible water saturation (Swo) decreased; furthermore, the slope of the curves of Krw/Krg in two‐phase saturation zone decreased and the two‐phase saturation zone increased, indicating that the gas relative flow ability increased. Variations of the permeability exist in sandstones with different pore‐throat combination types, which indicate the type‐III sandstones are better reservoirs, followed by type‐II sandstones and type‐I sandstones. As an important factor affecting the reservoir quality, the pore‐throat combination type in sandstones is the cumulative expression of lithology and diagenetic modifications with strong heterogeneity.