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

Abstract

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 (S wo ) decreased; furthermore, the slope of the curves of K rw /K rg 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 porethroat 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.

Mechanism Affecting the Pore Differentiation Characteristics of Fine-Grained Tight Sandstones: A Case Study of Permian Shanxi Formation in Ordos Basin

“…Ordos basin is the second largest petroliferous basin in China, located in the middle of Chinese mainland, and superimposed by multistage prototype basins (Figure a) . The present Ordos Basin is surrounded by mountains, and Yishan slope in the middle is the largest tectonic unit of the basin (Figure b).…”

“…Ordos basin is the second largest petroliferous basin in China, located in the middle of Chinese mainland, and superimposed by multistage prototype basins (Figure 1a). 27 The present Ordos Basin is surrounded by mountains, and Yishan slope in the middle is the largest tectonic unit of the basin 40 (Figure 1b). The area studied is located in Mizhi, east of Yishan slope, Ordos Basin (Figure 1c), and the targeted horizon is the Lower Permian Shanxi Formation.…”

“…At present, various methods have been selected to study the pore structure of fine-grained sandstone, and three of them are most frequently used . The first one is to manifest the morphological characteristics of pores based on observation and analysis of the microscopic images. − The second one is to calculate the numerical value of gas adsorption and fluid injection, such as a high-pressure mercury injection (HPMI) experiment , and nuclear magnetic resonance (NMR) experiment. − The third one is three-dimensional numerical reconstruction and simulation using ray detection, such as CT scanning, and small angle scattering experiments …”

Dynamic capillary pressure analysis of tight sandstone based on digital rock model

“…Recent years, many researches have been carried out on pore segmentation of tight sandstone. From the qualitative evaluation of pores develops into the quantitative evaluation of pore distribution at the micron-nanometer level (Loucks et al, 2009;Andrew et al, 2014;Liu et al, 2018;Zhu et al, 2018;Li et al, 2019;Qu et al, 2020;Shan et al, 2020;, from 2D analysis relying on scanning electron microscopy (SEM) develops into 3D feature analysis using focused ion beam scanning electron microscopy (FIB-SEM) (Curtis et al, 2012;Tartakovsky et al, 2015;Shaina et al, 2016;Alexandra et al, 2020;Chen et al, 2020;Choi et al, 2020;Goral et al, 2020). But most of these researches are focus on imaging technology, the essence of the pore segmentation method is still based on the gray-scale threshold without any significantly improvement (Kang et al, 2019;Shou et al, 2020).…”