Study on the Pore Structure and Fractal Dimension of Tight Sandstone in Coal Measures

Abstract

The pore structure directly controls the tight reservoir’s physical properties and plays a crucial role in gas charging and accumulation. To characterize the pore structure accurately, it is critical for the exploration and development of tight gas. In this paper, the pore structure of a tight sandstone reservoir in the Carboniferous Benxi Formation coal-bearing strata in the Ordos Basin was characterized by X-ray diffraction, thin sections, scanning electron microscopy, high-pressure mercury intrusion, and X-ray–computed tomography techniques. The porosity and permeability of the Benxi Formation reservoir are 0.12–12.53% and 0.0003–33.59 mD, respectively. The type of pores is dominated by secondary pores, followed by primary pores and microcracks. The reservoirs are divided into three types based on the high-pressure mercury injection curve pattern and the displacement pressure. The average pore radii of Type I and Type II reservoirs are similar, and the pore volume, average radius of throats, and pore–throat connectivity ratio of Type II reservoirs are all higher than those of Type I from the results displayed by micron CT. The pores of the reservoir are divided into small pores (<1 μm), mesopores (0.1–1 μm), and macropores (>1 μm) according to their diameters. In type I reservoirs, three types of pores are found, and the type II reservoirs have mainly mesopores and macropores. There are almost no large pores in Type III reservoirs. Type I has the best porosity and the smallest heterogeneity, while Type II has stronger heterogeneity than Type I, according to the findings when combining the high-pressure mercury intrusion experiment with fractal theory. Total fractal dimension has a weak negative correlation with porosity and a positive correlation with permeability. The contribution rate of mesopores and macropores to porosity is more affected by heterogeneity. The seepage of a reservoir is mainly affected by the throat radius and throat connectivity ratio under the same porosity condition. With a larger throat radius and a higher throat connection ratio, the seepage capacity will be stronger. As the content of clay minerals increases, the heterogeneity of the reservoir increases and permeability decreases.

Review on Applications of X-ray Computed Tomography for Coal Characterization: Recent Progress and Perspectives

“…Its establishment not only solves the problem of excessive computation during the simulation, but also enables the statistics of parameters such as throat length and coordination number, which are important for studying the connectivity of coal. The pore space and pore throat can be extracted separately in this model, which can describe the distribution of pore space and pore throat more clearly . Meanwhile, the maximum sphere algorithm is used to calculate the coordination number of pore network and the pore throat distribution of coal sample, , which is the fundamental for coal permeability calculation.…”

“…The pore space and pore throat can be extracted separately in this model, which can describe the distribution of pore space and pore throat more clearly. 60 Meanwhile, the maximum sphere algorithm is used to calculate the coordination number of pore network and the pore throat distribution of coal sample, 61,62 which is the fundamental for coal permeability calculation.…”

Virtual Special Issue of Recent Research Advances in China: Unconventional Gas

“…A review by Lu et al presents a summary and perspective on efficient exploitation of CBM in China. The original research articles on this topic cover several aspects related to CBM, including gas adsorption, methane production, , numerical modeling and simulation, , pore-scale measurements, , characterization studies, , etc.…”

“…A review by Lu et al 62 presents a summary and perspective on efficient exploitation of CBM in China. The original research articles on this topic cover several aspects related to CBM, including gas adsorption, 63 methane production, 68,71 numerical modeling and simulation, 64,66 porescale measurements, 65,69 characterization studies, 67,70 etc. This VSI is part of the series of VSIs on recent advances in selected energy research areas, launched by Energy & Fuels in January 2021.…”

Study on the Pore and Crack Change Characteristics of Bituminous Coal and Anthracite after Different Temperature Gradient Baking

“…The fractal theory is an effective method to analyze the structure and geometric characteristics of materials. − Ni et al studied the pore structure of coal samples treated with nitric acid by SEM and LTNA and used fractal dimensions to quantitatively analyze the pore structure of coal samples, revealing the fractal dimension and SSA, total pore volume, and nitrogen adsorption with positive and negative correlations . Wang et al used the fractal theory to study the pore differences between raw coal and structural coal and found that D 1 increases with the decrease of the particle size.…”