Investigation on the Structure and Fractal Characteristics of Nanopores in High-Rank Coal: Implications for the Methane Adsorption Capacity

Abstract

The structure and fractal characteristics of nanopores of high-rank coal were investigated using an approach that integrates N2 adsorption and field emission scanning electron microscopy (FE-SEM). The results indicated that the high-rank coal of the Shanxi Formation has a complex pore-fracture network composed of organic matter pores, mineral-related pores, and microfractures. The pore type of high-rank coal tends to be complicated, and the main pore types are inkbottle pores and open pores, which are more conducive to methane enrichment. The Ro,max has a negative relationship with the total pore volume. In addition, the ash and inertinite contents show a positive correlation with the average pore size (APS), while the fixed carbon content exhibits a negative relationship with the APS. The pore structure of high-rank coal is controlled not only by the degree of metamorphism but also by coal composition, which leads to the variation in pore structure becoming more complicated. With the increase in coal metamorphism, high-rank coal with high amounts of fixed carbon content generally possesses a higher irregularity in pore structure. No obvious relationship was observed between D2 and the coal components, which indicates that the pore structure, ash content, moisture content and other factors controlled by the metamorphism of coal have different effects on D2 that lead to this inapparent relationship. A negative relationship exists between adsorption volume and D1, which indicates that the high irregularity of the pore structure is not conducive to methane absorption and that no obvious correlation exists between the adsorption volume and D2. In the high-rank coal, the high D1 value represents the complexity and heterogeneity of the pore structure and represents a low adsorption affinity for methane molecules; in addition, D2 has no effect on the methane adsorption capacity.

Correlation Evaluation and Schematic Analysis of Influencing Factors Affecting Pore and Fracture Connectivity on the Microscale and Their Application Discussion in Coal Reservoir Based on X-ray CT Data

“…The characterization methods of pore/fracture structures are summarized as follows: (1) pore size distribution: CO 2 /N 2 adsorption method and mercury injection method; , (2) 2D morphology observation: optical microscopy and scanning electron microscopy; , (3) 3D imaging analysis: X-ray computed tomography (i.e., CT), focused ion beam scanning electron microscopy (i.e., FIB-SEM). , As a nondestructive and high resolution scanning technology, 3D imaging scanning is the most direct and accurate method to reconstruct the pore/fracture structure, which can realize the extraction and reconstruction of interconnected pores/fractures and the analysis of characteristic parameters of geometric and topological structures. , FIB-SEM can only analyze the nanoscale structures; the testing cost is high, and the testing period is long. Therefore, X-ray CT technology can be used to quantitatively analyze the pore/fracture structures and its characteristic parameters of the coal reservoir.…”

Insights into the Nanoscale Microstructure Diversity of Different Rank Coals

“…Yu et al revealed the structure and fractal characteristics of micropores and mesopores in low- and middle-rank tectonic-deformed coals by combining the absorption experiment and fractal theory. Subsequently, several scholars have also demonstrated that the fractal dimension affects porosity, gas adsorption capacity, and permeability. , …”

“…Subsequently, several scholars have also demonstrated that the fractal dimension affects porosity, gas adsorption capacity, and permeability. 26,27 To sum up, great progress has been achieved in the area of coal pore characterization. 16,27,28 However, the relatively high micropore content of coal is the main place of gas occurrence, and studies applying the fractal theory to microporosity have rarely been reported.…”

“…26,27 To sum up, great progress has been achieved in the area of coal pore characterization. 16,27,28 However, the relatively high micropore content of coal is the main place of gas occurrence, and studies applying the fractal theory to microporosity have rarely been reported. Furthermore, a majority of the test coal samples were taken from the same mine without considering the comprehensive impact of the geological process and coalification.…”