Characterizing the Complexity Assembly of Pore Structure in a Coal Matrix: Principle, Methodology, and Modeling Application

Abstract

The pore structure in a coal matrix is a dual-porosity system where fractures and pores coexist and feature scale-invariance properties, which would affect the occurrence and migration of coalbed methane (CBM) significantly. Therefore, it is of fundamental importance to well define complexity types and effectively characterize their assembly mechanism of pore structure in a coal matrix. Here we identify the pore structure in a coal matrix as a dual-complexity system consisting of original complexity and behavioral complexity independent to each other, where the former determines the scaling types of single-or dual-porosity structure, while the latter dominates the scale-invariance properties of self-similarity, self-affinity, and multifractality. Next we clarify the essentials of scale-invariance properties and unify the definition of behavioral complexity. By employing Voronoi diagrams, we develop a dual-porosity coupling algorithm to describe the original complexity and set up a mathematical framework to characterize the complexity assembly in fractal dual-porosity media. For modeling demonstration, we select some typical coal samples from different reservoirs in China, extract the scale-invariance parameters, and establish fractal topography based on mercury intrusion porosimetry (MIP) and N 2 adsorption data. Using experimental tests, mathematical derivation, and numerical simulations in combination, we reveal the principle and methodology for the characterization of the complexity assembly in a coal matrix.

Mineral Composition and Full-Scale Pore Structure of Qianjiadian Sandstone-Type Uranium Deposits: Application for In Situ Leaching Mining

“…Both adsorption and desorption curves of samples are consistent when the relative pressure is lower than 0.5, while the hysteresis loops appear when the relative pressure exceeds 0.5. According to the classification of hysteresis loops by the IUPAC, the adsorption-desorption hysteresis loops of samples CS and FS belong to the type H2b; it indicates that the pores in these samples are the most composed of ink bottle pores and slit pores [51][52][53], which make the injected liquid N 2 cannot be discharged smoothly; the adsorption-desorption hysteresis loop of sample SM can be classified as the type H3, meaning that the sample SM contains more slit pores and plate pores with one end open; these pores can quickly exhaust the injected liquid N 2 , but there is a little residue; the adsorption-desorption hysteresis loops of samples MS, AS, and SS can be categorized as the type H4 [54], which demonstrates that the plate pores with two ends open develop in these samples. The mercury intrusion/extrusion curves of the samples are shown in Figure 7.…”

Influence of Shear Displacement on Fluid Flow and Solute Transport in a 3D Rough Fracture

“…However, it is still a challenge to directly measure fluid flow and solute transport processes within fractures with small apertures and rough surfaces. Benefiting from the development of computer computing power, numerical simulations that can provide pore-scale details are widely adopted to investigate fluid flow in fractures or porous media [10,[26][27][28][29].…”

Effect of the Coal Molecular Structure on the Micropore Volume and the Coalbed Methane Content

“…CBM is mainly stored in coal’s micropores in the adsorbed state, while a small quantity of free-state CBM exists in pores and fractures. − The spatial arrangement of coal’s complex macromolecular polymers and their combinations is widely investigated using X-ray diffraction, − high-resolution transmission electron microscopy (HRTEM), − Raman spectroscopy, numerical simulation, and so forth. HRTEM plays an important role in macromolecular structure studies, providing fringe images and allowing one to directly obtain macromolecular structural parameters .…”