Enhanced permeability mechanism in coal seams through liquid nitrogen immersion: multi-scale pore structure analysis

Abstract

The geological structure of coal seams in China is remarkably varied and complex, with coalbed methane reservoirs marked by significant heterogeneity and low permeability, creating substantial technical challenges for efficient extraction. This study systematically investigates the impact of liquid nitrogen immersion (LNI) on the coal’s pore structure and its mechanism of enhancing permeability with a combination of quantitative nuclear magnetic resonance (NMR) analysis, nitrogen adsorption experiments, and fractal dimension calculations. The results demonstrate that LNI can damage the coal’s pore structure and promote fracture expansion through thermal stress induction and moisture phase transformation, thereby enhancing the permeability of coal seams. The T2 peak area in the NMR experiments on coal samples subjected to LNI treatment shows a significant increase, the Brunauer-Emmett-Teller (BET) specific surface area decreases to 6.02 m²/g, and the Barrett-Joyner-Halenda (BJH) total pore volume increases to 14.99 mm³/g. Furthermore, changes in fractal dimensions (D1 rising from 2.804 to 2.837, and D2 falling from 2.757 to 2.594) indicate a notable enhancement in the complexity of the pore structure. With increasing LNI cycles, the adsorption capacity of the coal samples diminishes, suggesting a significant optimization of the pore structure. This optimization is particularly evident in the reconstruction of the micropore structure, which in turn greatly enhances the complexity and connectivity of the sample’s pore network. In summary, the study concludes that LNI technology can effectively improve the permeability of coal seams and the extraction efficiency of coalbed methane by optimizing the micropore structure and enhancing pore connectivity, which offers a potential method for enhancing the permeability of gas-bearing coal seams and facilitating the development and utilization of coalbed methane.

1. Introduction

China has abundant coalbed methane resources within its coal seams [1−3]. A forecast by the National Energy Administration of China in 2022 estimates these potential resources to be around 2.6 trillion cubic meters [4]. “Coal mine gas” also known as coalbed methane, is a crucial energy source that needs to be developed and utilized expediently not only for the safe operation of coal mines but also for the availability of clean energy sources [5−7]. Despite its potential, coalbed methane is predominantly found in an adsorbed state within the intricate micropore structures of the coal seams. The complex geological setting and the inherently low permeability of these seams result in less-than-optimal extraction rates [8−11]. To enhance the extraction efficiency, enhancement treatments are imperative for coal seams prior to the coalbed methane extraction process [12−16]. Liquid nitrogen emerges as a key player in this context and serves as a cryogenic medium capable of swiftly reducing the temperature of coal seams owing to its extremely low temperatures. This rapid cooling prompts the moisture within the seams to freeze and expand, creating fractures that facilitate increased permeability and subsequently improve the extraction efficiency of coalbed methane [17−19]. Moreover, the volumetric expansion that occurs as liquid nitrogen vaporizes further contributes to the formation and development of these fractures, thereby promoting the release of more coalbed methane. This technological application not only enhances the extraction process but also sets the stage for a more efficient utilization of this valuable energy resource [20−23].

The pore structure of coal is a key factor in the storage and migration of coalbed methane. LNI technology induces thermal stress