Differences of the Pore Structure and Methane Adsorption/Desorption between Vitrain and Durain of Low-Rank Coals: Case Study in the Huanglong Coalfield, Southern Ordos Basin, China

Abstract

Coal reservoirs are highly heterogeneous, and different macrolithotypes of coal have differences in material composition and pore structure, resulting in different coalbed methane (CBM) adsorption/desorption performance and gas-water migration characteristics. In order to explore the pore development characteristics of different macrolithotypes in low-rank coal and their influence on adsorption and desorption, the low-rank coal of the Yan’an Formation in the Huanglong coalfield is taken as the sample of the research object. Vitrain and durain are collected and then separated. The results show that the porosity of coal is 2.92%–11.81%, and the total porosity is relatively developed. The porosity of durain is slightly higher than that of vitrain, the pore throat of durain is thicker, and the macropores are more developed. The Brunauer-Emmett-Teller (BET) specific surface area (SSA) and Barrett-Joyner-Halenda (BJH) total pore volume (TPV) of vitrain is slightly larger than durain, the micropores are more developed, and most of them are semiclosed and ink-bottle-shaped pores. The pore-fracture connectivity of the durain is better than vitrain via nuclear magnetic resonance (NMR). Vitrain has more developed micropores than durain, with larger specific surface area and stronger adsorption capacity. In the process of methane adsorption/desorption, the phenomenon of desorption hysteresis generally exists. Moreover, the pore connectivity of durain is relatively good, the degree of desorption hysteresis is weaker than vitrain, and the theoretical desorption efficiency is higher than vitrain. Temperature has a significant influence on adsorption and desorption of methane. The increase in temperature is beneficial to desorption and weakens the hysteresis effect of desorption. At the same temperature, the isosteric adsorption heat in the desorption process is greater than the adsorption process, which further explains the incomplete reversibility of the adsorption and desorption process. Under the same conditions, the isosteric adsorption heat of vitrain in the adsorption process is greater than durain, and adsorption capacity of vitrain is stronger than durain. Similarly, the isosteric adsorption heat of vitrain in the desorption process is greater than durain, and desorption needs to absorb more heat. The desorption capacity is weaker than durain.