Methane Adsorption Interpreting with Adsorption Potential and Its Controlling Factors in Various Rank Coals

Feng Qiu,Yidong Cai,Yongkai Qiu,Dameng Liu,Ning Liu

doi:10.3390/pr8040390

Abstract

Water content, metamorphism (coal rank) particle size, and especially pore structure, strongly influence the adsorption capacity of coal to methane. To understand the mechanism of methane adsorption in different rank coals, and its controlling factors, isothermal adsorption experiments with different coal ranks, moisture contents and particle sizes at the temperature of 303.15 K were conducted. In addition, the pore structures of coals were investigated through N2 adsorption/desorption experiments at the low-temperature of 77 K for selected coals from the Junggar Basin of NW China, Qinshui Basin and Ordos Basin of north China. Moreover, the adsorption potential of methane on the surface of the coal matrix was calculated, the controlling factors of which were discussed. The obtained methane isothermal adsorption result shows that the Langmuir volume (VL) of coal is independent of the particle size, and decreases with the increase of moisture content, which decreases first and then increases when the coal rank increases. Combined with the pore structure by the N2 adsorption at 77 K, VL increases with the increase of pore surface area and pore volume of coal pores. Besides, the adsorption potential of all selected coals decreased with the increase of the methane adsorption volume, showing a negative relationship. The interesting phenomena was found that the surface adsorption potential of the coal matrix decreases with the increase of moisture content, and increases with the decrease of particle size at the same pressure. With the same adsorption amount, the adsorption potential on the surface of coal matrix decreases first, and then increases with the increase of coal rank, reaching a minimum at Ro,m of 1.38%, and enlarging with the increase of pore surface area and the pore volume of coal pores. These findings may have significant implications for discovering CBM accumulation areas and enhancing CBM recovery.

Highlights

With the increasing energy consumption, the exploration and development of unconventional oil and gas has been promoted [1], among which coalbed methane (CBM), as a kind of unconventional natural gas, has great exploitation potential [2]
Over 80% of CBM is present in the adsorbed state in the coal matrix, and a small amount exists in the pores in the free state [3,4,5]
Since methane exists in the coal seam mainly in the adsorption state, the isothermal adsorption becomes the key way to study the methane adsorption on the coal matrix

Summary

Introduction

With the increasing energy consumption, the exploration and development of unconventional oil and gas has been promoted [1], among which coalbed methane (CBM), as a kind of unconventional natural gas, has great exploitation potential [2]. Over 80% of CBM is present in the adsorbed state in the coal matrix, and a small amount exists in the pores in the free state [3,4,5]. Accurate evaluation of the CBM adsorption amount in the coal seam is the key to accurately estimate the geological reserves, which is of great significance for the CBM development. Since methane exists in the coal seam mainly in the adsorption state, the isothermal adsorption becomes the key way to study the methane adsorption on the coal matrix. The methane isothermal adsorption curve can be obtained by measuring the adsorption amount of methane on the coal matrix under different pressures. Pressure, coal metamorphism and moisture content have significant influence on the adsorption amount of methane [6]. Understanding the factors controlling on methane adsorption is crucial for the adsorption capacity of coal to methane

Methods

Results

Conclusion