Fractal characterization of pore structure and its influence on CH4 adsorption and seepage capacity of low-rank coals

Abstract

The pore structures of coal can directly affect the adsorption and seepage capacity of coalbed methane (CBM), which therefore is an important influence on CBM exploration and development. In this study, the pore structures of low-rank coals from the Middle Jurassic Xishanyao Formation in the southern Junggar Basin were analyzed, and the fractal dimensions (D1, D2, D3 and D4 corresponding to pore sizes of 0−5 nm, 5−100 nm, 100−1000 nm and 1000−20,000 nm, respectively) were calculated to quantitatively describe these coal pore structures. The results show that Xishanyao coal is characterized by open pore morphology, good pore connectivity and well-developed seepage pores and microfractures, which is beneficial to CBM seepage. The D1 and D2 can be used to characterize the pore surface and structure of adsorption pores respectively. The D3 and D4 can be used to represent the pore structure of seepage pores. Compared with adsorption pores, the structure of seepage pores is more affected by the change of coal rank. The D1 is better than D2 in characterizing the methane adsorption capacity. When D1 > 2.2, D1 is positively correlated with Langmuir volume (VL) and negatively correlated with Langmuir pressure (PL), while D2 shows a weak opposite trend. The coals with the higher D1 and lower D2 are associated with a higher VL, indicating the coal reservoir with more complex pore surfaces and simpler pore structures has stronger methane adsorption capacity. D4 is better than D3 in characterizing the methane seepage capacity. The porosity and permeability of coal reservoirs increases with the increase of D4, while D3 displays an opposite trend, which is mainly related to the well-developed microfractures. The well-developed fracture system enhances the seepage capacity of the Xishanyao coal reservoir. This study reveals the fractal characteristics of pore structure and its significant influence on adsorption and seepage capacity of low-rank coal.

Coalbed Methane Enrichment Characteristics and Exploration Target Selection in the Zhuozishan Coalfield of the Western Ordos Basin, China

“…Both coal seams belong to medium-rank coal. Previous studies have shown that the gas content increases with the coal rank, , which can be attributed to the increase of micropore and pore specific surface area . The No.…”

“…Previous studies have shown that the gas content increases with the coal rank, 45 , 46 which can be attributed to the increase of micropore and pore specific surface area. 47 The No. 16 coal seam may have higher gas contents than the No.…”

“…The ash yield is closely related to the terrigenous clastic input during peat accumulation, which is controlled by hydrodynamic conditions and fluctuating water levels in the peat swamp . As for the same rank coals, the higher ash yields tend to be associated with the lower gas adsorption capacity of the coal reservoir. , During the deposition of the Shanxi Formation, the depositional environments of the coalfield were mainly the fluvial plain and delta plain, where the active hydrodynamic conditions would favor the input of terrigenous siliciclastics. Therefore, the average ash yield in the No.…”