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 1000−20000 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 (V_L) and negatively correlated with Langmuir pressure (P_L), while D₂ shows a weak opposite trend. The coals with the higher D₁ and lower D₂ are associated with a higher V_L, 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.

Keywords

southern Junggar Basin / Middle Jurassic / low-rank coal / coalbed methane / pore structure / fractal dimensions

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