Fractal characterization of pore structure for coal macrolithotypes in the Hancheng area, southeastern Ordos Basin, China

Abstract

Coalbed methane (CBM) reservoir pore structure is extremely complex and heterogeneous. With the fractal theory, the geometric and structural characteristics of the pore structure could be quantified. However, few studies have aimed to understand the fractal features of compositional-based macrolithotypes (bright, semi-bright, semi-dull, and dull coals). In this work, samples from the Hancheng area, in the southeastern Ordos Basin, China, were collected to investigate the fractal features of pore structures for macrolithotypes and calculate fractal dimensions (Da1 and Da2 for adsorption pores; Ds1 and Ds2 for seepage pores) based on low temperature N2 adsorption and mercury intrusion data, respectively. Meanwhile, the impact of the fractal dimensions on the gas adsorption capacity and the seepage ability were discussed. The results demonstrate that when bright coal turns into dull coal, the vitrinite content, helium permeability, and gas adsorption capacity gradually decrease, while the coal density, mineral content, and ash yield increase. Since the Da1 and Ds1 either deviate from the natural fractal dimension (2 < D < 3) or have a poor correlation with macrolithotypes, Da2 and Ds2 are regarded as the adsorption and seepage pore fractal dimensions, respectively. Of all the coal macrolithotypes, bright coal has the smallest BET specific surface area and micropore proportion, which results in the smallest fractal dimension Da2, while for the dull coal, this is just the contrary; due to the higher mineral content, a higher Da2 of the dull coal indicates a more complicated adsorption pore structure, and the coal with a larger Langmuir volume has a smaller Da2. For the seepage pores, when the bright coal turns into dull coal, the macropore proportion gradually decreases, but the pore proportion tends toward equilibrium, and the mercury withdrawal efficiency increases; the dull coal has a larger Ds2, indicating a more complex seepage pore structure than that in the bright coal. Due to the decreasing macropore proportion and mineral filling, Ds2 demonstrates an obvious negative correlation with the helium permeability and macropore proportion.