Multifractal Investigation on Multi-scale Pore Structure Heterogeneity of High Rank Coal Reservoirs

Pore structure determination is the key to estimating gas-in-place and potential primary recovery of coalbed methane. In this study, various fluid intrusion technologies were applied to determine pore structures of coal samples at different scales. Subsequently, the multifractal theory was applied to evaluate quantitatively the multi-scale pore structure heterogeneity. Results show that the pore size distributions (PSDs) of macropores and micropores were characterized by unimodal and bimodal distributions, respectively, and bimodal and multimodal distributions were found in mesopores. Micropores provide the majority of pore surface area and total pore volume for coals. Moreover, the pore structure of coals had significant multifractal characteristics, and the relevant multifractal parameters were the sensitive indicators for determining the heterogeneity of different probability measure areas. The multi-scale pore structure heterogeneity was strongly related to the mineral components of coal reservoirs and the higher the clay content, the larger heterogeneity of mesopores in the scale of high probability measure areas. Additionally, the multifractal parameter Delta f(alpha) reflected directly the relative development degree of different scale pores and thus can be used to determine the effect of pore structure heterogeneity on permeability. A negative value of Delta f(alpha) for macropores indicated that the part of macropores corresponding to the relatively larger scale in low probability measure area dominated the heterogeneity of PSD and determined the permeability of the coals. The findings provide a better understanding of pore structure heterogeneity and create a bridge between pore structure and gas flow behavior of coal reservoirs.

Automated summary

Determining pore structure is crucial for estimating coalbed methane and primary recovery. This study used various fluid intrusion technologies to determine the pore structures of coal samples at different scales and applied multifractal theory to quantitatively evaluate the heterogeneity. The results revealed different distributions of pore size in macropores, micropores, and mesopores, and the clay content had a significant impact on the heterogeneity. Micropores were found to provide the majority of pore surface area and total volume. Moreover, the pore structure exhibited multifractal characteristics and the multifractal parameters were sensitive indicators for determining heterogeneity.