A Novel Approach to Obtain Fractal Dimension in Coals by LFNMR: Insights from the T2 Peak and T2 Geometric Mean

Pore structural parameters of coalbed methane (CBM) reservoirs can be vital in establishing their gas adsorption behaviors and gas transport capabilities. The fractal dimension of pore structure in coals by low-field nuclear magnetic resonance (LFNMR) measurements are not well understood. The LFNMR parameters, including the amplitude weighted mean on a logarithmic dimension (T2g), the highest periodicity on the transverse relaxation time (T2) spectrum (T2p), bulk volume irreducible (BVI), free fluid index (FFI), and the radio of BVI and FFI, for seven coal rank samples were determined. Accessible pore and inaccessible pore are distinguished and porosity-related characteristics were investigated. We propose a novel model to obtain the fractal dimension of three type of pores by the LFNMR. The accuracy of this model was validated by the statistically significant correlations. We suggested fractal dimensions 2.65, 2.93, and 2.86 corresponding to accessible, inaccessible, and total pores, respectively. Results of fractal dimension analysis were purposed for the investigation of the relationship between petrophysics properties and pore structure heterogeneity. The fractal dimension of accessible pores presented distinctly correlated with the T2p and connectivity. Moreover, the strong positive relationship between fractal and vitrinite reflectance documented that that accessible pore characterization had contributed to lower fractal dimensions and less complex pore structure in the lower metamorphic coals. Consequently, this method is a new theory that can be used to better understand the heterogeneity of accessible, inaccessible, and total pores. These findings contribute to the further understanding of pore structural heterogeneity and provide practical guidelines to achieve gas storage properties and gas transport behaviors, which are critical but much-needed for CBM reservoirs.