Insights into fractal characteristics of pores in different rank coals by nuclear magnetic resonance (NMR)

Low-field nuclear magnetic resonance (NMR), as a nondestructive and time saving method, has been widely used to evaluate the petrophysical properties of coalbed methane (CBM) reservoirs. To better understand the impacts of pore structures on permeability, NMR T₂ spectrums of different rank coals and their meaning to porosity and permeability were investigated. Fractal dimensions of coal pores with NMR T₂ spectrum and their correlations with porosity, R_o,m, and permeability were synthetically analyzed. The results show that T₂ spectrums can be divided into three types named as unimodal, bimodal, and trimodal based on the spectrum peaks. The percentage of spectrum peak P1 increases with increasing R_{o, m} and the connectivity of different peaks can effectively reflect the permeability. The irreducible fluid and movable fluid porosity have an opposite relationship with increasing percentage of centrifugal fluid and increasing R_o,m. Fractal dimensions of adsorption pores (D1, 0.82–1.94) and seepage pores (D2, 2.50–2.99) demonstrate that coal pores have multiple fractals. Due to the coalification jump, both D1 and D2 show binomial relationship with R_o,m at the inflection section of 1.3–1.5% R_o,m. Generally, movable fluid porosity, irreducible fluid porosity, and total porosity show exponentially decreased or binomial relationship with the increasing D1 and D2. The development of microfractures could be the main cause of abnormal tendency between D2 and total porosity in the high-rank coals. The movable fluid porosity that reflects the proportion of seepage pores decreases with the increasing D2, which have a significant impact on low permeability coals. Most seepage pores that contributed permeability is lower than 0.1 mD, which contributes less than 20% to total permeability. However, seepage pores that contributed permeability may have an important role on CBM production, especially at the middle to late stage.