Investigating the Fractal Characteristics of Pore-Fractures in Bituminous Coals and Anthracites through Fluid Flow Behavior

The characteristics of pore-fractures are the key petrophysical properties used to assess and evaluate coalbed methane (CBM) reservoirs and include pore types, structure types, porosity/percentage, and pore-fracture space properties. To study the storage and seepage capability of a CBM reservoir, based on 18 coal samples with the maximum vitrinite reflectance (R-o,R-max) in the range of 1.06-3.04%, we used the nuclear magnetic resonance (NMR) method and fractal analysis to investigate the effect of coalification on the characteristics of pore-fractures. First, the pore-fracture space in a coal reservoir includes irreducible fluid space and moveable fluid space. We built up the moveable fluid NMR fractal based on the saturated/irreducible fluid NMR fractals. Saturated fluid fractal (D-w), irreducible fluid fractal (D-ir), and moveable fluid fractal (D-M) have the following relationship: DM > Dll. > Dw. Additionally, DI, has a huge fluctuating value area with the maximum vitrinite reflectance (R-o,R-max) varying between 1.93% and 2.06%. The curve lg(T-2) and lg(V) for a moveable fluid fractal has four typical types. The shapes C, Z, S, and T correspond to four structure types (A, B, C, and D), which provide significant guidance for CBM exploitation. Moreover, the saturated-irreducible T-2 distribution has a special shape, known as the pendular ring, which affects the seepage ability and permeability. Second, we classify the pore-fracture types based on the saturated fluid NMR fractal and the T2 distribution as follows: adsorbed pore (T-2 < 0.4 ms, < 10 nm), transition pore (T-2 = 0.4-2.5 ms, 10-100 nm), seepage pore-(T, > 2.5 ms, > 100 nm). At R-o,R-max = 1.06-3.04%, the percentages of pore-fracture types show that coalification have three coalification jumps at R-o,R-max = 1.3%, 2.0%, and 2.8%. The porosities of pore-fracture types show that the thermal volatilization of fillers in pore-fractures begins at R.,,a. = 1.5% and disappears at R-o,R-max = 2.5%. Different fluid space fractals demonstrate that when 120,max < 2.0%, the coal reservoir tends to be compressed and consolidated, whereas when R-o,R-max > 2.0% the coal reservoir tends to be orderly and uniform. BVM/BVI is the ratio of the irreducible fluid space (BVI) and the movable fluid space (BVM). Finally, the permeability indicates a positive exponential function with BVM/BVI for seepage pores, and it exhibits a negative exponential function with BVM/BVI for adsorbed pores and transition pores. The NMR method combining fractal analysis with coalification enables us to characterize pore-fractures and the effects of coalification on pore-fractures quantitatively, which contributes to guiding CBM exploitation. Moreover, the relationship between permeability and BVM/BVI is beneficial for predicting the favorable areas for CBM exploitation.