Fractal analysis of the dynamic variation in pore-fracture systems under the action of stress using a low-field NMR relaxation method: An experimental study of coals from western Guizhou in China

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To quantitatively analyze the dynamic variation in pore-fracture systems under the action of stress, a low-field nuclear magnetic resonance (NMR) method was used to determine the stress sensitivity of six coal samples from western Guizhou in China. The NMR T2 distributions show that the coals become considerably more compact due to the reduction of porosity with increasing coal rank and that the adsorption space (≤100 nm) with short relaxation times (≤2.5 ms) gradually become dominant. With increasing confining pressures from 0 to 12 MPa, a significant decrease in peak areas with long relaxation times (>2.5 ms) can be observed. Additionally, the lower rank coals have a higher stress sensitivity, and an increase in the adsorption space occurs due to the compression and closure of the seepage space (>100 nm). When the pressure unloads (12 MPa→0 MPa), the destruction of the pore and fracture structures cannot be completely reversed. Moreover, the adsorption space fractal D1 (T2 ≤ 2.5 ms), seepage space fractal D2 (T2 > 2.5 ms) and the total pore space fractal DNMR (0 < T2 ≤ 10000 ms) were calculated with NMR fractal theory. The D1 is generally less than 2, which does not conform to fractal geometry theory. The D2 and DNMR have positive linear correlations with the coal rank, the specific surface area of adsorption space and the Langmuir volume, while both of them decrease with increasing porosity and permeability. The higher D2 and DNMR indicate stronger adsorption capacity. With increasing confining pressure, D2 increases gradually because of the closure of the seepage space. Furthermore, the cleat compressibility (Cf) calculated with the NMR results showed that Cf is not a constant factor but exhibits a decreasing trend as the pressure increases. The permeability at different pressures was also calculated with the Cf and initial permeability, which decrease exponentially with the increase of pressure. The negative linear relationship between the dynamic permeability and D2 indicated that coals with high D2 have low flow capability. The results of this study provide a visual method to quantify the stress sensitively of coal reservoirs within different pore sizes.