Effects of natural micro-fracture morphology, temperature and pressure on fluid flow in coals through fractal theory combined with lattice Boltzmann method

The fluid flow behaviors during the production of coalbed methane (CBM) are generally restricted by the pre-existing natural fractures in coal seams. To better understand the effect of natural micro-fracture morphology on the flow capability, nine coals collected from Ordos Basin were subjected to optical microscope observations to obtain micro-fractures morphology. And then, the box-counting method (BCM) was used to quantify the complexity of the micro-fracture network planar distribution. Besides, the lattice Boltzmann method (LBM) was adopted to simulate the flow in the complex micro-fracture network under different pressures and temperatures. Finally, factors affecting the flow capability in micro-fracture were elaborated. The results show that the micro-fractures generally present dendritic, reticular, filamentous and orthogonal structures. The natural micro-fracture morphology has a remarkable impact on flow behavior, in which the presence of dominant channels with a length of ~498.26 μm and a width of ~10.96 μm has a significant contribution to permeability, while the orthogonal micro-fracture network normally is not conducive to fluid flow. The fractal dimension extracted from the nine coals varies from 1.321 to 1.584, and the permeability calculated from LBM method varies from 0.147 to 0.345 D; in contrast to other studies, a non-monotonic change, an inverted U-shaped, of permeability on fractal dimension was observed. Moreover, permeability decreases as pressure increases and increases with increasing temperature due to the physical properties of methane and coal matrix. Therefore, this work may contribute to understanding the process of hydrofracturing and hydrothermal methods for improving CBM reservoirs during enhancing CBM recovery.