Characteristics of pore permeability of highly metamorphic bone coal

The study of pore penmeability characteristics of stone coal(sapropel coal)has great significance for deepened understanding of adsorption and desorption of shale gas. On the basis of the principle of fractal geometry, the fractal models for describing the different types of pore size and capillary pressure curve were developed and fractal dimension was divided into the penetration fractal dimension and the proliferation fractal dimension. According to the capillary pressure data, the fraction dimension is calculated on a log-log plot of intrusion volume versus pressure. The results show that the penetration fractal dimension(D_s) is from 2.524 to 2.931, and has positive relationship with volatile matter ratio, moisture ratio, ash content and tortuosity, while negative relationship with metamorphic grade and mean pore diameter. The proliferation fractal dimension(D_k) is from 2.400 to 2.934, and has positive relationship with metamorphic grade, volatile matter ratio, moisture ratio and mean pore diameter, and negative relationship with ash content and tortuosity. In the respect of physical properties of bone coal, the penetration fractal dimension increases with porosity, permeability and proliferation fractal dimension is negatively correlated. This shows that the more heterogeneous penetration hole, the greater the porosity of coal, and the degree of uniformity of the diffusing pore is important to the permeability transformation in reservoir.