Fractal analysis in pore structure of coal under conditions of CO2 sequestration process

A high pressure supercritical CO2 (HP-ScCO2) geochemical reactor was used to simulate CO2 sequestration into deep coal seam under around 40 °C and 9.8 MPa for 72 h and fractal analysis were employed to study the mercury intrusion data of 4 different coal rank samples before and after the ScCO2-H2O treatment, focusing on the pore structure. It is revealed from the mercury porosimetry data that after exposure to the ScCO2-H2O fluid, the true density of coal samples are changed as well as total pore volume and porosity most importantly in the increase of micro-pore range. Fractal analysis is introduced to distinguish inter- and intraparticle pores at lower mercury intrusion pressure and to identify the initial pressure associated with the coal compressibility. Three values of fractal dimension (D1, D2 and D3) are obtained under different pressure ranges, which can be classified corresponding to three different mercury intrusion processes. Varied D1 values are mainly due to the accumulation mode of samples in the penetrometer and can be used to distinguish the interpore and intrapore intrusion process at lower pressure range of mercury intrusion. D2 values represents the mercury intrusion into intrapores. D3 value is decreasing as coal rank increased and can be used to describe the initial pressure when coal samples begin to be compressed or deformed at higher pressure of mercury intrusion. The experiments revealed that CO2 sequestration process changed the physical properties of coal samples, especially in compression resistance. Coal rank and ash content in coal are very important factors which will affect the variation of coal structure during CO2 sequestration.