Pore Structure and Compressibility of Coal Matrix with Elevated Temperatures by Mercury Intrusion Porosimetry

Abstract

To gain a better understanding of the effect of heat (e.g., magma intrusion, geothermal fluids and enhanced coal-bed methane recovery process) on coal reservoir properties, the pore structure and compressibility of coal matrix for low rank coal (0.69% Ro, m) with elevated temperatures were investigated by using multiple methods, including thermogravimetry-mass spectrometry (TG-MS), scanning electron microscope (SEM), N2 adsorption/desorption at 77 K and mercury intrusion porosimetry (MIP). The results from TG-MS showed that moisture and partial volatiles were removed from the coal matrix, and pore structure almost remained unchanged during the low heat treatment (25∼200°C). The micropores and transition pores consisted of more than 80% of the total pore volume based on the MIP. The pore structure was slightly changed following the temperature increase to 400°C, and the bound moisture and partial organics in the coal were released and decomposed by the increased heat, respectively. When temperature reached 400°C, organic matter decomposition of the coal released a large amount of hydrocarbon and micromolecule gases. The meso- and macropore in the coal were significantly developed, occupying ∼35% of the total pore volume. Although there was no large change in generated gas composition after 600°C, the pore volume and structures, including pore size distribution, pore volume and pore connectivity, were significantly changed based on the MIP. The pore structure acquired from MIP exhibited a deviation when the mercury intruded pressure reached 10 MPa. A fractal model was introduced to correct the MIP data and acquire the pore compressibility of the coal matrix. The results showed that the pore compressibility decreased with increasing pressure and temperature. Thus, this study provides significant implications of the pore structure evolution of underground coals that encounter heating.