Pore Structure and Compressibility Characteristics of Heat-Treated Coals by N2 Adsorption/Desorption and Mercury Intrusion Porosimetry

To improve the physical properties of a coal reservoir with heat treatment and enhance the coalbed methane recovery, the characteristics of gas generation and pore structure evolution of different rank coals during heat treatment were investigated by combining the thermogravimetry–mass spectrometry, N2 adsorption/desorption, and mercury intrusion porosimetry analyses. The impacts of these characteristics on pore compressibility were also studied. The results indicate that the macromolecular organic matter in coals begins to decompose into hydrocarbon gases (such as CH4 and C2H4) at temperature ranging from 350 to 600 °C, and then the production peaks of CO and CO2 exist at 600–800 °C in different rank coals and are accompanied by the generation of H2 and H2O, which results from the decomposition of carbonate minerals and the polycondensation reaction. The pore structure and heterogeneity of different rank coals treated at 200 °C remain stable except for the enlargement of the pore size because of the slight thermal expansion of the coal matrix and the removal of moistures/partial volatiles. However, as the temperature rises to 400 °C, the partial adsorption pores of low-rank coal (LRC) are closed, while the adsorption pore volume of medium-rank coal (MRC) increases, which may relate to the continuous decomposition of the volatile matter and the outburst of small-molecule gas. The massive seepage pores and microfractures are extensively developed in LRC (90.6% vol) and MRC (61.26% vol) treated at 600 °C, which provide an important flow pathway for coalbed methane recovery. In comparison, the pore structure and heterogeneity of high-rank coal (HRC) change indistinctly at 400 and 600 °C because of the original high metamorphic degree. Moreover, the pore compressibility values show a descending trend as the coal rank increases, corresponding to 2.45 × 10–4 to 3.09 × 10–2 MPa–1 for LRC, 9.43 × 10–4 to 4.03 × 10–2 MPa–1 for MRC, and 2.76 × 10–4 to 6.9 × 10–4 MPa–1 for HRC when the pressure and temperature range from 14.5 to 206 MPa and 25 to 600 °C, respectively. Meanwhile, the pore compressibility of coals shows a remarkable positive correlation with the pore volume fraction of micropores and transition pores, which can provide a large amount of compressible space at the high-pressure stage.