Microfracturing of coal due to interaction with CO2 under unconfined conditions

• Published in: Fuel (Guildford) 2012-07, Vol.97, p.569-584
• Main Authors: Hol, Sander, Spiers, Christopher J., Peach, Colin J.
• Format: Article
• Language: English
• Subjects: Applied sciences; Crack; ECBM; Energy; Energy. Thermal use of fuels; Exact sciences and technology; Fuels; Permeability; Strain; Stress
• ISSN: 0016-2361; 1873-7153
• DOI: 10.1016/j.fuel.2012.02.030

► Microfractures form in unconfined coal samples after exposure to CO2. ► Fracturing caused by heterogeneous swelling and differential accessibility of coal macerals. ► Plasticisation is unlikely to play a significant role. ► In situ stress conditions during ECBM prevent microfractures to open or propagate. ► Permeability during ECBM operations will remain low. Laboratory experiments conducted in the past century have shown that exposure of coal to CO2 under unconfined, hydrostatic conditions leads to reversible adsorption and swelling. However, several authors also report irreversible changes in sorption capacity, sample volume, equilibration time and brittle failure strength. Some relate these effects to the formation of microfractures, while others consider “structural rearrangements” in the macromolecular structure of coal to be responsible. In this study, we investigate the magnitude of irreversible swelling effects and changes in equilibration time in high volatile bituminous coal (Brzeszcze, Seam 364, Poland), and attempt to explain the results in terms of the operative microphysical processes. We also assess the implications for Enhanced Coalbed Methane (ECBM) operations. Our approach involves detailed dilatometry experiments conducted on fresh, unconfined, mm-scale coal matrix cylinders at CO2 pressures up to 100MPa, and at 40.0°C. Exposure of our samples to CO2 produced reversible and irreversible strains resulting predominantly from competition between adsorption-induced swelling and elastic compression. During the first or second cycle of exposure, substantial hysteresis was observed in volumetric behaviour, notably at CO2 pressures above 35–40MPa. After two or three upward and downward CO2 pressure cycles, the measured strain response became fully reversible. Equilibration with CO2 took about four times longer during the first CO2 pressurisation cycle than in subsequent CO2 pressurisation cycles. Microstructural analysis and comparison showed that microfractures formed in the coal during first exposure to CO2. From the microstructural and mechanical data, we infer that microfracturing was responsible for enhanced CO2 penetration into the present samples. This, in turn allowed more homogeneous access of CO2, and caused adsorption-induced swelling of matrix material not previously accessed by CO2. We further infer that the enhanced penetration, sorption and swelling, in turn, resulted in the observed hysteresis in dimensional response and in the