Mechanical response and mineral dissolution of anthracite induced by supercritical CO2 saturation: Influence of saturation time

Injection of CO2 into coal seams is a critical path to achieve the goals of enhanced coalbed methane recovery and carbon sequestration. Under the in-situ reservoir conditions, CO2 is likely to be stored in the form of supercritical state. Up to date, little understanding is reported regarding the influence of supercritical CO2 (SC-CO2) saturation time on mechanical properties and structural changes of coal. In this work, anthracite specimens are prepared and saturated by SC-CO2 fluids at 40 celcius and 11 MPa for saturation time up to 15 days. Uniaxial compression measurements, X-ray diffraction (XRD) tests and mercury intrusion porosimetry (MIP) were performed to explore the mechanical response and microstructural changes of coal. The results show that after SC-CO2 interaction, coal mechanical properties are remarkably altered, including the reductions of uniaxial compression strength (up to 65.42%), elasticity modulus (up to 59.07%) and tensile strength (up to 56.45%). Changes in coal mechanical behaviour induced by SC-CO2 interaction are time-dependent and mainly take place within 9 days, then followed by only a slight further reduction (approximately 4-6%). MIP results indicate that as the increase of interaction time, porosity and pore specific surface area are continuously increased from 12.12 to 18.36% and 3.56 to 6.75 m(2)/g, respectively. The reductions in coal mechanical strength are closely related to the increase of mineral consumption and coal porosity. Mineral dissolution, hydrocarbon extraction and differential swelling are mainly responsible for the softening effect of SC-CO2 on coal. This study provides clearer insights into the process of CO2 geological sequestration.

Automated summary

Injection of CO2 into coal seams is a critical method for enhanced coalbed methane recovery and carbon sequestration. This study investigates the influence of supercritical CO2 saturation time on the mechanical properties and structural changes of coal. The results show that after interaction with supercritical CO2, the mechanical properties of coal are significantly altered, with reductions in compressive strength, elasticity modulus, and tensile strength. These changes are time-dependent and primarily occur within 9 days of saturation. Additionally, the porosity and pore specific surface area of coal increase with interaction time.