Coupled Damage Effects and Microscopic Mechanism of CO2–Alkaline Water on the Physicochemical Properties of Anthracite

Abstract

CO2–alkaline water two-phase displacing gas and wetting coal (CADW) can significantly enhance coalbed methane (CBM) extraction efficiency and prevent coal and gas outburst. Nevertheless, both the residual CO2 and alkaline water damage coal. To study the coupled damage effects of CO2 and alkaline water on the physicochemical properties of anthracite, the variation in coal physicochemical structure was quantitatively examined, and thereby the underlying microscopic mechanism was elucidated. The experimental results indicated that (1) Direct water injection exhibited minimal impact on anthracite minerals, resulting in a minor reduction in mineral content. In QC coal leachate, HCO3– was the dominant ion (69.97% of total ions). However, following the CADW treatment, coal experienced enhanced water dissolution and acid–base erosion, leading to an evident reduction in mineral content. In QC coal leachate, HCO3– became negligible, whereas contents of Na+, NH4+, Cl–, and CO32– increased significantly, with CO32– constituting 32.93% of the total ions. (2) After direct water injection, the content of hydrophilic groups in anthracite decreased markedly. Specifically, in QC coal, −OH and C═O contents decreased by 72.96 and 34.78%, respectively. However, following the CADW treatment, hydrophilic group content increased substantially. Contents of −OH and C═O in QC coal increased by 19.03 and 47.83%, respectively. (3) Direct water injection induced minor modification in the pore structure of anthracite; the specific surface area and total pore volume of QC coal increased by 6.19 and 8.06%, respectively. In contrast, the CO2–alkaline water exhibited pronounced pore increment and enlargement effects on anthracite, developing the pore structure significantly. After the CADW treatment, the specific surface area and total pore volume of QC coal increased by 15.46 and 37.44%, respectively. (4) Compared with direct water injection, the CADW significantly reduced the mechanical strength of anthracite. Specifically, in QC coal, the elastic modulus and uniaxial compressive strength decreased by 60.24 and 53.35%, respectively, and the peak strain increased by 12.80%. These findings provide valuable insights for enhancing CBM extraction efficiency and preventing coal and gas outburst.