Influence of supercritical CO2-H2O fluid on seepage pore structure of coals

Abstract

In the process of CO2 storage in deep coal seam, the change of seepage pore structure in water-bearing coal seam by supercritical CO2 （SCCO2） greatly determines the recovery of CH4. Thus, the interaction of SCCO2-H2O mixture fluid with low water rate （mass ratio of SCCO2∶H2O =10∶1） with three rank coals （long flame coal, gas coal and anthracite） was simulated on a SCCO2 geo-reaction system under conditions of temperature of 45 ℃ and pressure of 12 MPa. Mercury intrusion method was used to address the response of seepage pore distribution to SCCO2-H2O exposure, and the influence of SCCO2-H2O on seepage pore heterogeneity of coal samples was discussed based on thermodynamic fractal model. The results show that SCCO2-H2O exposure causes an increase in porosity and pore volume of all coal samples. As a result, the permeability of coals increases. Nevertheless, the response of seepage pores to SCCO2-H2O varies with coal rank. After SCCO2-H2O exposure, the seepage pore volume of long flame coal shows a remarkable increase, the meso-porosity of gas coal rapidly increases, while slight increase in macropore and mesopore is found for anthracite. The increase in seepage pore volume of coal samples can be attributed to the chemical reaction, dissolution and mobilization of mineral matters, water loss, and sorption swelling. The content and distribution of mineral matters greatly determine the change in seepage pore structure of various rank coals. SCCO2-H2O interaction degrades the seepage pore heterogeneity of coal samples and thus smoothen pore structure, improve pore connectivity and increase permeability.

Keywords

• coalbed methane
• supercritical co2
• mercury intrusion method
• pore structure
• seepage pore