Expansion deformation and permeability characteristics of bituminous coal with different moisture content after CO2 adsorption

Abstract

Injecting CO2 into deep unexploitable coal seams is a means with dual benefits of enhanced coalbed methane exploitation (CO2-ECBM) and CO2 geological storage (CGS). The permeability change of the coal seam after CO2 injection is an important factor for the CO2-ECBM and CGS. However, the impact of the interaction of CO2–H2O-coal in the coal seam on the adsorption swelling and permeability characteristics is poorly understood. In this study, we quantitatively analyzed the adsorption characteristics, permeability characteristics, and microstructure changes of coal samples under CO2–H2O-coal interaction by Geotechnical Consulting & Testing Systems (GCTS), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD). The results show that the increase of moisture content inhibits the adsorption of CO2 due to the clusters formed by water molecules occupy the position of adsorption pores, reducing the expansion rate of coal samples. The reaction rate content of carbonate minerals in coal samples increases with the increase of moisture content after subcritical CO2 treatment, leading to an increase in the relative content of clay minerals, which can enlarge the pore volume. However, the subcritical CO2 treatment cannot completely eliminate the negative impact of loading and unloading processes on the mesopore and macropore, and therefore show a decrease of the pore volume and permeability. The dissolution of carbonate mineral in coal samples is more severe after supercritical CO2 (ScCO2), which can effectively enlarge seepage pores and thus improves the permeability. The clay mineral content increases slightly at 5.6% moisture content, indicating that it also reacts significantly after ScCO2 treatment. For instance, the permeability growth rate of 2.8% moisture content coal sample reaches 421.74% at 8 MPa effective stress, which is the best moisture content for permeability enhancement. In addition, the coal samples with 5.6% moisture content show the transformation from mesopores and macropores to micropores and small pores after subcritical CO2 and ScCO2 treatment, which provides more adsorption sites for CGS.