Changes of Coal Strength under Hydrostatic Pressure Caused by CO₂ and N₂ Injection

Abstract

A project of CO₂ injection in the Ishikari coal basin (Yubari-shi, Hokkaido, Japan) since 2002 has shown that injection of CO₂ into coal seams affected their permeability. Adsorption of CO₂ caused swelling of the coal matrix and lowered the permeability of coal seams, probably because of closure of cleats and pores in the coal seams. However, a temporary recovery of permeability was observed after subsequent injection of N₂, suggesting that some amount of adsorbed CO₂ was replaced by N₂ and the closure of cleats and pores were mitigated. However, the changes of mechanical properties of coal in the process are not yet fully understood. In this study, we injected CO₂ and N₂ into coal specimens recovered from the Ishikari coal basin and observed the changes of coal strength under hydrostatic pressure and temperatures typical of a coal basin. We conducted triaxial compressive strength tests on the coal specimens to clarify the effects of CO₂ and N₂ on the mechanical properties. Our results are summarized below.

1) We verified that swelling and shrinkage of the coal were associated with adsorption and desorption, respectively, of CO₂.

2) The triaxial compressive strength tests showed that injection of CO₂ caused decreases of peak strength and Young's modulus.

3) Both coal strength and Young's modulus recovered when previously injected CO₂ was replaced by N₂.