Gas saturated coal failure envelope establishment through single core multistage triaxial loading tests

Coal properties may behavior in a different way under in situ reservoir conditions from other traditional reservoirs because of the combined effects of effective stress and sorbing-gas induced matrix property changes.Multistage triaxial testing can generate a full failure envelope using a single plug core, especially for the test when there is insufficient number of cores to run the standard triaxial tests. In this paper, a series of multistage triaxial compression testes of gas saturated coal cores were conducted based on ISRM standard procedures to determine the mechanical properties of reservoir coal formations as a function of confining stress and gas pressure.Coal strength deterioration due to phase-dependent gas adsorption was investigated.It demonstrated that coal strength with adsorbing gas injection had a positive correlation with confining stress; coalspecimen showed plasticization under a higher confining stress condition; anthracite coal has a strong adsorption effect on CO₂ and the free gas phase CO₂ adsorption can cause a dramatic peak strength deterioration; coal strength deterioration was exacerbated with respect to gas pressure. However, as the gas was in subcritical phase, gas density increasing induced hardening effect may be dominant; supercritical CO₂ led to a strong softening on coal skeleton and resulted in a plasticization of coal under a quite small confining stress condition.