Physical Properties of Low-Rank Coal Samples from the Powder River Basin, Wyoming

We characterize the mechanical properties of coal samples from the Powder River Basin (Wyoming, USA) by conducting laboratory experiments. We present results from laboratory measurements of adsorption, static and dynamic elastic moduli, and permeability as a function of effective stress, pore pressure, and gas species. Notably, we observe that CO2 adsorption causes the static bulk modulus to decrease by a factor of two, while simultaneously causing the dynamic bulk modulus to increase by several percent. Permeability of both intact and powdered samples decreases by approximately an order of magnitude in the presence of CO2, which is consistent with observations of adsorption-related swelling of the coal matrix. Interestingly, CO2 appears to change the constitutive behavior of coal; helium saturated samples exhibit elastic behavior, while CO2 saturated samples exhibit viscous, anelastic behavior, as evidenced by creep strain observations.