Evaluating geological sequestration of CO 2 in bituminous coals: The southern Sydney Basin, Australia as a natural analogue

Abstract

Carbon dioxide contents of coals in the Sydney Basin vary both aerially and stratigraphically. In places, the coal seam gas is almost pure CO 2 that was introduced from deep magmatic sources via faults and replaced pre-existing CH 4. In some respects this process is analogous to sequestration of anthropogenic CO 2. Laboratory studies indicate that CO 2:CH 4 storage capacity ratios for Sydney Basin coals are up to ∼2 and gas diffusivity is greater for CO 2 by a factor of up to 1.5. Present-day distribution of CO 2 in the coals is controlled by geological structure, depth and a combination of hydrostatic and capillary pressures. Under present-day P– T conditions, most of the CO 2 occurs in solution at depths greater than about 650 m; at shallower depths, larger volumes of CO 2 occur in gaseous form and as adsorbed molecules in the coal due to rapidly decreasing CO 2 solubility. The CO 2 has apparently migrated up to structural highs and is concentrated in anticlines and in up-dip sections of monoclines and sealing faults. CO 2 sequestered in coal measure sequences similar to those of the Sydney Basin may behave in a similar way and, in the long term, equilibrate according to the prevailing P– T conditions. In situ CO 2 contents of Sydney Basin coals range up to 20 m 3/t. Comparisons of adsorption isotherm data measured on ground coal particles with in situ gas contents of Sydney Basin coals indicate that the volumes of CO 2 stored do not exceed ∼60% of the total CO 2 storage capacity. Therefore, the maximum CO 2 saturation that may be achieved during sequestration in analogous coals is likely to be considerably lower than the theoretical values indicated by adsorption isotherms.