Evaluation of the mechanical behaviour of brine+CO2 saturated brown coal under mono-cyclic uni-axial compression

Abstract

Abstract CO2-sequestration and ECBM techniques necessitate injection of CO2 into coal reservoirs that are saturated under various pore fluid conditions, resulting in alterations in mechanical properties. In this study, we evaluate the effect of pore fluid chemistry and the interaction time on the coal mechanical degradation. Uni-axial compressive strength (UCS) tests, ARAMIS photogrammetric analysis, micro-CT imaging, and Fourier transform infrared spectroscopy (FT-IR) are combinedly used to evaluate such alterations and to interpret the causative factors in water, water\xa0+\xa0CO2, 10% brine\xa0+\xa0CO2 and 20% brine\xa0+\xa0CO2 saturated coal. The results indicate that irrespective of the pore fluid chemistry, mechanical parameters including UCS, Young’s modulus and brittleness index reduce significantly in saturated coal, due to moisture adsorption-induced softening effect. The presence of CO2 in pore fluids causes additional alterations in each mechanical property, due to the corrosive chemical interactions occur in acidic environments, CO2 adsorption-induced energy reduction and plasticization-induced alterations in coal macromolecular structure. The direct comparison of FT-IR spectrums of natural and CO2-interacted coal concludes that CO2 interaction causes alterations in the coal macromolecular structure possibly causing the mechanical degradation of coal mass due to plasticization effect and the extraction of pore-constricting hydrocarbons. The volumetric strain analysis and micro-CT image-based 3D-reconstruction infer that water, water\xa0+\xa0CO2 and 10% brine\xa0+\xa0CO2 saturation increase the ductile properties of coal, resulting in a dilatancy deformation and extensive fracturing, upon mechanical loading. In contrast, the higher order of NaCl concentrations (i.e. 20% brine) in pore fluid causes NaCl crystallization in coal, resulting in an elevated brittleness and consequently altering the sample deformation, failure pattern and fracturing mechanism. Although, softening effect and chemical interactions cause continues mechanical degradation with increasing saturation time in all saturation conditions, a significant strength reduction occurs at a short-term saturation period and the latter longer saturations have caused only gradual strength reductions, probably due to the rapid CO2 adsorption process on to coal matrix.