Effects of water and brine saturation on mechanical property alterations of brown coal

Abstract

The adsorption of moisture or brine into coal causes the coal mass mechanical properties to be significantly altered, which can greatly affect the coal mining and coal seam gas extraction process. A study was therefore initiated to investigate the influence of moisture and brine saturations (5-25%) on brown coals' strength through a series of unconfined compressive strength tests, with the aid of acoustic emission, optical 3-D deformation analysis and scanning electron microscopy. According to the results, the coal mass is weakened by up to 26% upon the adsorption of moisture and water saturated samples show no crack propagation, whereas brine saturation enhances coal strength by up to 21% and delays crack propagation due to the crystallization of sodium chloride. Besides, a high brine concentration (25%) greatly improves coal mass strength but impairs the increase of Young's modulus due to its corrosive nature, which is consistent with the values of maximum strain at failure of the tested samples (3.9%, 3.1% and 3.6% for 5%, 15% and 25% brine saturated samples, respectively). In addition, because of the precipitation of sodium chloride in coal and the increase of conductivity of pore fluid, more acoustic emission signals are detected for brine saturated samples, while water saturated samples exhibit mush less acoustic release compared to the unsaturated samples.