Mechanisms of Weakening in Coal-Measure Rocks by Hyperalkaline Fluids

We explore mechanisms controlling dramatic strength loss in coal-measure sandstones and mudstones in the far-from-equilibrium aqueous environment of the Northwest China coalfields. Strength loss in roof and floor increases long-term deformation and jeopardizes stability of underground excavations and mine safety. We investigate mechanisms of mechanical deterioration under hyperalkaline conditions through an integrated program of strength testing with concurrent imaging by acoustic emission (AE), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Uniaxial compressive strengths (UCS) of sandy mudstones are reduced by ~ 84.4% and elastic modulus by ~ 83.1% after 5d of immersion in pH = 12 solution with strength decay scaling exponentially with immersion time. Total AE energy to failure decreases with increasing alkalinity as failure mode transitions from tensile-dominated splitting to composite tensile-shear rupture—focused in areas of high-cumulative-energy and high-number-density of AE events. Porosity increases sharply by 217.3% (pH = 12) and the fractal dimension of the porous medium rises from 1.75 to 1.86, indicating increased pore connectivity and a more complex structure. Microstructural analysis identifies test-concurrent dissolution of silicates (quartz and chlorite) and the formation of clay minerals such as kaolinite. These observations suggest that grouting of the excavation disturbed zone surrounding roadways would reduce infiltration of reactive fluids and thus weakening. The influence of such hyperalkaline fluids will also be important in CO₂ sequestration in deep reservoirs; permeability enhancement through hydraulic fracturing in unconventional reservoirs; rational design and stability control of coal mine underground dams and reservoirs; as well as the safe utilization of underground space in abandoned mines.