Pore and strength damage evolution mechanism of coal induced by the circulating water immersion effect

As the main bearing structure of the reservoir in the goaf of the mine, the damage instability mechanism under the influence of multi-field coupling of water immersion directly restricts the long-term safe and stable operation of the reservoir in the goaf. In this paper, nuclear magnetic resonance (NMR)was used to study the T2 spectrum, pore throat, porosity change and nuclear magnetic image evolution of coal samples under different times of cyclic immersion under'unilateral' immersion conditions. With the increase of cyclic immersion times, the number of pores, pore throat ratio and porosity of coal samples maintained an increasing trend, which increased by 67.18%, 3.48% and 3.49% respectively. The nuclear magnetic resonance imaging further obtained the permeability and pore change law of water molecules in different times of unilateral cyclic immersion of coal samples: the water molecules in unilateral cyclic immersion of coal samples gradually flow from the immersion side to the internal of coal samples, and finally expand to the whole coal samples, further resulting in an increase in porosity. With the increase of cyclic soaking times, the uniaxial compressive strength and residual strength of coal samples decreased gradually. The average peak strength of coal samples decreased from 15.74 MPa to 11.76 MPa, 9.65 MPa and 8.41 MPa, respectively. The average uniaxial compressive strength of coal samples decreased by 46.56% compared with that of initial coal samples. The average residual strength decreased from 5.55 MPa to 3.08, 2.44 and 0 MPa, respectively. Long-term cyclic immersion has a significant softening effect on coal samples. Based on the experimental results, the evolution law of internal pores and the morphological characteristics of uniaxial compression failure of coal samples under the increasing number of unilateral cyclic immersion were analyzed, and the mechanism of water immersion damage and failure of coal samples was revealed. The experimental results provide a scientific basis for the stability control of reservoir spatial structure in goaf.