Study on the dynamic evolution of coal pore under variable pressure water injection environment

The effects of different water injection pressures and confining pressures on the initial pore structure of coal during the water injection process in coal seams are investigated to improve the application effects of water injection technology in dust reduction and outburst prevention. Coal samples from the 2-2 coal seam of Xinqiao Coal Mine are studied based on the low-field nuclear magnetic resonance equipment and the supporting liquid injection and high temperature and pressure circulation experimental system. Water injection experiments on coal samples under three conditions: fixed water injection pressure and fixed confining pressure, variable water injection pressure and fixed confining pressure, and variable confining pressure, are carried out. The dynamic evolution process of coal pore size distribution, porosity and pore volume is analyzed. The results show that the initial pore T₂ spectrum of coal samples presents a three-peak structure, with micropores having the largest proportion, followed by mesopores and macropores. Under the influence of water injection pressure and confining pressure, the coal pores show dynamic changes during the water injection process. Pressure water mainly affects the coal pore by expanding the initial pore of the coal body, increasing the number of pores and forming new pores. The radius of the newly formed pores ranges from 0.19×10⁵−1.62×10⁵ nm. The injection of water increases the porosity and pore volume of various types of pores, among which the average increase of macropore porosity reaches 106.31%, and the average pore volume increases by 262.08×10⁻³ cm³, which is the largest increase. The average increase of micropore porosity is 5.42%, and the average pore volume increases by 57.30×10⁻³ cm³, which is the smallest increase. The porosity and pore volume of mesopores and small pores also increase, with the porosity increasing by 8.13% and 9.83%, and the pore volume increasing by 69.68×10⁻³ cm³ and 61.93×10⁻³ cm³, respectively. At the same time, increasing the water injection pressure can effectively shorten the water injection time. The pore evolution law under variable water injection pressure and fixed water injection pressure conditions is similar, and a sudden increase of macropores occurs when the water injection pressure reaches 11 MPa. The confining pressure mainly affects the coal pores by compressing the micropores and small pores, with a decrease of 29.18% and 28.66%, respectively.