Study on water seepage law of confined coal body and optimization of water injection parameters

Abstract

To optimize water injection parameters, it is necessary to understand the water seepage characteristics of coal and establish an evaluation method for the coal seam water injection effect. Using low-field nuclear magnetic resonance (NMR) technology and a peripheral loading pressure experimental system, real-time Carr-Purcell-Meiboom-Gill sequence tests were conducted on coal samples under different water injection schemes. The dynamic migration characteristics of pressurized water in coal under a constant water injection pressure and injection flow were studied. The results showed that under a fixed water injection pressure, the change in water content with injection time was in concordance with first-order kinetics, indicating that the process is controlled by diffusion. Combined with the fitting results of the wetting rate constant and water injection pressure, the best water injection pressure was obtained. Moderate water injection flow, an increase in water injection volume, and an extension of the water injection time can improve the wetting effect. Under constant water pressure, water seepage in the coal includes two processes: the rapid migration of free water to the pore fractures of coal and the spontaneous water absorption process in the coal. Under constant water flow, the water seepage process in the coal is consistent with the Boltzmann equation. Based on the time constant and inflection point of the time growth obtained by the Boltzmann equation, the seepage process was divided into the water inflow, water transportation, and water adsorption stages. The peak area of the adsorbed water is proportional to the mass of coal moisture. This relationship can realize the mutual conversion between the NMR water signal quantity and moisture content in coal.