Seepage characteristics of fracture and dead-end pore structure in coal at micro- and meso-scales

Abstract

Coal contains a large number of internal pores and fractures that directly affect the seepage process of water injected into it. But the influence of dead-end pores on the seepage process is incompletely explored. To this end, we constructed structure models of “fractures and dead-end pores” in gas coal and non-stick coal with different pore sizes and connectivity based on computed tomography with 3-dimentional reconstruction and artificial pore technology. We simulated the dynamic seepage process of water injected into different coals using the Navier-Stokes equation, and obtained the distribution characteristics of water seepage in the “fractures and dead-end pores” structure at initial velocity of 0.03 mm/s, 0.04 mm/s, 0.05 mm/s and 0.06 mm/s for models with different pore sizes. The results showed that the seepage velocities in natural pores, fractures and artificial dead-end pores all exhibited a trend of “decreasing after increasing and stabilizing” with the increase of time. We then introduced the concepts of “peak velocity” and “stable flow velocity” to describe unsteady seepage conditions. In the dead-end pores, with the increase of the equivalent size, the peak velocity of the two different coals showed different variation patterns and the stable flow velocity increased nonlinearly. Changes in pore sizes affected not only the seepage characteristics in the pores, but also the stable flow velocity in the fractures. Specifically, as the pore size increased, the stable flow velocity changed from “gradually increasing” to “gradually decreasing”. Changes in connectivity affected the stable flow velocity in the “fractures and dead-end pores”.