Numerical study of the impact mechanism of decoupling charge on blasting-enhanced permeability in low-permeability sandstones

More than seventy percent of sandstone-type uranium mine occur in the compact sandstone reservoir with extremely low permeability in China, there is no doubt that it has seriously restricted the utilization of uranium resource. Thus, improving the permeability of sandstone reservoir is the key factor for enhancing the uranium deposits recovery. Obviously, blasting fracturing technique is one of the most promising options for enhancing the permeability of sandstone reservoir. This paper emphatically studies the influence rules of decoupling charge on the improvement of permeability in sandstone reservoir through combining two-dimensional particle flow code (PFC2D) with Universal Distinct Element Code (UDEC). Firstly, the discrete fracture network (DFN) induced by blasting are simulated by PFC2D under the condition of six decoupling coefficient of charges. Secondly, isolate fractures and the fracture clusters that are not connected to the borehole are neglected because of their invalidation for the migration of uranium mine dissolved in leaching liquid from the sandstone reservoir into the borehole, while the fracture clusters that are beneficial for uranium deposits recovery are exported from the PFC2D and imported into the UDEC to establish the numerical model for seepage analysis. Finally, through simulating the flow of leaching liquid in the useful fracture clusters, the average permeability coefficient at different distance to the center of borehole are derived from Darcy law. It is significant to note that the action from the explosive stress wave and the detonation gas are considered simultaneously when the initiation and propagation of fractures induced by blasting are simulated in PFC2D. From the studies in this paper, it is concluded that the effective area for uranium mine recovery increase first and then decrease with the increase of decoupling coefficients, which indicates that the effective distribution area do not increase monotonically with the increase of decoupling coefficient and there must be an optimal decoupling coefficient for enhancing the permeability of sandstone reservoir.