The coupling evolution mechanism of gas-water migration and the temperature field during hot nitrogen injection into coal rock after volume fracturing

Volume fracturing is a pivotal technology for exploiting coalbed methane in “three-low, one-high” reservoirs, though water retention often limits effectiveness. This research establishes a multi-phase flow–thermal coupling model integrating fluid dynamics, mass transfer, and heat transport to simulate thermal nitrogen injection into a three-level fracture network. The study examines multi-phase flow evolution and thermal response under varying injection conditions. Results reveal a three-stage displacement process: initial breakthrough through dominant channels, followed by mixed network flow, and finally stabilized gas propagation. Higher injection pressure significantly expands gas coverage and reduces residual water, while low pressure promotes water trapping. The temperature field evolution, mainly controlled by pressure, shows the most pronounced change in the main fractures. Flow behavior is hierarchically structured: primary fractures exhibit stepwise decline, secondary fractures display strong fluctuations, and tertiary fractures face initiation constraints. Increasing injection pressure enhances driving force, moderates flow decay in primary fractures, reduces instability in secondary ones, and facilitates activation of tertiary fractures. This process activates complex fracture networks and delays productivity decline, providing a theoretical foundation for post-fracturing water removal and enhanced permeability strategies.