Influence mechanism of stress and N2 injection pressure on water displacement efficiency: A visualization experimental study based on low-field nuclear magnetic resonance

Gas injection displacement is an effective way to overcome the water lock effect and boost coalbed methane recovery. Using the in situ low-field nuclear magnetic resonance system, gas-driven water tests were conducted under varying gas pressures and stresses to explore the displacement mechanism of saturated water in coal's multi-scale pores and fractures. Findings show: (1) the Sujiagou samples consist of adsorption pore (AP, 0 < T2 < 1 ms), percolation pore (PP, 1 ms < T2 < 10 ms), and migration pore (MP, T2 > 10 ms). As gas pressure rises, the displacement contribution rate shifts from AP-dominated to MP-dominated. The optimal gas injection pressure balances pore contributions, enhancing inter-pore connectivity and displacement efficiency. Too low pressure causes inefficient displacement due to fluid blockage, while too high pressure leads to premature breakthrough and rapid failure. (2) Low stress ensures balanced pore contributions and high efficiency. When stress increases from low to medium, AP closes, causing displacement failure and a shift to low-scale structures with MP contributing 86.8%. At high stress, AP expansion aids desorption but PP contributes only 13%, resulting in low efficiency. (3) The optimal pressure boosts fluid drivability, mitigates the decline of water phase relative permeability, and extends the effective displacement cycle. Low stress favors early displacement but hinders continuous driving of residual fluids, with gas-phase relative permeability dominating later and causing displacement failure. The research provides new insights for gas injection to break through the water lock effect and improve the gas drainage rate.