Abstract: In recent years, major breakthroughs have been achieved in deep coalbed methane (CBM) development. However, gas production varies significantly among deep CBM wells across various blocks and even in various well areas in the same block. One of the reasons lies in the differences in dynamic permeability changes of deep coal reservoirs in different blocks, while the permeability stress sensitivity and velocity sensitivity properties remain unclear. The full-diameter coal core plugs taken from deep formations in Linxing-Shenfu block have been used to conduct porosity–permeability measurements, stress sensitivity experiments, and velocity sensitivity experiments, obtaining permeabilities under different effective stresses and flow velocities. Based on the measured stress sensitivity data, the applicability of exponential, power-law, and logarithmic stress sensitivity models has been compared and evaluated, which enables to select the optimal permeability stress sensitivity model and establish a new standard for stress sensitivity evaluation. Additionally, the measured velocity sensitivity data has been applied to determine the parameters of permeability velocity sensitivity model, revealing the deep coal permeability velocity sensitivity property in Linxing-Shenfu block. The study results indicate that: (1) The deep coal seams in Linxing-Shenfu block exhibit typical characteristics of low porosity and low permeability, with the porosity range of 3.9%–9.1% (average of 6.5%) and gas permeability under low pressure of 0.9–2.1 mD (average of 1.63 mD). (2) The dynamic permeability changes with pressure follow an exponential trend,with the stress sensitivity index of 0.33–0.52 MPa-1. Based on the new stress sensitivity evaluation criteria (which classify stress sensitivity into six levels—none, weak, moderately weak, moderately strong, strong, and extremely strong using stress sensitivity boundaries of 0.01 MPa^-1, 0.08 MPa^-1, 0.15 MPa^-1, 0.23 MPa^-1, and 0.45 MPa^-1), the stress sensitivity degree of deep coal seams is classified as strong or extremely strong, so special consideration should be given to stress sensitivity effects in the process of optimization design of production regimes. (3) As the fluid flow rate increases, permeability decreases sharply initially and then declines slowly. The fitted critical plugging velocity (v_cr2) ranges in 0.01– 0.11 m/d, the maximum damage degree (D_v,max) is 0.81–0.96, and the damage rate index (n) is 0.40–1.05. Furthermore, the lower the original coal permeability, the lower the critical plugging velocity, the higher the maximum damage degree, and the smaller the damage rate index. The research findings provide a theoretical basis for production capacity evaluation and production regime design of deep CBM wells.

Key words: Linxing–Shenfu block, deep coal seam, permeability change, stress sensitivity, velocity sensitivity, experiment, model fitting