Experimental study on permeability evolution of deep high-stressed coal under major horizontal stress unloading paths

Abstract

Both bulk stress (σ_ii) and stress path (SP) significantly affect the transportation characteristics of deep gas during reservoir pressure depletion. Therefore, the experimental study of horizontal stress unloading on seepage behavior of gas-bearing coal under constant σ_ii-constraints is performed. The results show that coal permeability is affected by horizontal stress anisotropy (σ_H≠σ_h), and the contribution of minor horizontal stress to permeability is related to the differential response of horizontal strain. The slippage phenomenon is prominent in deep high-stress regime, especially in low reservoir pressure. σ_ii and SP jointly determine the manifestation of slippage effect and the strength of stress sensitivity (γ) of permeability. Deep reservoir implies an incremental percentage of slip-based permeability, and SP weakens the slippage effect by changing the elastic–plastic state of coal. However, γ is negatively correlated with slippage effect. From the Walsh model, narrow (low aspect-ratio) fractures within the coal under unloading SP became the main channel for gas seepage, and bring the effective stress coefficient of permeability (χ) less than 1 for both low-stress elastic and high-stress damaged coal. With the raise of the effective stress, the effect of pore-lined clay particles on permeability was enhanced, inducing an increase in χ for high-stress elastic coal.