Effective stress effect and slippage effect of gas migration in deep coal reservoirs

Abstract

The deep coal reservoir is in a high in-situ stress regime, and the drop in pore pressure during the coalbed methane extraction can induce significant slippage effect and effective stress change. Both of them play a competitive role in permeability evolution. In our research, coal seepage experiments under variable bulk stress and pore pressure paths are performed to investigate the slippage effect and effective stress effect during gas migration in deep coal seams. The results show that the gas flow pattern changes from effective stress–control to slippage effect–control during the increase of bulk stress. Because of the dual pore structure and rich clay, coal presents microscopically inhomogeneous assemblage in mechanical and seepage properties. It makes the effective stress coefficient (ESC) for permeability, κ, always greater than 1, up to 1.94–2.64. By substituting the elastic modulus of the bedding for that of the clay in the coal, the clay-particle model can be used to explain the κ > 1. The linear-Klinkenberg equation is no longer applicable to the correction of gas slippage effect in deep coal reservoir, but conforms to the double-slip-Klinkenberg equation. Slippage effect weakens the stress sensitivity of permeability. As the effective stress increases, the slippage coefficient b increases accordingly, and the slippage effect becomes more pronounced. The gas flow pattern in deep coal reservoir is basically in the region of slippage flow, and the slippage effect cannot be ignored in gas migration. Furthermore, the anisotropy of κ is weakened in deep coal reservoirs, and the intrinsic permeability of coal, k∞, has the strongest stress sensitivity to σ3.