Research on the slippage effect of deep reservoir rock seepage:A case study of coal and sandstone
The pore size of tight reservoir rock subjected to deep high stresses approximates the average free path of gas molecules,leading to occurrence of gas slippage.This phenomenon can exert a significant impact on the sustained and stable production of gas(or coalbed methane)over the long term.The helium injection experiments on coal and sandstone under deep high stresses were investigated.The results demonstrate that helium migration exhibits a significant slippage effect throughout the entire extraction cycle within a high-stress coal,even under reservoir pressures reaching up to 8 MPa.Under identical conditions,the seepage of sandstone typically remains unaffected by the slippage effect.When correcting the apparent permeability(ka)of coal under deep high stresses,the double-slip Klinkenberg equation provides more accurate results compared to the linear Klinkenberg equation and the quadratic Klinkenberg correction equation.Furthermore,the linear relationship has been established between the inherent permeability(k∞)of coal and the effective average stress(σvme),which is caused by the weakened heterogeneity and the enhanced homogeneity within the internal structure of coal subjected to high stresses.The seepage behavior of homogeneous sandstone exhibits a linear mechanical response,validating the rationality of correcting the coal slippage effect.As the inherent permeability of deep coal k∞ decreases linearly with σvme,the stress-induced permeability exhibits a slight decline under stress loading,ultimately resulting in a decrease in the slippage effect as σvme increases.Based on the principles of double Hooke's law,a permeability model was established.The model comprehensively considered the linear law of coal fracture opening under high stress and the exponential attenuation law of slippage permeability,resulting in excellent fitting outcomes across diverse boundary conditions.
mining engineeringslippage effectlinear responsedouble-slip Klinkenberg equationpermeability model
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