深部储层岩石渗流的滑脱效应研究——以煤和砂岩为例

Research on the slippage effect of deep reservoir rock seepage:A case study of coal and sandstone

深部高应力作用下致密储层岩石因其孔径与气体分子平均自由程相当,会诱发气体的滑脱效应,进而对瓦斯(煤层气)长期稳产产生重要影响。基于此,开展深部高应力作用下煤和砂岩的氦气注入试验。研究结果表明:在全抽采周期,高应力煤体中氦气运移始终表现出显著的滑脱效应,即使在高达8 MPa的储层压力下也不可避免,砂岩渗流则基本上不受滑脱效应影响。相较于线性Klinkenberg方程和二次型Klinkenberg修正方程,双滑移Klinkenberg方程更适用于深部高应力煤体的表观渗透率ka修正,进而得到煤体固有渗透率k_(∞)与有效平均应力σ_(vme)之间满足线性关系,这是由高应力状态下煤体内结构异质性减弱、均质性增强所致。均质性砂岩的渗流行为表现为线性力学响应特性,证明了煤体滑脱效应修正的合理性。由于深部煤体k_(∞)随σ_(vme)呈线性弱衰减规律,导致应力型渗透率在应力加载作用下略有降低,最终表现为滑脱效应随着σ_(vme)增加而减弱的反常现象。以双胡克定律为理论基础,建立综合考虑高应力煤体裂隙开度呈线性变化和滑脱型渗透率呈指数衰减规律的渗透率模型,并在不同边界条件下取得了良好的拟合效果。

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(k_(a))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(σ_(vme)),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 withσ_(vme),the stress-induced permeability exhibits a slight decline under stress loading,ultimately resulting in a decrease in the slippage effect asσ_(vme)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.