再谈滑脱效应

Another discussion on slippage effect

岩石的气测渗透率高于液测渗透率,且具有压力依赖性,该现象被称作滑脱效应或Klinkenberg效应。通过理论和测试资料分析,并结合流体力学原理,对该现象进行深入研究后认为,滑脱效应是一个错误认识。气测渗透率的压力依赖性是由于在计算渗透率时气体黏度取值不当所致,气体黏度在低压下随压力变化很大,但计算渗透率时却选用了定值。滑脱效应将使气体的黏度无法测量,从而出现测试悖论。气体分子时刻在做不规则的热运动,会不停地与孔隙壁面发生碰撞,致使气体无法出现滑脱。岩石渗透率的气测值高于液测值,是测试介质的分子尺度与孔隙尺度对比的结果。地下不存在离散形式的自由分子流。孔隙中只存在几个甲烷分子的地层没有开采价值,不应该作为研究对象。滑脱效应对生产实践没有任何指导意义,建议今后不再对气测渗透率进行滑脱校正。

Gas permeability of rock is higher than liquid permeability with high dependence of pressure, which is considered to be caused by slippage effect or Klinkenberg effect. Slippage effect means that gas flows through pores without any friction with the wall of pores, which is a wrong view to permeability measurement. The high pressure dependence of gas permeability is the result of misuse of gas viscosity. Gas viscosity changes dramatically with pressure under low pressure condition, but a certain gas viscosity is chosen to determine the gas permeability in laboratory.Slippage effect makes the measurement of gas viscosity impossible. Irregular thermal motions of gas molecules lead to collide into walls of pores at random, which prevents the gas molecules slip from walls of pores. The gas permeability of rocks is higher than liquid permeability, which is the result of comparison of fluid molecule size with the pores of rock. There is no free molecular motion in discrete form underground at all. A pore with only a few methane molecules in it is not worth being exploited, and is not worth being studied. Slippage effect does not make any sense for production practice. It is suggested not to take the slippage correction for gas permeability measurement.