致密气藏气体克努森扩散特征

Characteristics ofKnudsen diffusion in tight gas reservoirs

从分子热运动角度推导建立了致密气藏真实气体运动的平均自由程计算公式,在此基础上,结合致密气储层温压和孔喉尺度等参数论证了致密气藏中克努森扩散的显著性。考虑克努森扩散对气体渗流的贡献作用,推导修正了达西运动方程,建立了基质表观渗透率计算模型,基于此模型分析了致密气藏中克努森扩散效应对渗流的贡献特征。研究结果表明:致密气储层中,气体分子运动的平均自由程范围为0.26~2.86μm,克努森数范围为0.4~71.5,致密气藏中克努森扩散效应不可忽略。克努森扩散效应的存在使储层表观渗透率与基质达西渗透率之间差别较大,且基质渗透率越小、压力越低,差别越大。因此,克努森扩散效应更容易产生在低压致密气藏以及气藏开发中后期。

The formula for calculating mean free path of gas molecules in tight gas reservoirs is established using molecular thermal motion theory.Based on this formula,significance of Knudsen diffusion in tight gas reservoirs is validated considering the temperature,pressure,and pore sizes in these reservoirs.The Darcy equation is corrected and then a calculation model of apparent permeability for matrix in tight gas reservoirs is developed by taking into account the contribution of Knudsen diffusion to gas flow.Results show that the range of mean free path of gas molecules and range of Knudsen number in tight gas reservoirs are 0.26~2.86μm,and 0.4~71.5,respectively.This indicates the great effect of Knudsen diffusion in tight gas reservoirs.Moreover,Knudsen diffusion leads to the disparity of apparent permeability and matrix permeability.And this disparity increases with decreasing of the matrix permeability and/or decreasing of the reservoir pressure.Hence,the Knudsen diffusion is more likely to appear in low-pressure and tight gas reservoirs.