土壤气相抽提过程中多孔介质扰动的数值分析

Numerical analysis of the the changes of porous media during soil vapor extraction

基于质量守恒与流体达西定律推导水气二相流动的连续性微分方程,进而结合饱和度～相对渗透率～毛细压力耦合关系构建二相流动数学模型,并建立多孔介质孔隙度变化与水气二相饱和度之间的数学关系,最终实现多孔介质扰动时空变化的定量表征.案例模拟分析结果表明:对于特定场地而言,抽提影响带的空间形态与抽提真空度密切相关,抽提真空度越大,影响半径及影响带内的气流速度越大,本案例中抽提真空度在11kPa和31kPa时的抽提影响半径分别达到8.5m和9m;在抽提过程中,孔隙度及渗透率随时间呈现先增加后稳定的显著变化,达到稳定所需的时长及其变幅则与离抽提段的空间距离成反相关,抽提压力为0.7 105Pa、特征参数C=0.8的情景模拟显示:距离抽提段1m的P1点在约40min后孔隙度达到稳定、增幅为0.0387,而较远的P4点,距抽提段水平距离为3m,约在60min后达到稳定、增幅为0.0031,相应地,P1和P4点介质渗透率分别从1.18×10-11m2增加至2.22×10-11与1.25×10-11m2;在相同抽提压力下,孔隙度增幅与关键参数C值成正相关,抽提压力为0.9×105Pa、C=0.1和0.8时的孔隙度最大增幅分别约为0.009和0.055;相同参数C条件下,孔隙度增幅与抽提压力成正相关,C=0.8、抽提压力为0.7×105Pa时的孔隙度最大增幅则达到0.066.

Numerical method was developed to study the change of porous media caused by physical air perturbation during soil vapor extraction （SVE）. This method incorporates a water-air two phase flow model to estimate dynamics of the saturation, the relative permeability and capillary pressure during SVE, and varations of porosity is interacted based on its relationship with the saturation in the two phase flow model. Then an application of this method is shown for a field SVE case. The results reveal that the influence radius of a single SVE welt were up to 8.5m and 9m at the extraction pressure of 0.9~105 Pa and 0.7x105 Pa respectively. That indicates influence radius is proportional to the extraction vacuum for a specified site. Both the porosity and permeability increased abruptly after SVE beginning and a steady state was then gradually achieved, whilst the increases induced in vadose zone weakens with distance and the time need to reach a peak delayed with increasing distance from the SVE screen. In this case, △Ф at P1 and P4 reached the maximum of 0.0387 in 40 min and 0.0031 in 60 min respectively, whilst the permeability at P1 and P4 increased from 1.18×10^11 m^2 to 2.22×10^-11 m^2 and 1.25×10^-11 m^2 correspondingly in such simulation scenario with the extraction pressure of 0.7E5Pa and parameter C =0.8. In addition, changes of porosity and permeability were suggested to be proportional to C value and extraction vacuum. The maximum △Ф were up to 0.009 （at C =0.1） and 0.055 （at C =0.8） under the same extraction pressure of 0.9×10^5 Pa, while it was up to 0.066 under the scenario with the extraction pressure of0.7×10^5 Pa and C =0.8.