水位波动下沙介质中饱和度-毛细压力关系滞后研究

HYSTERESIS OF SATURATION-CAPILLARY PRESSURE RELATIONSHIP UNDER FLUCTUATING WATER LEVEL IN SANDY MEDIUM

通过对气-水两相沙介质中干燥和湿润交替过程中的饱和度-毛细压力(S-p)关系的测定,研究了干燥和湿润交替过程中沙介质内非湿润相流体(空气)运移滞后问题。研究结果表明干燥和湿润过程之间的滞后程度受沙介质干燥-湿润循环次数影响不明显;滞后程度随着循环过程中湿润过程初始水饱和度的增加而逐渐减少。通过利用Van Genuchten(VG)模型对S-p关系进行的拟合和分析发现,水位波动引起的所有干燥过程中,反映侵入压大小的α值均很接近,从实测数据及拟合参数两方面均表明了沙介质的侵入压大致是一常数,不受干燥过程的路径和初始水饱和度的影响;湿润过程中参数α随着本过程初始水饱和度的升高而不断降低,这也表明了参数α值与干燥过程与湿润过程之间滞后性存在正相关。VG模型不但可以描述一般的S-p关系曲线,同时也可用于水位波动复杂条件下干燥和湿润过程中S-p关系的描述。

Saturation-capillary pressure （S-p） relationships of consecutive drainage-imbibition cycles in an air-water two-phase sandy medium were measured for investigation of hysteresis between drainage and imbibition processes. Results show that the degree of hysteresis was obviously not related to the number of the drainage-imbibition cycles, and it decreased with increasing the initial water saturation of the imbibition process in the drainage-imbibition cycle. The S-p relations measured in the tests were fitted with the Van Genuchten （VG） model and parameters of the VG model were ana- lyzed. Results also show that parameter a related to the entry pressure approximated to a constant in all the drainage processes. Both the measured S-p relations and the parameter a indicate that the entry pressure of a medium is a constant, independent from the effects of path and initial water saturation of the drainage process. In the imbibition processes, the parameter a decreased with increasing initial water saturation, which corresponds positively to the change in hysteresis. The VG model is applicable to the description not only of simple S-p relations, but also of S-p relations under complicated drainage-imbibition cycles.