A Physical Experiment On Air Entrapment In The Rise Of Water Table

When the phreatic level rises,there will be a part of air entrapped in the pore with the phreatic surface rises,which occupies the pore space to block pores connectivity,impeded the flow of water and even have an important effect on the permeability of the medium with the form of entrapped air.It has profound implications in studying the formation mechanism,influencing factors of air entrapped,and analyzing its influence on the permeability of media for the movement of phreatic flow.In this study,four kinds of media with different lithologies through the indoor experiment were used to simulate the test for air entrapped and measuring hydraulic conductivity when the phreatic level rises.Through it we can analyze the influencing factors of air entrapped,then use methods of fitting empirical formula,non-saturation methods,circular tube flow methods,and film flow and angular flow methods to quantitatively the relationship of entrapped air saturation and hydraulic conductivity.The achievements of the thesis include:(1)Fine-grained media traps more air than the coarse-grained media during the waterflooding process,because fine-grained media tend to be entrapped due to snap-off mechanism that because of the small size,pores and wide distribution range,and on the other hand,the entrapped air is not easy to escape wtih low pressure in fine-grained media.The results of the same medium with different displacement rate tests show that the more entrapped air was closed in medium with smaller displacement rate,that because faster the rate was,the more difficult to generate bubbles and the bubble formation rate was correspondingly lower.Further more,a faster rate would promote the flow between the pores to more easily displace the gas bubbles out of the pores of the medium.(2)The results of the determination of the hydraulic conductivity with different air saturation show that the saturation of entrapped air becomes larger with the decreases of hydraulic conductivity.A best fitting empirical formula is proposed in this paper based on the relationship between air saturation and hydraulic conductivity is a power function.(3)The method of the unsaturated hydraulic conductivity fitting shows that the model of Brooks-Core(1964)unsaturated permeability coefficient is closest to the measured value,but after comparing the parameters,it is found that the fitting parameter is inconsistent with the actual calculated parameter,which explains that the permeation of the air entrapped process has the same form as the calculation of the unsaturated permeation,but it is not the true sense of the unsaturated permeation.(4)The study of the relationship between the entrapped air and the hydraulic conductivity of the circular tube flow shows that the hydraulic conductivity obtained from the circular tube flow calcul ation has a large difference from the measured value,so the circular tube flow is not completely suitable for the theoretical calculation and analysis of the air entrapped process.Although the conceptual model based on film flow and angular flow can describe the entrap mechanism well,when calculating the permeability coefficient,the calculation result and the actual result do not fit well because of the lack of consideration in the pore scale and resistance.